Independent Submission D. Savage
Request for Comments: 7868 J. Ng
Category: Informational S. Moore
ISSN: 2070-1721 Cisco Systems
D. Slice
Cumulus Networks
P. Paluch
University of Zilina
R. White
LinkedIn
May 2016
Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP)
Abstract
This document describes the protocol design and architecture for
Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP is a
routing protocol based on Distance Vector technology. The specific
algorithm used is called "DUAL", a Diffusing Update Algorithm as
referenced in "Loop-Free Routing Using Diffusing Computations"
(Garcia-Luna-Aceves 1993). The algorithm and procedures were
researched, developed, and simulated by SRI International.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7868.
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Table of Contents
1. Introduction ....................................................5
2. Conventions .....................................................5
2.1. Requirements Language ......................................5
2.2. Terminology ................................................5
3. The Diffusing Update Algorithm (DUAL) ...........................9
3.1. Algorithm Description ......................................9
3.2. Route States ..............................................10
3.3. Feasibility Condition .....................................11
3.4. DUAL Message Types ........................................13
3.5. DUAL Finite State Machine (FSM) ...........................13
3.6. DUAL Operation -- Example Topology ........................18
4. EIGRP Packets ..................................................20
4.1. UPDATE Packets ............................................21
4.2. QUERY Packets .............................................21
4.3. REPLY Packets .............................................22
4.4. Exception Handling ........................................22
4.4.1. Active Duration (SIA) ..............................22
4.4.1.1. SIA-QUERY .................................23
4.4.1.2. SIA-REPLY .................................24
5. EIGRP Operation ................................................25
5.1. Finite State Machine ......................................25
5.2. Reliable Transport Protocol ...............................25
5.2.1. Bandwidth on Low-Speed Links .......................32
5.3. Neighbor Discovery/Recovery ...............................32
5.3.1. Neighbor Hold Time .................................32
5.3.2. HELLO Packets ......................................33
5.3.3. UPDATE Packets .....................................33
5.3.4. Initialization Sequence ............................34
5.4. Topology Table ............................................36
5.4.1. Route Management ...................................36
5.4.1.1. Internal Routes ...........................37
5.4.1.2. External Routes ...........................37
5.4.2. Split Horizon and Poison Reverse ...................38
5.4.2.1. Startup Mode ..............................38
5.4.2.2. Advertising Topology Table Change .........39
5.4.2.3. Sending a QUERY/UPDATE ....................39
5.5. EIGRP Metric Coefficients .................................39
5.5.1. Coefficients K1 and K2 .............................40
5.5.2. Coefficient K3 .....................................40
5.5.3. Coefficients K4 and K5 .............................40
5.5.4. Coefficient K6 .....................................41
5.5.4.1. Jitter ....................................41
5.5.4.2. Energy ....................................41
5.6. EIGRP Metric Calculations .................................41
5.6.1. Classic Metrics ....................................41
5.6.1.1. Classic Composite Formulation .............42
5.6.1.2. Cisco Interface Delay Compatibility .......43
5.6.2. Wide Metrics .......................................43
5.6.2.1. Wide Metric Vectors .......................44
5.6.2.2. Wide Metric Conversion Constants ..........45
5.6.2.3. Throughput Calculation ....................45
5.6.2.4. Latency Calculation .......................46
5.6.2.5. Composite Calculation .....................46
6. EIGRP Packet Formats ...........................................46
6.1. Protocol Number ...........................................46
6.2. Protocol Assignment Encoding ..............................47
6.3. Destination Assignment Encoding ...........................47
6.4. EIGRP Communities Attribute ...............................48
6.5. EIGRP Packet Header .......................................49
6.6. EIGRP TLV Encoding Format .................................51
6.6.1. Type Field Encoding ................................52
6.6.2. Length Field Encoding ..............................52
6.6.3. Value Field Encoding ...............................52
6.7. EIGRP Generic TLV Definitions .............................52
6.7.1. 0x0001 - PARAMETER_TYPE ............................53
6.7.2. 0x0002 - AUTHENTICATION_TYPE .......................53
6.7.2.1. 0x02 - MD5 Authentication Type ............54
6.7.2.2. 0x03 - SHA2 Authentication Type ...........54
6.7.3. 0x0003 - SEQUENCE_TYPE .............................54
6.7.4. 0x0004 - SOFTWARE_VERSION_TYPE .....................55
6.7.5. 0x0005 - MULTICAST_SEQUENCE_TYPE ...................55
6.7.6. 0x0006 - PEER_INFORMATION_TYPE .....................55
6.7.7. 0x0007 - PEER_ TERMINATION_TYPE ....................56
6.7.8. 0x0008 - TID_LIST_TYPE .............................56
6.8. Classic Route Information TLV Types .......................57
6.8.1. Classic Flag Field Encoding ........................57
6.8.2. Classic Metric Encoding ............................57
6.8.3. Classic Exterior Encoding ..........................58
6.8.4. Classic Destination Encoding .......................59
6.8.5. IPv4-Specific TLVs .................................59
6.8.5.1. IPv4 INTERNAL_TYPE ........................60
6.8.5.2. IPv4 EXTERNAL_TYPE ........................60
6.8.5.3. IPv4 COMMUNITY_TYPE .......................62
6.8.6. IPv6-Specific TLVs .................................62
6.8.6.1. IPv6 INTERNAL_TYPE ........................63
6.8.6.2. IPv6 EXTERNAL_TYPE ........................63
6.8.6.3. IPv6 COMMUNITY_TYPE .......................65
6.9. Multiprotocol Route Information TLV Types .................66
6.9.1. TLV Header Encoding ................................66
6.9.2. Wide Metric Encoding ...............................67
6.9.3. Extended Metrics ...................................68
6.9.3.1. 0x00 - NoOp ...............................69
6.9.3.2. 0x01 - Scaled Metric ......................70
6.9.3.3. 0x02 - Administrator Tag ..................70
6.9.3.4. 0x03 - Community List .....................71
6.9.3.5. 0x04 - Jitter .............................71
6.9.3.6. 0x05 - Quiescent Energy ...................71
6.9.3.7. 0x06 - Energy .............................72
6.9.3.8. 0x07 - AddPath ............................72
6.9.3.8.1. AddPath with IPv4 Next Hop .....73
6.9.3.8.2. AddPath with IPv6 Next Hop .....74
6.9.4. Exterior Encoding ..................................75
6.9.5. Destination Encoding ...............................76
6.9.6. Route Information ..................................76
6.9.6.1. INTERNAL TYPE .............................76
6.9.6.2. EXTERNAL TYPE .............................76
7. Security Considerations ........................................77
8. IANA Considerations ............................................77
9. References .....................................................77
9.1. Normative References ......................................77
9.2. Informative References ....................................78
Acknowledgments ...................................................79
Authors' Addresses ................................................80
1. Introduction
This document describes the Enhanced Interior Gateway Routing
Protocol (EIGRP), a routing protocol designed and developed by Cisco
Systems, Inc. DUAL, the algorithm used to converge the control plane
to a single set of loop-free paths is based on research conducted at
SRI International [3]. The Diffusing Update Algorithm (DUAL) is the
algorithm used to obtain loop freedom at every instant throughout a
route computation [2]. This allows all routers involved in a
topology change to synchronize at the same time; the routers not
affected by topology changes are not involved in the recalculation.
This document describes the protocol that implements these functions.
2. Conventions
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
2.2. Terminology
The following is a list of abbreviations and terms used throughout
this document:
ACTIVE State:
The local state of a route on a router triggered by any event that
causes all neighbors providing the current least-cost path to fail
the Feasibility Condition check. A route in Active state is
considered unusable. During Active state, the router is actively
attempting to compute the least-cost loop-free path by explicit
coordination with its neighbors using Query and Reply messages.
Address Family Identifier (AFI):
Identity of the network-layer protocol reachability information
being advertised [12].
Autonomous System (AS):
A collection of routers exchanging routes under the control of one
or more network administrators on behalf of a single
administrative entity.
Base Topology:
A routing domain representing a physical (non-virtual) view of the
network topology consisting of attached devices and network
segments EIGRP uses to form neighbor relationships. Destinations
exchanged within the Base Topology are identified with a Topology
Identifier value of zero (0).
Computed Distance (CD):
Total distance (metric) along a path from the current router to a
destination network through a particular neighbor computed using
that neighbor's Reported Distance (RD) and the cost of the link
between the two routers. Exactly one CD is computed and
maintained per the [Destination, Advertising Neighbor] pair.
CR-Mode
Conditionally Received Mode
Diffusing Computation:
A distributed computation in which a single starting node
commences the computation by delegating subtasks of the
computation to its neighbors that may, in turn, recursively
delegate sub-subtasks further, including a signaling scheme
allowing the starting node to detect that the computation has
finished while avoiding false terminations. In DUAL, the task of
coordinated updates of routing tables and resulting best path
computation is performed as a diffusing computation.
Diffusing Update Algorithm (DUAL):
A loop-free routing algorithm used with distance vectors or link
states that provides a diffused computation of a routing table.
It works very well in the presence of multiple topology changes
with low overhead. The technology was researched and developed at
SRI International [3].
Downstream Router:
A router that is one or more hops away from the router in question
in the direction of the destination.
EIGRP:
Enhanced Interior Gateway Routing Protocol.
Feasibility Condition:
The Feasibility Condition is a sufficient condition used by a
router to verify whether a neighboring router provides a loop-free
path to a destination. EIGRP uses the Source Node Condition
stating that a neighboring router meets the Feasibility Condition
if the neighbor's RD is less than this router's Feasible Distance.
Feasible Distance (FD):
Defined as the least-known total metric to a destination from the
current router since the last transition from ACTIVE to PASSIVE
state. Being effectively a record of the smallest known metric
since the last time the network entered the PASSIVE state, the FD
is not necessarily a metric of the current best path. Exactly one
FD is computed per destination network.
Feasible Successor:
A neighboring router that meets the Feasibility Condition for a
particular destination, hence, providing a guaranteed loop-free
path.
Neighbor/Peer:
For a particular router, another router toward which an EIGRP
session, also known as an "adjacency", is established. The
ability of two routers to become neighbors depends on their mutual
connectivity and compatibility of selected EIGRP configuration
parameters. Two neighbors with interfaces connected to a common
subnet are known as adjacent neighbors. Two neighbors that are
multiple hops apart are known as remote neighbors.
PASSIVE state:
The local state of a route in which at least one neighbor
providing the current least-cost path passes the Feasibility
Condition check. A route in PASSIVE state is considered usable
and not in need of a coordinated re-computation.
Network Layer Reachability Information (NLRI):
Information a router uses to calculate the global routing table to
make routing and forwarding decisions.
Reported Distance (RD):
For a particular destination, the value representing the router's
distance to the destination as advertised in all messages carrying
routing information. RD is not equivalent to the current distance
of the router to the destination and may be different from it
during the process of path re-computation. Exactly one RD is
computed and maintained per destination network.
Sub-Topology:
For a given Base Topology, a sub-topology is characterized by an
independent set of routers and links in a network for which EIGRP
performs an independent path calculation. This allows each sub-
topology to implement class-specific topologies to carry class-
specific traffic.
Successor:
For a particular destination, a neighboring router that meets the
Feasibility Condition and, at the same time, provides the least-
cost path.
Stuck In Active (SIA):
A destination that has remained in the ACTIVE State in excess of a
predefined time period at the local router (Cisco implements this
as 3 minutes).
Successor-Directed Acyclic Graph (SDAG):
For a particular destination, a graph defined by routing table
contents of individual routers in the topology, such that nodes of
this graph are the routers themselves and a directed edge from
router X to router Y exists if and only if router Y is router X's
successor. After the network has converged, in the absence of
topological changes, SDAG is a tree.
Topology Change / Topology-Change Event:
Any event that causes the CD for a destination through a neighbor
to be added, modified, or removed. As an example, detecting a
link-cost change, receiving any EIGRP message from a neighbor
advertising an updated neighbor's RD.
Topology Identifier (TID):
A number that is used to mark prefixes as belonging to a specific
sub-topology.
Topology Table:
A data structure used by EIGRP to store information about every
known destination including, but not limited to, network prefix /
prefix length, FD, RD of each neighbor advertising the
destination, CD over the corresponding neighbor, and route state.
Type, Length, Value (TLV):
An encoding format for information elements used in EIGRP messages
to exchange information. Each TLV-formatted information element
consists of three generic fields: Type identifying the nature of
information carried in this element, Length describing the length
of the entire TLV triplet, and Value carrying the actual
information. The Value field may, itself, be internally
structured; this depends on the actual type of the information
element. This format allows for extensibility and backward
compatibility.
Upstream Router:
A router that is one or more hops away from the router in
question, in the direction of the source of the information.
VID:
VLAN Identifier
Virtual Routing and Forwarding (VRF):
Independent Virtual Private Network (VPN) routing/forwarding
tables that coexist within the same router at the same time.
3. The Diffusing Update Algorithm (DUAL)
The Diffusing Update Algorithm (DUAL) constructs least-cost paths to
all reachable destinations in a network consisting of nodes and edges
(routers and links). DUAL guarantees that each constructed path is
loop free at every instant including periods of topology changes and
network reconvergence. This is accomplished by all routers, which
are affected by a topology change, computing the new best path in a
coordinated (diffusing) way and using the Feasibility Condition to
verify prospective paths for loop freedom. Routers that are not
affected by topology changes are not involved in the recalculation.
The convergence time with DUAL rivals that of any other existing
routing protocol.
3.1. Algorithm Description
DUAL is used by EIGRP to achieve fast loop-free convergence with
little overhead, allowing EIGRP to provide convergence rates
comparable, and in some cases better than, most common link state
protocols [10]. Only nodes that are affected by a topology change
need to propagate and act on information about the topology change,
allowing EIGRP to have good scaling properties, reduced overhead, and
lower complexity than many other interior gateway protocols.
Distributed routing algorithms are required to propagate information
as well as coordinate information among all nodes in the network.
Unlike basic Bellman-Ford distance vector protocols that rely on
uncoordinated updates when a topology change occurs, DUAL uses a
coordinated procedure to involve the affected part of the network
into computing a new least-cost path, known as a "diffusing
computation". A diffusing computation grows by querying additional
routers for their current RD to the affected destination, and it
shrinks by receiving replies from them. Unaffected routers send
replies immediately, terminating the growth of the diffusing
computation over them. These intrinsic properties cause the
diffusing computation to self-adjust in scope and terminate as soon
as possible.
One attribute of DUAL is its ability to control the point at which
the diffusion of a route calculation terminates by managing the
distribution of reachability information through the network.
Controlling the scope of the diffusing process is accomplished by
hiding reachability information through aggregation (summarization),
filtering, or other means. This provides the ability to create
effective failure domains within a single AS, and allows the network
administrator to manage the convergence and processing
characteristics of the network.
3.2. Route States
A route to a destination can be in one of two states: PASSIVE or
ACTIVE. These states describe whether the route is guaranteed to be
both loop free and the shortest available (the PASSIVE state) or
whether such a guarantee cannot be given (the ACTIVE state).
Consequently, in PASSIVE state, the router does not perform any route
recalculation in coordination with its neighbors because no such
recalculation is needed.
In ACTIVE state, the router is actively involved in re-computing the
least-cost loop-free path in coordination with its neighbors. The
state is reevaluated and possibly changed every time a topology
change is detected. A topology change is any event that causes the
CD to the destination over any neighbor to be added, changed, or
removed from EIGRP's topology table.
More exactly, the two states are defined as follows:
o Passive
A route is considered to be in the Passive state when at least one
neighbor that provides the current least-total-cost path passes
the Feasibility Condition check that guarantees loop freedom. A
route in the PASSIVE state is usable and its next hop is perceived
to be a downstream router.
o Active
A route is considered to be in the ACTIVE state if neighbors that
do not pass the Feasibility Condition check provide lowest-cost
path, and therefore the path cannot be guaranteed loop free. A
route in the ACTIVE state is considered unusable and this router
must coordinate with its neighbors in the search for the new loop-
free least-total-cost path.
In other words, for a route to be in PASSIVE state, at least one
neighbor that provides the least-total-cost path must be a Feasible
Successor. Feasible Successors providing the least-total-cost path
are also called "Successors". "successors". For a route to be in PASSIVE state, at
least one Successor successor must exist.
Conversely, if the path with the least total cost is provided by
routers that are not Feasible Successors (and thus not Successors), successors),
the route is in the ACTIVE state, requiring re-computation.
Notably, for the definition of PASSIVE and ACTIVE states, it does not
matter if there are Feasible Successors providing a worse-than-least-
total-cost path. While these neighbors are guaranteed to provide a
loop-free path, that path is potentially not the shortest available.
The fact that the least-total-cost path can be provided by a neighbor
that fails the Feasibility Condition check may not be intuitive.
However, such a situation can occur during topology changes when the
current least-total-cost path fails and the next-least-total-cost
path traverses a neighbor that is not a Feasible Successor.
While a router has a route in the ACTIVE state, it must not change
its Successor successor (i.e., modify the current SDAG) nor modify its own
Feasible Distance or RD until the route enters the PASSIVE state
again. Any updated information about this route received during
ACTIVE state is reflected only in CDs. Any updates to the Successor, successor,
FD, and RD are postponed until the route returns to PASSIVE state.
The state transitions from PASSIVE to ACTIVE and from ACTIVE to
PASSIVE are controlled by the DUAL FSM and are described in detail in
Section 3.5.
3.3. Feasibility Condition
The Feasibility Condition is a criterion used to verify loop freedom
of a particular path. The Feasibility Condition is a sufficient but
not a necessary condition, meaning that every path meeting the
Feasibility Condition is guaranteed to be loop free; however, not all
loop-free paths meet the Feasibility Condition.
The Feasibility Condition is used as an integral part of DUAL
operation: every path selection in DUAL is subject to the Feasibility
Condition check. Based on the result of the Feasibility Condition
check after a topology change is detected, the route may either
remain PASSIVE (if, after the topology change, the neighbor providing
the least cost path meets the Feasibility Condition) or it needs to
enter the ACTIVE state (if the topology change resulted in none of
the neighbors providing the least cost path to meet the Feasibility
Condition).
The Feasibility Condition is a part of DUAL that allows the diffused
computation to terminate as early as possible. Nodes that are not
affected by the topology change are not required to perform a DUAL
computation and may not be aware a topology change occurred. This
can occur in two cases:
First, if informed about a topology change, a router may keep a route
in PASSIVE state if it is aware of other paths that are downstream
towards the destination (routes meeting the Feasibility Condition).
A route that meets the Feasibility Condition is determined to be loop
free and downstream along the path between the router and the
destination.
Second, if informed about a topology change for which it does not
currently have reachability information, a router is not required to
enter into the ACTIVE state, nor is it required to participate in the
DUAL process.
In order to facilitate describing the Feasibility Condition, a few
definitions are in order.
o A Successor successor for a given route is the next hop used to forward data
traffic for a destination. Typically, the successor is chosen
based on the least-cost path to reach the destination.
o A Feasible Successor is a neighbor that meets the Feasibility
Condition. A Feasible Successor is regarded as a downstream
neighbor towards the destination, but it may not be the least-cost
path but could still be used for forwarding data packets in the
event equal or unequal cost load sharing was active. A Feasible
Successor can become a successor when the current successor
becomes unreachable.
o The Feasibility Condition is met when a neighbor's advertised
cost, (RD) to a destination is less than the FD for that
destination, or in other words, the Feasibility Condition is met
when the neighbor is closer to the destination than the router
itself has ever been since the destination has entered the PASSIVE
state for the last time.
o The FD is the lowest distance to the destination since the last
time the route went from ACTIVE to PASSIVE state. It should be
noted it is not necessarily the current best distance; rather, it
is a historical record of the best distance known since the last
diffusing computation for the destination has finished. Thus, the
value of the FD can either be the same as the current best
distance, or it can be lower.
A neighbor that advertises a route with a cost that does not meet the
Feasibility Condition may be upstream and thus cannot be guaranteed
to be the next hop for a loop-free path. Routes advertised by
upstream neighbors are not recorded in the routing table but saved in
the topology table.
3.4. DUAL Message Types
DUAL operates with three basic message types: QUERY, UPDATE, and
REPLY.
o UPDATE - sent to indicate a change in metric or an addition of a
destination.
o QUERY - sent when the Feasibility Condition fails, which can
happen for reasons like a destination becoming unreachable or the
metric increasing to a value greater than its current FD.
o REPLY - sent in response to a QUERY or SIA-QUERY
In addition to these three basic types, two additional sub-types have
been added to EIGRP:
o SIA-QUERY - sent when a REPLY has not been received within one-
half of the SIA interval (90 seconds as implemented by Cisco).
o SIA-REPLY - sent in response to an SIA-QUERY indicating the route
is still in ACTIVE state. This response does not stratify the
original QUERY; it is only used to indicate that the sending
neighbor is still in the ACTIVE state for the given destination.
When in the PASSIVE state, a received QUERY may be propagated if
there is no Feasible Successor found. If a Feasible Successor is
found, the QUERY is not propagated and a REPLY is sent for the
destination with a metric equal to the current routing table metric.
When a QUERY is received from a non-successor in ACTIVE state, a
REPLY is sent and the QUERY is not propagated. The REPLY for the
destination contains a metric equal to the current routing table
metric.
3.5. DUAL Finite State Machine (FSM)
The DUAL FSM embodies the decision process for all route
computations. It tracks all routes advertised by all neighbors. The
distance information, known as a metric, is used by DUAL to select
efficient loop-free paths. DUAL selects routes to be inserted into a
routing table based on Feasible Successors. A successor is a
neighboring router used for packet forwarding that has a least-cost
path to a destination that is guaranteed not to be part of a routing
loop.
When there are no Feasible Successors but there are neighbors
advertising the destination, a recalculation must occur to determine
a new successor.
The amount of time it takes to calculate the route impacts the
convergence time. Even though the recalculation is not processor
intensive, it is advantageous to avoid recalculation if it is not
necessary. When a topology change occurs, DUAL will test for
Feasible Successors. If there are Feasible Successors, it will use
any it finds in order to avoid any unnecessary recalculation.
The FSM, which applies per destination in the topology table,
operates independently for each destination. It is true that if a
single link goes down, multiple routes may go into ACTIVE state.
However, a separate SDAG is computed for each destination, so loop-
free topologies can be maintained for each reachable destination.
Figure 1 illustrates the FSM:
+------------+ +-----------+
| \ / |
| \ / |
| +=================================+ |
| | | |
|(1)| Passive |(2)|
+-->| |<--+
+=================================+
^ | ^ ^ ^ |
(14)| |(15)| |(13)| |
| (4)| |(16)| | (3)|
| | | | | +------------+
| | | | | \
+-------+ + + | +-------------+ \
/ / / | \ \
/ / / +----+ \ \
| | | | | |
| v | | | v
+==========+(11) +==========+ +==========+(12) +==========+
| Active |---->| Active |(5) | Active |---->| Active |
| | (9)| |---->| | (10)| |
| Oij=0 oij=0 |<----| Oij=1 oij=1 | | Oij=2 oij=2 |<----| Oij=3 oij=3 |
+--| | +--| | +--| | +--| |
| +==========+ | +==========+ | +==========+ | +==========+
| ^ |(5) | ^ | ^ ^ | ^
| | +-----|------|---------|----+ | | |
+------+ +------+ +---------+ +---------+
(6,7,8) (6,7,8) (6,7,8) (6,7,8)
Figure 1: DUAL Finite State Machine
Legend:
i Node that is computing route
j Destination node or network
k Any neighbor of node i
Oij
oij QUERY origin flag
0 = metric increase during ACTIVE state
1 = node i originated
2 = QUERY from, or link increase to, successor during ACTIVE state
3 = QUERY originated from successor
rijk REPLY status flag for each neighbor k for destination j
1 = awaiting REPLY
0 = received REPLY
lik = the link connecting node i to neighbor k
The following describes in detail the state/event/action transitions
of the DUAL FSM. For all steps, the topology table is updated with
the new metric information from either QUERY, REPLY, or UPDATE
received.
(1) A QUERY is received from a neighbor that is not the current
successor. The route is currently in PASSIVE state. As the
Successor
successor is not affected by the QUERY, and a Feasible Successor
exists, the route remains in PASSIVE state. Since a Feasible
Successor exists, a REPLY MUST be sent back to the originator of
the QUERY. Any metric received in the QUERY from that neighbor
is recorded in the topology table and FC the Feasibility Check (FC)
is run to check for any change to current successor.
(2) A directly connected interface changes state (connects,
disconnects, or changes metric), or similarly an UPDATE or QUERY
has been received with a metric change for an existing
destination, the route will stay in the PASSIVE state if the
current successor is not affected by the change, or it is no
longer reachable and there is a Feasible Successor. In either
case, an UPDATE is sent with the new metric information if it
has changed.
(3) A QUERY was received from a neighbor who is the current
successor and no Feasible Successors exist. The route for the
destination goes into ACTIVE state. A QUERY is sent to all
neighbors on all interfaces that are not split horizon. Split
horizon takes effect for a query or update from the successor it
is using for the destination in the query. The QUERY origin
flag is set to indicate the QUERY originated from a neighbor
marked as successor for route. The REPLY status flag is set for
all neighbors to indicate outstanding replies.
(4) A directly connected link has gone down or its cost has
increased, or an UPDATE has been received with a metric
increase. The route to the destination goes to ACTIVE state if
there are no Feasible Successors found. A QUERY is sent to all
neighbors on all interfaces. The QUERY origin flag is to
indicate that the router originated the QUERY. The REPLY status
flag is set to 1 for all neighbors to indicate outstanding
replies.
(5) While a route for a destination is in ACTIVE state, and a QUERY
is received from the current successor, the route remains in
ACTIVE state. The QUERY origin flag is set to indicate that
there was another topology change while in ACTIVE state. This
indication is used so new Feasible Successors are compared to
the metric that made the route go to ACTIVE state with the
current successor.
(6) While a route for a destination is in ACTIVE state and a QUERY
is received from a neighbor that is not the current successor, a
REPLY should be sent to the neighbor. The metric received in
the QUERY should be recorded.
(7) If a link cost changes, or an UPDATE with a metric change is
received in ACTIVE state from a non-successor, the router stays
in ACTIVE state for the destination. The metric information in
the UPDATE is recorded. When a route is in the ACTIVE state,
neither a QUERY nor UPDATE are ever sent.
(8) If a REPLY for a destination, in ACTIVE state, is received from
a neighbor or the link between a router and the neighbor fails,
the router records that the neighbor replied to the QUERY. The
REPLY status flag is set to 0 to indicate this. The route stays
in ACTIVE state if there are more replies pending because the
router has not heard from all neighbors.
(9) If a route for a destination is in ACTIVE state, and a link
fails or a cost increase occurred between a router and its
successor, the router treats this case like it has received a
REPLY from its successor. When this occurs after the router
originates a QUERY, it sets the QUERY origin flag to indicate
that another topology change occurred in ACTIVE state.
(10) If a route for a destination is in ACTIVE state, and a link
fails or a cost increase occurred between a router and its
successor, the router treats this case like it has received a
REPLY from its successor. When this occurs after a successor
originated a QUERY, the router sets the QUERY origin flag to
indicate that another topology change occurred in ACTIVE state.
(11) If a route for a destination is in ACTIVE state, the cost of the
link through which the successor increases, and the last REPLY
was received from all neighbors, but there is no Feasible
Successor, the route should stay in ACTIVE state. A QUERY is
sent to all neighbors. The QUERY origin flag is set to 1.
(12) If a route for a destination is in ACTIVE state because of a
QUERY received from the current successor, and the last REPLY
was received from all neighbors, but there is no Feasible
Successor, the route should stay in ACTIVE state. A QUERY is
sent to all neighbors. The QUERY origin flag is set to 3.
(13) Received replies from all neighbors. Since the QUERY origin
flag indicates the successor originated the QUERY, it
transitions to PASSIVE state and sends a REPLY to the old
successor.
(14) Received replies from all neighbors. Since the QUERY origin
flag indicates a topology change to the successor while in
ACTIVE state, it need not send a REPLY to the old successor.
When the Feasibility Condition is met, the route state
transitions to PASSIVE.
(15) Received replies from all neighbors. Since the QUERY origin
flag indicates either the router itself originated the QUERY or
FC was not satisfied with the replies received in ACTIVE state,
FD is reset to infinite value and the minimum of all the
reported metrics is chosen as FD and route transitions back to
PASSIVE state. A REPLY is sent to the old-successor if Oij oij
flags indicate that there was a QUERY from successor.
(16) If a route for a destination is in ACTIVE state because of a
QUERY received from the current successor or there was an
increase in distance while in ACTIVE state, the last REPLY was
received from all neighbors, and a Feasible Successor exists for
the destination, the route can go into PASSIVE state and a REPLY
is sent to the successor if Oij oij indicates that QUERY was
received from the successor.
3.6. DUAL Operation -- Example Topology
The following topology (Figure 2) will be used to provide an example
of how DUAL is used to reroute after a link failure. Each node is
labeled with its costs to destination N. The arrows indicate the
successor (next hop) used to reach destination N. The least-cost
path is selected.
N
|
(1)A ---<--- B(2)
| |
^ |
| |
(2)D ---<--- C(3)
Figure 2: Stable Topology
In the case where the link between A and D fails (Figure 3);
N N
| |
A ---<--- B A ---<--- B
| | | |
X | ^ |
| | | |
D ---<--- C D ---<--- C
Q-> <-R
N
|
(1)A ---<--- B(2)
|
^
|
(4)D --->--- C(3)
Figure 3: Link between A and D Fails
Only observing the destination provided by node N, D enters the
ACTIVE state and sends a QUERY to all its neighbors, in this case
node C.
C determines that it has a Feasible Successor and replies
immediately with metric 3.
C changes its old successor of D to its new single successor B
and the route to N stays in PASSIVE state.
D receives the REPLY and can transition out of ACTIVE state
since it received replies from all its neighbors.
D now has a viable path to N through C.
D selects C as its successor to reach node N with a cost of 4.
Notice that nodes A and B were not involved in the recalculation
since they were not affected by the change.
Let's consider the situation in Figure 4, where Feasible Successors
may not exist. If the link between node A and B fails, B goes into
ACTIVE state for destination N since it has no Feasible Successors.
Node B sends a QUERY to node C. C has no Feasible Successors, so it
goes active for destination N; and since C has no neighbors, it
replies to the QUERY, deletes the destination, and returns to the
PASSIVE state for the unreachable route. As C removes the (now
unreachable) destination from its table, C sends REPLY to its old
successor. B receives this REPLY from C, and determines this is the
last REPLY it is waiting on before determining what the new state of
the route should be; on receiving this REPLY, B deletes the route to
N from its routing table.
Since B was the originator of the initial QUERY, it does not have to
send a REPLY to its old successor (it would not be able to any ways,
because the link to its old successor is down). Note that nodes A
and D were not involved in the recalculation since their successors
were not affected.
N N
| |
(1)A ---<--- B(2) A ------- B Q
| | | | |^ ^
^ ^ ^ | v| |
| | | | | |
(2)D C(3) D C ACK R
Figure 4: No Feasible Successors When Link between A and B Fails
4. EIGRP Packets
EIGRP uses five different packet types to handle session management
and pass DUAL Message types:
HELLO Packets (includes ACK)
QUERY Packets (includes SIA-Query)
REPLY Packets (includes SIA-Reply)
REQUEST Packets
UPDATE Packets
EIGRP packets are directly encapsulated into a network-layer
protocol, such as IPv4 or IPv6. While EIGRP is capable of using
additional encapsulation (such as AppleTalk, IPX, etc.) no further
encapsulation is specified in this document.
Support for network-layer protocol fragmentation is not supported,
and EIGRP will attempt to avoid a maximum size packets that exceed
the interface MTU by sending multiple packets that are less than or
equal to MTU-sized packets.
Each packet transmitted will use either multicast or unicast network-
layer destination addresses. When multicast addresses are used, a
mapping for the data link multicast address (when available) must be
provided. The source address will be set to the address of the
sending interface, if applicable.
The following network-layer multicast addresses and associated data
link multicast addresses:
224.0.0.10 for IPv4 "EIGRP Routers" [13]
FF02:0:0:0:0:0:0:A for IPv6 "EIGRP Routers" [14]
They will be used on multicast-capable media and will be media
independent for unicast addresses. Network-layer addresses will be
used and the mapping to media addresses will be achieved by the
native protocol mechanisms.
4.1. UPDATE Packets
UPDATE packets carry the DUAL UPDATE message type and are used to
convey information about destinations and the reachability of those
destinations. When a new neighbor is discovered, unicast UPDATE
packets are used to transmit a full table to the new neighbor, so the
neighbor can build up its topology table. In normal operation (other
than neighbor startup such as a link cost changes), UPDATE packets
are multicast. UPDATE packets are always transmitted reliably. Each
TLV destination will be processed individually through the DUAL state
machine. FSM.
4.2. QUERY Packets
A QUERY packet carries the DUAL QUERY message type and is sent by a
router to advertise that a route is in ACTIVE state and the
originator is requesting alternate path information from its
neighbors. An infinite metric is encoded by setting the Delay delay part
of the metric to its maximum value.
If there is a topology change that causes multiple destinations to be
marked ACTIVE, EIGRP will build one or more QUERY packets for all
destinations present. The state of each route is recorded
individually, so a responding QUERY or REPLY need not contain all the
same destinations in a single packet. Since EIGRP uses a reliable
transport mechanism, route QUERY packets are also guaranteed be
reliably delivered.
When a QUERY packet is received, each destination will trigger a DUAL
event, and the state machine will run individually for each route.
Once the entire original QUERY packet is processed, then a REPLY or
SIA-REPLY will be sent with the latest information.
4.3. REPLY Packets
A REPLY packet carries the DUAL REPLY message type and will be sent
in response to a QUERY or SIA-QUERY packet. The REPLY packet will
include a TLV for each destination and the associated vector metric
in its own topology table.
The REPLY packet is sent after the entire received QUERY packet is
processed. When a REPLY packet is received, there is no reason to
process the packet before an acknowledgment is sent. Therefore, an
acknowledgment is sent immediately and then the packet is processed.
The sending of the acknowledgment is accomplished either by sending
an ACK packet or by piggybacking the acknowledgment onto another
packet already being transmitted.
Each TLV destination will be processed individually through the DUAL
state machine.
FSM. When a QUERY is received for a route that doesn't exist in our
topology table, a REPLY with an infinite metric is sent and an entry
in the topology table is added with the metric in the QUERY if the
metric is not an infinite value.
If a REPLY for a designation not in the Active state, or not in the
topology table, EIGRP will acknowledge the packet and discard the
REPLY.
4.4. Exception Handling
4.4.1. Active Duration (SIA)
When an EIGRP router transitions to ACTIVE state for a particular
destination, a QUERY is sent to a neighbor and the ACTIVE timer is
started to limit the amount of time a destination may remain in an
ACTIVE state.
A route is regarded as SIA when it does not receive a REPLY within a
preset time. This time interval is broken into two equal periods
following the QUERY, and up to three additional "busy" periods in
which an SIA-QUERY packet is sent for the destination.
This process is begun when a router sends a QUERY to its neighbor.
After one-half the SIA time interval (default implementation is 90
seconds), the router will send an SIA-QUERY; this must be replied to
with either a REPLY or SIA-REPLY. Any neighbor that fails to send
either a REPLY or SIA-REPLY with-in one-half the SIA interval will
result in the neighbor being deemed to be "stuck" in the active
state.
Cisco also limits the number of SIA-REPLY messages allowed to three.
Once the timeout occurs after the third SIA-REPLY with the neighbor
remaining in an ACTIVE state (as noted in the SIA-Reply message), the
neighbor being deemed to be "stuck" in the active state.
If the SIA state is declared, DUAL may take one of two actions;
a) Delete the route from that neighbor, acting as if the neighbor
had responded with an unreachable REPLY message from the
neighbor.
b) Delete all routes from that neighbor and reset the adjacency
with that neighbor, acting as if the neighbor had responded
with an unreachable message for all routes.
Implementation note: Cisco currently implements option (b).
4.4.1.1. SIA-QUERY
When a QUERY is still outstanding and awaiting a REPLY from a
neighbor, there is insufficient information to determine why a REPLY
has not been received. A lost packet, congestion on the link, or a
slow neighbor could cause a lack of REPLY from a downstream neighbor.
In order to try to ascertain if the neighboring device is still
attempting to converge on the active route, EIGRP may send an SIA-
QUERY packet to the active neighbor(s). This enables an EIGRP router
to determine if there is a communication issue with the neighbor or
if it is simply still attempting to converge with downstream routers.
By sending an SIA-QUERY, the originating router may extend the
effective active time by resetting the ACTIVE timer that has been
previously set, thus allowing convergence to continue so long as
neighbor devices successfully communicate that convergence is still
underway.
The SIA-QUERY packet SHOULD be sent on a per-destination basis at
one-half of the ACTIVE timeout period. Up to three SIA-QUERY packets
for a specific destination may be sent, each at a value of one-half
the ACTIVE time, so long as each are successfully acknowledged and
met with an SIA-REPLY.
Upon receipt of an SIA-QUERY packet, an EIGRP router should first
send an ACK and then continue to process the SIA-QUERY information.
The QUERY is sent on a per-destination basis at approximately one-
half the active time.
If the EIGRP router is still active for the destination specified in
the SIA-QUERY, the router should respond to the originator with the
SIA-REPLY indicating that active processing for this destination is
still underway by setting the ACTIVE flag in the packet upon
response.
If the router receives an SIA-QUERY referencing a destination for
which it has not received the original QUERY, the router should treat
the packet as though it was a standard QUERY:
1) Acknowledge the receipt of the packet
2) Send a REPLY if a Successor successor exists
3) If the SIA-QUERY is from the successor, transition to the
ACTIVE state if and only if a Feasibility Condition check fails
and send an SIA-REPLY with the ACTIVE bit set
4.4.1.2. SIA-REPLY
An SIA-REPLY packet is the corresponding response upon receipt of an
SIA-QUERY from an EIGRP neighbor. The SIA-REPLY packet will include
a TLV for each destination and the associated vector metric in the
topology table. The SIA-REPLY packet is sent after the entire
received SIA-QUERY packet is processed.
If the EIGRP router is still ACTIVE for a destination, the SIA-REPLY
packet will be sent with the ACTIVE bit set. This confirms for the
neighbor device that the SIA-QUERY packet has been processed by DUAL
and that the router is still attempting to resolve a loop-free path
(likely awaiting responses to its own QUERY to downstream neighbors).
The SIA-REPLY informs the recipient that convergence is complete or
still ongoing; it is an explicit notification that the router is
still actively engaged in the convergence process. This allows the
device that sent the SIA-QUERY to determine whether it should
continue to allow the routes that are not converged to be in the
ACTIVE state or if it should reset the neighbor relationship and
flush all routes through this neighbor.
5. EIGRP Operation
EIGRP has four basic components:
o Finite State Machine
o Reliable Transport Protocol
o Neighbor Discovery/Recovery
o Route Management
5.1. Finite State Machine
The detail of DUAL, the State Machine used by EIGRP, is covered in
Section 3.5.
5.2. Reliable Transport Protocol
The reliable transport is responsible for guaranteed, ordered
delivery of EIGRP packets to all neighbors. It supports intermixed
transmission of multicast and unicast packets. Some EIGRP packets
must be transmitted reliably and others need not. For efficiency,
reliability is provided only when necessary.
For example, on a multi-access network that has multicast
capabilities, such as Ethernet, it is not necessary to send HELLOs
reliably to all neighbors individually. EIGRP sends a single
multicast HELLO with an indication in the packet informing the
receivers that the packet need not be acknowledged. Other types of
packets, such as UPDATE packets, require acknowledgment and this is
indicated in the packet. The reliable transport has a provision to
send multicast packets quickly when there are unacknowledged packets
pending. This helps ensure that convergence time remains low in the
presence of varying speed links.
DUAL assumes there is lossless communication between devices and thus
must depend on the transport protocol to guarantee that messages are
transmitted reliably. EIGRP implements the reliable transport
protocol to ensure ordered delivery and acknowledgment of any
messages requiring reliable transmission. State variables such as a
received sequence number, acknowledgment number, and transmission
queues MUST be maintained on a per-neighbor basis.
The following sequence number rules must be met for the EIGRP
reliable transport protocol to work correctly:
o A sender of a packet includes its global sequence number in the
sequence number field of the fixed header. The sequence number
wraps around to one when the maximum value is exceeded
(sequence number zero is reserved for unreliable transmission).
The sender includes the receivers sequence number in the
acknowledgment number field of the fixed header.
o Any packets that do not require acknowledgment must be sent
with a sequence number of 0.
o Any packet that has an acknowledgment number of 0 indicates
that sender is not expecting to explicitly acknowledge
delivery. Otherwise, it is acknowledging a single packet.
o Packets that are network-layer multicast must contain
acknowledgment number of 0.
When a router transmits a packet, it increments its sequence number
and marks the packet as requiring acknowledgment by all neighbors on
the interface for which the packet is sent. When individual
acknowledgments are unicast addressed by the receivers to the sender
with the acknowledgment number equal to the packets sequence number,
the sender SHALL clear the pending acknowledgment requirement for the
packet from the respective neighbor.
If the required acknowledgment is not received for the packet, it
MUST be retransmitted. Retransmissions will occur for a maximum of 5
seconds. This retransmission for each packet is tried 16 times,
after which, if there is no ACK, the neighbor relationship is reset
with the peer that didn't send the ACK.
The protocol has no explicit windowing support. A receiver will
acknowledge each packet individually and will drop packets that are
received out of order.
Implementation note: The exception to this occurs if a duplicate
packet is received, and the acknowledgment for the original packet
has been scheduled for transmission, but not yet sent. In this case,
EIGRP will not send an acknowledgment for the duplicate packet, and
the queued acknowledgment will acknowledge both the original and
duplicate packet.
Duplicate packets are also discarded upon receipt. Acknowledgments
are not accumulative. Therefore, an ACK with a non-zero sequence
number acknowledges a single packet.
There are situations when multicast and unicast packets are
transmitted close together on multi-access broadcast-capable
networks. The reliable transport mechanism MUST ensure that all
multicasts are transmitted in order and not mix the order among
unicast and multicast packets. The reliable transport provides a
mechanism to deliver multicast packets in order to some receivers
quickly, while some receivers have not yet received all unicast or
previously sent multicast packets. The SEQUENCE_TYPE TLV in HELLO
packets achieves this. This will be explained in more detail in this
section.
Figure 5 illustrates the reliable transport protocol on point-to-
point links. There are two scenarios that may occur: an UPDATE-
initiated packet exchange or a QUERY-initiated packet exchange.
This example will assume no packet loss.
Router A Router B
An Example UPDATE Exchange
<----------------
UPDATE (multicast)
A receives packet SEQ=100, ACK=0
Add packet to A's retransmit list
---------------->
ACK (unicast)
SEQ=0, ACK=100 Receive ACK
Process UPDATE Delete packet from A's retransmit list
An Example QUERY Exchange
<----------------
QUERY (multicast)
A receives packet SEQ=101, ACK=0
Process QUERY Add packet to A's retransmit list
---------------->
REPLY (unicast)
SEQ=201, ACK=101 Process ACK
Delete packet from A's retransmit
list
Process REPLY packet
<----------------
ACK (unicast)
A receives packet SEQ=0, ACK=201
Figure 5: Reliable Transfer on Point-to-Point Links
The UPDATE exchange sequence requires UPDATE packets sent to be
delivered reliably. The UPDATE packet transmitted contains a
sequence number that is acknowledged by a receipt of an ACK packet.
If the UPDATE or the ACK packet is lost on the network, the UPDATE
packet will be retransmitted.
This example will assume there is heavy packet loss on a network.
Router A Router B
<----------------
UPDATE (multicast)
A receives packet SEQ=100, ACK=0
Add packet to A's retransmit list
---------------->
ACK (unicast)
SEQ=0, ACK=100 Receive ACK
Process UPDATE Delete packet from A's retransmit list
<--/LOST/--------------
UPDATE (multicast)
SEQ=101, ACK=0
Add packet to A's retransmit list
Retransmit Timer Expires
<----------------
Retransmit UPDATE (unicast)
SEQ=101, ACK=0
Keep packet on A's retransmit list
---------------->
ACK (unicast)
SEQ=0, ACK=101 Receive ACK
Process UPDATE Delete packet from A's retransmit list
Figure 6: Reliable Transfer on Lossy Point-to-Point Links
Reliable delivery on multi-access LANs works in a similar fashion to
point-to-point links. The initial packet is always multicast and
subsequent retransmissions are unicast addressed. The
acknowledgments sent are always unicast addressed. Figure 7 shows an
example with four routers on an Ethernet.
Router B -----------+
|
Router C -----------+------------ Router A
|
Router D -----------+
An Example UPDATE Exchange
<----------------
A send UPDATE (multicast)
SEQ=100, ACK=0
Add packet to B's retransmit list
Add packet to C's retransmit list
Add packet to D's retransmit list
---------------->
B sends ACK (unicast)
SEQ=0, ACK=100 Receive ACK
Process UPDATE Delete packet from B's retransmit list
---------------->
C sends ACK (unicast)
SEQ=0, ACK=100 Receive ACK
Process UPDATE Delete packet from C's retransmit list
---------------->
D sends ACK (unicast)
SEQ=0, ACK=100 Receive ACK
Process UPDATE Delete packet from D's retransmit list
An Example QUERY Exchange
<----------------
A sends UPDATE (multicast)
SEQ=101, ACK=0
Add packet to B's retransmit list
Add packet to C's retransmit list
Add packet to D's retransmit list
---------------->
B sends REPLY (unicast) <----------------
SEQ=511, ACK=101 A sends ACK (unicast to B)
Process UPDATE SEQ=0, ACK=511
Delete packet from B's retransmit list
---------------->
C sends REPLY (unicast) <----------------
SEQ=200, ACK=101 A sends ACK (unicast to C)
Process UPDATE SEQ=0, ACK=200
Delete packet from C's retransmit list
---------------->
D sends REPLY (unicast) <----------------
SEQ=11, ACK=101 A sends ACK (unicast to D)
Process UPDATE SEQ=0, ACK=11
Delete packet from D's retransmit list
Figure 7: Reliable Transfer on Multi-Access Links
And finally, a situation where numerous multicast and unicast packets
are sent close together in a multi-access environment is illustrated
in Figure 8.
Router B -----------+
|
Router C -----------+------------ Router A
|
Router D -----------+
<----------------
A sends UPDATE (multicast)
SEQ=100, ACK=0
---------------/LOST/-> Add packet to B's retransmit list
B sends ACK (unicast) Add packet to C's retransmit list
SEQ=0, ACK=100 Add packet to D's retransmit list
---------------->
C sends ACK (unicast)
SEQ=0, ACK=100 Delete packet from C's retransmit list
---------------->
D sends ACK (unicast)
SEQ=0, ACK=100 Delete packet from D's retransmit list
<----------------
A sends HELLO (multicast)
SEQ=0, ACK=0, SEQ_TLV listing B
B receives Hello, does not set CR-Mode
C receives Hello, sets CR-Mode
D receives Hello, sets CR-Mode
<----------------
A sends UPDATE (multicast)
SEQ=101, ACK=0, CR-Flag=1
---------------/LOST/-> Add packet to B's retransmit list
B sends ACK (unicast) Add packet to C's retransmit list
SEQ=0, ACK=100 Add packet to D's retransmit list
B ignores UPDATE 101 because the CR-Flag
is set and it is not in CR-Mode
---------------->
C sends ACK (unicast)
SEQ=0, ACK=101
---------------->
D sends ACK (unicast)
SEQ=0, ACK=101
<----------------
A resends UPDATE (unicast to B)
SEQ=100, ACK=0
B packet duplicate
--------------->
B sends ACK (unicast) A removes packet from retransmit list
SEQ=0, ACK=100
<----------------
A resends UPDATE (unicast to B)
SEQ=101, ACK=0
--------------->
B sends ACK (unicast) A removes packet from retransmit list
SEQ=0, ACK=101
Figure 8: Reliable Transfer on Multi-Access Links
with Conditional Receive
Initially, Router A sends a multicast addressed UPDATE packet on the
LAN. B and C receive it and send acknowledgments. Router B receives
the UPDATE, but the acknowledgment sent is lost on the network.
Before the retransmission timer for Router B's packet expires, there
is an event that causes a new multicast addressed UPDATE to be sent.
Router A detects that there is at least one neighbor on the interface
with a full queue. Therefore, it MUST signal that neighbor not to
receive the next packet or it would receive the retransmitted packet
out of order. If all neighbors on the interface have a full queue,
then EIGRP should reschedule the transmission of the UPDATE once the
queues are no longer full.
Router A builds a HELLO packet with a SEQUENCE_TYPE TLV indicating
all the neighbors that have full queues. In this case, the only
neighbor address in the list is Router B. The HELLO packet is sent
via multicast unreliably out the interface.
Routers C and D process the SEQUENCE_TYPE TLV by looking for their
own addresses in the list. If not found, they put themselves in CR-
Mode.
Router B does not find its address in the SEQUENCE TLV peer list, so
it enters CR-Mode. Packets received by Router B with the CR-flag CR-Flag
MUST be discarded and not acknowledged.
Later, Router A will unicast transmit both packets 100 and 101
directly to Router B. Router B already has 100, so it discards and
acknowledges it.
Router B then accepts and acknowledges packet 101. Once an
acknowledgment is received, Router A can remove both packets from
Router B's transmission list.
5.2.1. Bandwidth on Low-Speed Links
By default, EIGRP limits itself to using no more than 50% of the
bandwidth reported by an interface when determining packet-pacing
intervals. If the bandwidth does not match the physical bandwidth
(the network architect may have put in an artificially low or high
bandwidth value to influence routing decisions), EIGRP may:
1. Generate more traffic than the interface can handle, possibly
causing drops, thereby impairing EIGRP performance.
2. Generate a lot of EIGRP traffic that could result in little
bandwidth remaining for user data. To control such
transmissions, an interface-pacing timer is defined for the
interfaces on which EIGRP is enabled. When a pacing timer
expires, a packet is transmitted out on that interface.
5.3. Neighbor Discovery/Recovery
Neighbor Discovery/Recovery is the process that routers use to
dynamically learn of other routers on their directly attached
networks. Routers MUST also discover when their neighbors become
unreachable or inoperative. This process is achieved with low
overhead by periodically sending small HELLO packets. As long as any
packets are received from a neighbor, the router can determine that
neighbor is alive and functioning. Only after a neighbor router is
considered operational can the neighboring routers exchange routing
information.
5.3.1. Neighbor Hold Time
Each router keeps state information about adjacent neighbors. When
newly discovered neighbors are learned the address, interface, and
hold time
Hold Time of the neighbor is noted. When a neighbor sends a HELLO,
it advertises its Hold Time. The Hold Time is the amount of time a
router treats a neighbor as reachable and operational. In addition
to the HELLO packet, if any packet is received within the hold time Hold Time
period, then the Hold Time period will be reset. When the Hold Time
expires, DUAL is informed of the topology change.
5.3.2. HELLO Packets
When an EIGRP router is initialized, it will start sending HELLO
packets out any interface on which EIGRP is enabled. HELLO packets,
when used for neighbor discovery, are normally sent multicast
addressed. The HELLO packet will include the configured EIGRP metric
K-values. Two routers become neighbors only if the K-values are the
same. This enforces that the metric usage is consistent throughout
the Internet. Also included in the HELLO packet is a Hold Time
value. This value indicates to all receivers the length of time in
seconds that the neighbor is valid. The default Hold Time will be
three times the HELLO interval. HELLO packets will be transmitted
every 5 seconds (by default). There may be a configuration command
that controls this value and therefore changes the Hold Time. HELLO
packets are not transmitted reliably, so the sequence number should
be set to 0.
5.3.3. UPDATE Packets
A router detects a new neighbor by receiving a HELLO packet from a
neighbor not presently known. To ensure unicast and multicast packet
delivery, the detecting neighbor will send a unicast UPDATE packet to
the new neighbor with no routing information (the NULL UPDATE
packet). The initial NULL UPDATE packet sent MUST have the INIT-flag INIT-Flag
set and contain no topology information.
Implementation note: The NULL UPDATE packet is used to ensure
bidirectional UNICAST packet delivery as the NULL UPDATE and the ACK
are both sent unicast. Additional UPDATE packets cannot be sent
until the initial NULL UPDATE packet is acknowledged.
The INIT-flag INIT-Flag instructs the neighbor to advertise its routes, and it
is also useful when a neighbor goes down and comes back up before the
router detects it went down. In this case, the neighbor needs new
routing information. The INIT-Flag informs the router to send it.
Implementation note: When a router sends an UPDATE with the INIT-Flag
set, and without the Restart (RS) Flag flag set in the header, the
receiving neighbor must also send an UPDATE with the INIT-Flag.
Failure to do so will result in a Cisco device posting a "stuck in
INIT state" error and subsequent discards.
5.3.4. Initialization Sequence
Router A Router B
(just booted) (up and running)
(1)---------------->
HELLO (multicast) <---------------- (2)
SEQ=0, ACK=0 HELLO (multicast)
SEQ=0, ACK=0
<---------------- (3)
UPDATE (unicast)
SEQ=10, ACK=0, INIT
(4)----------------> UPDATE 11 is queued
UPDATE (unicast)
SEQ=100, ACK=10, INIT <---------------- (5)
UPDATE (unicast)
SEQ=11, ACK=100
All UPDATES sent
(6)--------------/lost/->
ACK (unicast)
SEQ=0, ACK=11
(5 seconds later)
<---------------- (7)
Duplicate received, UPDATE (unicast)
packet discarded SEQ=11, ACK=100
(8)--------------->
ACK (unicast)
SEQ=0, ACK=11
Figure 9: Initialization Sequence
(1) Router A sends a multicast HELLO and Router B discovers it.
(2) Router B sends an expedited HELLO and starts the process of
sending its topology table to Router A. In addition, Router B
sends the NULL UPDATE packet with the INIT-Flag. The second
packet is queued, but it cannot be sent until the first is
acknowledged.
(3) Router A receives the first UPDATE packet and processes it as a
DUAL event. If the UPDATE contains topology information, the
packet will be processed and stored in a topology table. Router
B sends its first and only UPDATE packet with an accompanied ACK.
(4) Router B receives UPDATE packet 100 from Router A. Router B can
dequeue packet 10 from A's transmission list since the UPDATE
acknowledged 10. It can now send UPDATE packet 11 and with an
acknowledgment of Router A's UPDATE.
(5) Router A receives the last UPDATE packet from Router B and
acknowledges it. The acknowledgment gets lost.
(6) Router B later retransmits the UPDATE packet to Router A.
(7) Router A detects the duplicate and simply acknowledges the
packet. Router B dequeues packet 11 from A's transmission list,
and both routers are up and synchronized.
5.3.5. Neighbor Formation
To prevent packets from being sent to a neighbor prior to verifying
multicast and unicast packet delivery is reliable, a three-way
handshake is utilized.
During normal adjacency formation, multicast HELLOs cause the EIGRP
process to place new neighbors into the neighbor table. Unicast
packets are then used to exchange known routing information and
complete the neighbor relationship (Section 5.2).
To prevent EIGRP from sending sequenced packets to neighbors that
fail to have bidirectional unicast/multicast, or one neighbor
restarts while building the relationship, EIGRP MUST place the newly
discovered neighbor in a "pending" state as follows:
when Router A receives the first multicast HELLO from Router B, it
places Router B in the pending state and transmits a unicast
UPDATE containing no topology information and SHALL set the
initialization bit. While Router B is in this state, A will send
it neither a QUERY nor an UPDATE. When Router A receives the
unicast acknowledgment from Router B, it will change the state
from "pending" to "up".
5.3.6. QUERY Packets during Neighbor Formation
As described above, during the initial formation of the neighbor
relationship, EIGRP uses a form of three-way handshake to verify both
unicast and multicast connectivity are working successfully. During
this period of neighbor creation, the new neighbor is considered to
be in the pending state, and it is not eligible to be included in the
convergence process.
Because of this, any QUERY received by an EIGRP router would not
cause a QUERY to be sent to the new (and pending) neighbor. It would
perform the DUAL process without the new peer in the conversation.
To do this, when a router in the process of establishing a new
neighbor receives a QUERY from a fully established neighbor, it
performs the normal DUAL Feasible Successor check to determine
whether it needs to REPLY with a valid path or whether it needs to
enter the ACTIVE process on the prefix.
If it determines that it must go active, each fully established
neighbor that participates in the convergence process will be sent a
QUERY packet, and REPLY packets are expected from each. Any pending
neighbor will not be expected to REPLY and will not be sent a QUERY
directly. If it resides on an interface containing a mix of fully
established neighbors and pending neighbors, it might receive the
QUERY, but it will not be expected to REPLY to it.
5.4. Topology Table
The topology table is populated by the Protocol-Dependent Modules
(PDMs) (IPv4/IPv6), and it is acted upon by the DUAL finite state
machine. Associated with each entry are the destination address, a
list of neighbors that have advertised this destination, and the
metric associated with the destination. The metric is referred to as
the "CD".
The CD is the best-advertised RD from all neighbors, plus the link
cost between the receiving router and the neighbor.
The "RD" is the CD as advertised by the Feasible Successor for the
destination. In other words, the Computed Distance, when sent by a
neighbor, is referred to as the "Reported Distance" and is the metric
that the neighboring router uses to reach the destination (its CD as
described above).
If the router is advertising a destination route, it MUST be using
the route to forward packets; this is an important rule that distance
vector protocols MUST follow.
5.4.1. Route Management
Within the topology table, EIGRP has the notion of internal and
external routes. Internal routes MUST be preferred over external
routes, independent of the metric. In practical terms, if an
internal route is received, the diffusing computation will be run
considering only the internal routes. Only when no internal routes
for a given destination exist will EIGRP choose the Successor successor from
the available external routes.
5.4.1.1. Internal Routes
Internal routes are destinations that have been originated within the
same EIGRP AS. Therefore, a directly attached network that is
configured to run EIGRP is considered an internal route and is
propagated with this information throughout the network topology.
Internal routes are tagged with the following information:
o Router ID of the EIGRP router that originated the route.
o Configurable administrator tag.
5.4.1.2. External Routes
External routes are destinations that have been learned from another
source, such as a different routing protocol or static route. These
routes are marked individually with the identity of their
origination. External routes are tagged with the following
information:
o Router ID of the EIGRP router that redistributed the route.
o AS number where the destination resides.
o Configurable administrator tag.
o Protocol ID of the external protocol.
o Metric from the external protocol.
o Bit flags for default routing.
As an example, suppose there is an AS with three border routers: BR1,
BR2, and BR3. A border router is one that runs more than one routing
protocol. The AS uses EIGRP as the routing protocol. Two of the
border routers, BR1 and BR2, also use Open Shortest Path First (OSPF)
[10] and the other, BR3, also uses the Routing Information Protocol
(RIP).
Routes learned by one of the OSPF border routers, BR1, can be
conditionally redistributed into EIGRP. This means that EIGRP
running in BR1 advertises the OSPF routes within its own AS. When it
does so, it advertises the route and tags it as an OSPF-learned route
with a metric equal to the routing table metric of the OSPF route.
The router-id is set to BR1. The EIGRP route propagates to the other
border routers.
Let's say that BR3, the RIP border router, also advertises the same
destinations as BR1. Therefore, BR3, redistributes the RIP routes
into the EIGRP AS. BR2, then, has enough information to determine
the AS entry point for the route, the original routing protocol used,
and the metric.
Further, the network administrator could assign tag values to
specific destinations when redistributing the route. BR2 can utilize
any of this information to use the route or re-advertise it back out
into OSPF.
Using EIGRP route tagging can give a network administrator flexible
policy controls and help customize routing. Route tagging is
particularly useful in transit ASes where EIGRP would typically
interact with an inter-domain routing protocol that implements global
policies.
5.4.2. Split Horizon and Poison Reverse
In some circumstances, EIGRP will suppress or poison QUERY and UPDATE
information to prevent routing loops as changes propagate though the
network.
The
Within Cisco, the split horizon rule states: suggests: "Never advertise a
route out of the interface through which it was learned". EIGRP
implements this to mean, "if you have a Successor successor route to a
destination, never advertise the route out the interface on which it
was learned".
The poison reverse rule states: "A route learned through an interface
will be advertised as unreachable through that same interface". As
with the case of Split Horizon, split horizon, EIGRP applies this rule only to
interfaces it is using for reaching the destination. Routes learned
though interfaces that EIGRP is NOT using to reach the destination
may have the route advertised out those interfaces.
In EIGRP, split horizon suppresses a QUERY, where as Reverse Poison poison reverse
advertises a destination as unreachable. This can occur for a
destination under any of the following conditions:
o two routers are in startup or restart mode
o advertising a topology table change
o sending a query
5.4.2.1. Startup Mode
When two routers first become neighbors, they exchange topology
tables during startup mode. For each destination a router receives
during startup mode, it advertises the same destination back to its
new neighbor with a maximum metric (Poison Route).
5.4.2.2. Advertising Topology Table Change
If a router uses a neighbor as the Successor successor for a given destination,
it will send an UPDATE for the destination with a metric of infinity.
5.4.2.3. Sending a QUERY/UPDATE
In most cases, EIGRP follows normal split-horizon rules. When a
metric change is received from the Successor successor via QUERY or UPDATE that
causes the route to go ACTIVE, the router will send a QUERY to
neighbors on all interfaces except the interface toward the
Successor.
successor.
In other words, the router does not send the QUERY out of the inbound
interface through which the information causing the route to go
ACTIVE was received.
An exception to this can occur if a router receives a QUERY from its
successor while already reacting to an event that did not cause it to
go ACTIVE, for example, a metric change from the Successor successor that did
not cause an ACTIVE transition, but was followed by the UPDATE/QUERY
that does result the router to transition to ACTIVE.
5.5. EIGRP Metric Coefficients
EIGRP allows for modification of the default composite metric
calculation (see Section 5.6) through the use of coefficients (K-
values). This adjustment allows for per-deployment tuning of network
behavior. Setting K-values up to 254 scales the impact of the scalar
metric on the final composite metric.
EIGRP default coefficients have been carefully selected to provide
optimal performance in most networks. The default K-values are as
follows:
K1 == K3 == 1
K2 == K4 == K5 == 0
K6 == 0
If K5 is equal to 0, then reliability quotient is defined to be 1.
5.5.1. Coefficients K1 and K2
K1 is used to allow path selection to be based on the bandwidth
available along the path. EIGRP can use one of two variations of
Throughput-based path selection.
o Maximum Theoretical Bandwidth: paths chosen based on the highest
reported bandwidth
o Network Throughput: paths chosen based on the highest "available"
bandwidth adjusted by congestion-based effects (interface reported
load)
By default, EIGRP computes the Throughput using the maximum
theoretical throughput Throughput expressed in picoseconds per kilobyte of data
sent. This inversion results in a larger number (more time)
ultimately generating a worse metric.
If K2 is used, the effect of congestion as a measure of load reported
by the interface will be used to simulate the "available throughput" Throughput"
by adjusting the maximum throughput. Throughput.
5.5.2. Coefficient K3
K3 is used to allow delay or latency-based path selection. Latency
and Delay delay are similar terms that refer to the amount of time it takes
a bit to be transmitted to an adjacent neighbor. EIGRP uses one-way-
based values either provided by the interface or computed as a factor
of the link s bandwidth.
5.5.3. Coefficients K4 and K5
K4 and K5 are used to allow for path selection based on link quality
and packet loss. Packet loss caused by network problems results in
highly noticeable performance issues or jitter Jitter with streaming
technologies, voice over IP, online gaming and videoconferencing, and
will affect all other network applications to one degree or another.
Critical services should pass with less than 1% packet loss. Lower
priority packet types might pass with less than 5% and then 10% for
the lowest of priority of services. The final metric can be weighted
based on the reported link quality.
The handling of K5 is conditional. If K5 is equal to 0, then
reliability quotient is defined to be 1.
5.5.4. Coefficient K6
K6 has been introduced with Wide Metric support and is used to allow
for Extended Attributes, which can be used to reflect in a higher
aggregate metric than those having lower energy usage. Currently
there are two Extended Attributes, jitter Jitter and energy, defined in the
scope of this document.
5.5.4.1. Jitter
Use of Jitter-based Path Selection results in a path calculation with
the lowest reported jitter. Jitter. Jitter is reported as the interval
between the longest and shortest packet delivery and is expressed in
microseconds. Higher values result in a higher aggregate metric when
compared to those having lower jitter Jitter calculations.
Jitter is measured in microseconds and is accumulated along the path,
with each hop using an averaged 3-second period to smooth out the
metric change rate.
Presently, EIGRP does not have the ability to measure jitter, Jitter, and, as
such, the default value will be zero (0). Performance-based
solutions such as PfR could be used to populate this field.
5.5.4.2. Energy
Use of Energy-based Path Selection results in paths with the lowest
energy usage being selected in a loop-free and deterministic manner.
The amount of energy used is accumulative and has results in a higher
aggregate metric than those having lower energy.
Presently, EIGRP does not report energy usage, and as such the
default value will be zero (0).
5.6. EIGRP Metric Calculations
5.6.1. Classic Metrics
The composite metric is based on bandwidth, delay, load, and
reliability. MTU is not an attribute for calculating the composite
metric, but carried in the vector metrics.
One of the original goals of EIGRP was to offer and enhance routing
solutions for IGRP. To achieve this, EIGRP used the same composite
metric as IGRP, with the terms multiplied by 256 to change the metric
from 24 bits to 32 bits.
5.6.1.1. Classic Composite Formulation
EIGRP calculates the composite metric with the following formula:
metric = 256 * ({(K1*BW) + [(K2*BW)/(256-LOAD)] + (K3*DELAY)} *
(K5/(REL+K4)))
In this formula, Bandwidth (BW) is the lowest interface bandwidth
along the path, and delay (DELAY) is the sum of all outbound
interface delays along the path. Load (LOAD) and reliability (REL)
values are expressed percentages with a value of 1 to 255.
Implementation note: Cisco IOS routers display reliability as a
fraction of 255. That is, 255/255 is 100% reliability or a perfectly
stable link; a value of 229/255 represents a 90% reliable link. Load
is a value between 1 and 255. A load of 255/255 indicates a
completely saturated link. A load of 127/255 represents a 50%
saturated link. These values are not dynamically measured; they are
only measured at the time a link changes.
Bandwidth is the inverse minimum bandwidth (in kbps) of the path in
bits per second scaled by a factor of 10^7. The formula for
bandwidth is as follows:
(10^7)/BWmin
Implementation note: When converting the real bandwidth to the
composite bandwidth, truncate before applying the scaling factor.
When converting the composite bandwidth to the real bandwidth, apply
the scaling factor before the division and only then truncate.
The delay is the sum of the outgoing interface delay (in tens of
microseconds) to the destination. A delay set to it maximum value
(hexadecimal 0xFFFFFFFF) indicates that the network is unreachable.
The formula for delay is as follows:
[sum of delays]
The default composite metric, adjusted for scaling factors, for EIGRP
is:
metric = 256 * { [(10^7)/ BWmin] + [sum of delays]}
Minimum Bandwidth (BWmin) is represented in kbps, and the "sum of
delays" is represented in tens of microseconds. The bandwidth and
delay for an Ethernet interface are 10 Mbps and 1 ms, respectively.
The calculated EIGRP bandwidth (BW) metric is then:
256 * (10^7)/BW = 256 * {(10^7)/10,000}
= 256 * 1000
= 256,000
And the calculated EIGRP delay metric is then:
256 * sum of delay = 256 * 100 * 10 microseconds
= 25,600 (in tens of microseconds)
5.6.1.2. Cisco Interface Delay Compatibility
For compatibility with Cisco products, the following table shows the
times in nanoseconds EIGRP uses for bandwidth and delay.
Bandwidth Classic Wide Metrics Interface
(kbps) Delay Delay Type
---------------------------------------------------------
9 500000000 500000000 Tunnel
56 20000000 20000000 56 kbps
64 20000000 20000000 DS0
1544 20000000 20000000 T1
2048 20000000 20000000 E1
10000 1000000 1000000 Ethernet
16000 630000 630000 TokRing16
45045 20000000 20000000 HSSI
100000 100000 100000 FDDI
100000 100000 100000 FastEthernet
155000 100000 100000 ATM 155 Mbps
1000000 10000 10000 GigaEthernet
2000000 10000 5000 2 Gig
5000000 10000 2000 5 Gig
10000000 10000 1000 10 Gig
20000000 10000 500 20 Gig
50000000 10000 200 50 Gig
100000000 10000 100 100 Gig
200000000 10000 50 200 Gig
500000000 10000 20 500 Gig
5.6.2. Wide Metrics
To enable EIGRP to perform the path selection for interfaces with
high bandwidths, both the EIGRP packet and composite metric formula
have been modified. This change allows EIGRP to choose paths based
on the computed time (measured in picoseconds) information takes to
travel though the links.
5.6.2.1. Wide Metric Vectors
EIGRP uses five "vector metrics": minimum throughput, Throughput, latency, load,
reliability, and MTU. These values are calculated from destination
to source as follows:
o Throughput - Minimum value
o Latency - accumulative
o Load - maximum
o Reliability - minimum
o MTU - minimum
o Hop count - accumulative
There are two additional values: jitter Jitter and energy. These two values
are accumulated from destination to source:
o Jitter - accumulative
o Energy - accumulative
These Extended Attributes, as well as any future ones, will be
controlled via K6. If K6 is non-zero, these will be additive to the
path's composite metric. Higher jitter Jitter or energy usage will result
in paths that are worse than those that either do not monitor these
attributes or that have lower values.
EIGRP will not send these attributes if the router does not provide
them. If the attributes are received, then EIGRP will use them in
the metric calculation (based on K6) and will forward them with those
routers values assumed to be "zero" and the accumulative values are
forwarded unchanged.
The use of the vector metrics allows EIGRP to compute paths based on
any of four (bandwidth, delay, reliability, and load) path selection
schemes. The schemes are distinguished based on the choice of the
key-measured network performance metric.
Of these vector metric components, by default, only minimum
throughput
Throughput and latency are traditionally used to compute the best
path. Unlike most metrics, minimum throughput Throughput is set to the minimum
value of the entire path, and it does not reflect how many hops or
low throughput Throughput links are in the path, nor does it reflect the
availability of parallel links. Latency is calculated based on one-
way delays and is a cumulative value, which increases with each
segment in the path.
Network Designer note: When trying to manually influence EIGRP path
selection though interface bandwidth/delay configuration, the
modification of bandwidth is discouraged for following reasons:
The change will only affect the path selection if the configured
value is the lowest bandwidth over the entire path. Changing the
bandwidth can have impact beyond affecting the EIGRP metrics. For
example, Quality of Service (QoS) also looks at the bandwidth on an
interface.
EIGRP throttles its packet transmissions so it will only use 50% of
the configured bandwidth. Lowering the bandwidth can cause EIGRP to
starve an adjacency, causing slow or failed convergence and control-
plane operation.
Changing the delay does not impact other protocols, nor does it cause
EIGRP to throttle back; changing the delay configured on a link only
impacts metric calculation.
5.6.2.2. Wide Metric Conversion Constants
EIGRP uses a number of defined constants for conversion and
calculation of metric values. These numbers are provided here for
reference
EIGRP_BANDWIDTH 10,000,000
EIGRP_DELAY_PICO 1,000,000
EIGRP_INACCESSIBLE 0xFFFFFFFFFFFFFFFFLL
EIGRP_MAX_HOPS 100
EIGRP_CLASSIC_SCALE 256
EIGRP_WIDE_SCALE 65536
When computing the metric using the above units, all capacity
information will be normalized to kilobytes and picoseconds before
being used. For example, delay is expressed in microseconds per
kilobyte, and would be converted to kilobytes per second; likewise,
energy would be expressed in power per kilobytes per second of usage.
5.6.2.3. Throughput Calculation
The formula for the conversion for Max-Throughput value directly from
the interface without consideration of congestion-based effects is as
follows:
(EIGRP_BANDWIDTH * EIGRP_WIDE_SCALE)
Max-Throughput = K1 * ------------------------------------
Interface Bandwidth (kbps)
If K2 is used, the effect of congestion as a measure of load reported
by the interface will be used to simulate the "available throughput" Throughput"
by adjusting the maximum throughput Throughput according to the formula:
K2 * Max-Throughput
Net-Throughput = Max-Throughput + ---------------------
256 - Load
K2 has the greatest effect on the metric occurs when the load
increases beyond 90%.
5.6.2.4. Latency Calculation
Transmission times derived from physical interfaces MUST be n units
of picoseconds, converted to picoseconds prior to being exchanged
between neighbors, or used in the composite metric determination.
This includes delay values present in configuration-based commands
(i.e., interface delay, redistribute, default-metric, route-map,
etc.).
The delay value is then converted to a "latency" using the formula:
Delay * EIGRP_WIDE_SCALE
Latency = K3 * --------------------------
EIGRP_DELAY_PICO
5.6.2.5. Composite Calculation
K5
metric =[(K1*Net-Throughput) + Latency)+(K6*ExtAttr)] * ------
K4+Rel
By default, the path selection scheme used by EIGRP is a combination
of Throughput and Latency where the selection is a product of total
latency and minimum throughput Throughput of all links along the path:
metric = (K1 * min(Throughput)) + (K3 * sum(Latency)) }
6. EIGRP Packet Formats
6.1. Protocol Number
The IPv6 and IPv4 protocol identifier number spaces are common and
will both use protocol identifier 88 [8] [9].
EIGRP IPv4 will transmit HELLO packets using either the unicast
destination of a neighbor or using a multicast host group address [7]
with a source address EIGRP IPv4 multicast address [13].
EIGRP IPv6 will transmit HELLO packets with a source address being
the link-local address of the transmitting interface. Multicast
HELLO packets will have a destination address of EIGRP IPv6 multicast
address [14]. Unicast packets directed to a specific neighbor will
contain the destination link-local address of the neighbor.
There is no requirement that two EIGRP IPv6 neighbors share a common
prefix on their connecting interface. EIGRP IPv6 will check that a
received HELLO contains a valid IPv6 link-local source address.
Other HELLO processing will follow common EIGRP checks, including
matching AS number and matching K-values.
6.2. Protocol Assignment Encoding
The External Protocol field is an informational assignment to
identify the originating routing protocol that this route was learned
by. The following values are assigned:
Protocols Value
IGRP 1
EIGRP 2
Static 3
RIP 4
HELLO 5
OSPF 6
ISIS 7
EGP 8
BGP 9
IDRP 10
Connected 11
6.3. Destination Assignment Encoding
Destinations types are encoded according to the IANA address family
number assignments. Currently only the following types are used:
AFI Description AFI Number
--------------------------------------
IP (IP version 4) 1
IP6 (IP version 6) 2
EIGRP Common Service Family 16384
EIGRP IPv4 Service Family 16385
EIGRP IPv6 Service Family 16386
6.4. EIGRP Communities Attribute
EIGRP supports communities similar to the BGP Extended Communities
RFC 4360 [4] extended type with Type Field field composed of 2 octets and
Value Field field composed of 6 octets. Each Community is encoded as an
8-octet quantity, as follows:
- Type Field: field: 2 octets
- Value Field: field: Remaining octets
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type high | Type low | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Value |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In addition to well-known communities supported by BGP (such as Site
of Origin), EIGRP defines a number of additional Community values in
the "Experimental Use" [5] range as follows:
Type high: 0x88
Type low:
Value Name Description
---------------------------------------------------------------
00 EXTCOMM_EIGRP EIGRP route information appended
01 EXTCOMM_DAD Data: AS + Delay
02 EXTCOMM_VRHB Vector: Reliability + Hop + BW
03 EXTCOMM_SRLM System: Reserve + Load + MTU
04 EXTCOMM_SAR System: Remote AS + Remote ID
05 EXTCOMM_RPM Remote: Protocol + Metric
06 EXTCOMM_VRR Vecmet: Rsvd + RouterID
6.5. EIGRP Packet Header
The basic EIGRP packet payload format is identical for both IPv4 and
IPv6, although there are some protocol-specific variations. Packets
consist of a header, followed by a set of variable-length fields
consisting of Type/Length/Value (TLV) triplets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Header Version | Opcode | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Virtual Router ID | Autonomous System Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Header Version: EIGRP Packet Header Format version. Current Version
is 2. This field is not the same as the TLV Version field.
Opcode: Indicates the type of the message. It will be one of the
following values:
EIGRP_OPC_UPDATE 1
EIGRP_OPC_REQUEST 2
EIGRP_OPC_QUERY 3
EIGRP_OPC_REPLY 4
EIGRP_OPC_HELLO 5
Reserved 6 (EIGRP_OPC_IPXSAP)
Reserved 7 (EIGRP_OPC_PROBE)
Reserved 8 (EIGRP_OPC_ACK)
Reserved 9
EIGRP_OPC_SIAQUERY 10
EIGRP_OPC_SIAREPLY 11
Checksum: Each packet will include a checksum for the entire contents
of the packet. The checksum will be the standard ones' complement
of the ones' complement sum. For purposes of computing the
checksum, the value of the checksum field is zero. The packet is
discarded if the packet checksum fails.
Flags: Defines special handling of the packet. There are currently
four defined flag bits.
Init Flag
INIT-Flag (0x01): This bit is set in the initial UPDATE sent to a
newly discovered neighbor. It instructs the neighbor to advertise
its full set of routes.
CR Flag
CR-Flag (0x02): This bit indicates that receivers should only accept
the packet if they are in Conditionally Received mode. A router
enters conditionally received Conditionally Received mode when it receives and processes
a HELLO packet with a Sequence SEQUENCE TLV present.
RS Flag
RS-Flag (0x04): The Restart flag is set in the HELLO and the UPDATE
packets during the restart period. The router looks at the RS
flag RS-
Flag to detect if a neighbor is restarting. From the restarting
routers perspective, if a neighboring router detects the RS flag RS-Flag
set, it will maintain the adjacency, and will set the RS flag RS-Flag in
its UPDATE packet to indicated it is doing a soft restart.
EOT Flag
EOT-Flag (0x08): The End-of-Table flag marks the end of the startup
process with a neighbor. If the flag is set, it indicates the
neighbor has completed sending all UPDATEs. At this point, the
router will remove any stale routes learned from the neighbor
prior to the restart event. A stale route is any route that
existed before the restart and was not refreshed by the neighbor
via and UPDATE.
Sequence Number: Each packet that is transmitted will have a 32-bit
sequence number that is unique with respect to a sending router.
A value of 0 means that an acknowledgment is not required.
Acknowledgment Number: The 32-bit sequence number that is being
acknowledged with respect to the receiver of the packet. If the
value is 0, there is no acknowledgment present. A non-zero value
can only be present in unicast-addressed packets. A HELLO packet
with a non-zero ACK field should be decoded as an ACK packet
rather than a HELLO packet.
Virtual Router Identifier (VRID): A 16-bit number that identifies the
virtual router with which this packet is associated. Packets
received with an unknown, or unsupported, value will be discarded.
Value Range Usage
0x0000 Unicast Address Family
0x0001 Multicast Address Family
0x0002-0x7FFF Reserved
0x8000 Unicast Service Family
0x8001-0xFFFF Reserved
Autonomous System Number: 16-bit unsigned number of the sending
system. This field is indirectly used as an authentication value.
That is, a router that receives and accepts a packet from a
neighbor must have the same AS number or the packet is ignored.
The range of valid AS numbers is 1 through 65,535.
6.6. EIGRP TLV Encoding Format
The contents of each packet can contain a variable number of fields.
Each field will be tagged and include a length field. This allows
for newer versions of software to add capabilities and coexist with
old versions of software in the same configuration. Fields that are
tagged and not recognized can be skipped over. Another advantage of
this encoding scheme is that it allows multiple network-layer
protocols to carry independent information. Therefore, if it is
later decided to implement a single "integrated" protocol, this can
be done.
The format of a {type, length, value} (TLV) is encoded as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type high | Type low | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The type values are the ones defined below. The length value
specifies the length in octets of the type, length, and value fields.
TLVs can appear in a packet in any order, and there are no
interdependencies among them.
Malformed TLVs contained in EIGRP messages are handled by silently
discarding the containing message. A TLV is malformed if the TLV
Length is invalid or if the TLV extends beyond the end of the
containing message.
6.6.1. Type Field Encoding
The type field is structured as follows: Type High: 1 octet that
defines the protocol classification:
Protocol ID VERSION
General 0x00 1.2
IPv4 0x01 1.2
IPv6 0x04 1.2
SAF 0x05 3.0
Multiprotocol 0x06 2.0
Type Low: 1 octet that defines the TLV Opcode; see TLV Definitions in
Section 3.
6.6.2. Length Field Encoding
The Length field is a 2-octet unsigned number, which indicates the
length of the TLV. The value includes the Type and Length fields.
6.6.3. Value Field Encoding
The Value field is a multi-octet field containing the payload for the
TLV.
6.7. EIGRP Generic TLV Definitions
Ver 1.2 Ver 2.0
PARAMETER_TYPE 0x0001 0x0001
AUTHENTICATION_TYPE 0x0002 0x0002
SEQUENCE_TYPE 0x0003 0x0003
SOFTWARE_VERSION_TYPE 0x0004 0x0004
MULTICAST_SEQUENCE_TYPE 0x0005 0x0005
PEER_INFORMATION_TYPE 0x0006 0x0006
PEER_TERMINATION_TYPE 0x0007 0x0007
PEER_TID_LIST_TYPE --- 0x0008
6.7.1. 0x0001 - PARAMETER_TYPE
This TLV is used in HELLO packets to convey the EIGRP metric
coefficient values: noted as "K-values" as well as the Hold Time
values. This TLV is also used in an initial UPDATE packet when a
neighbor is discovered.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0001 | 0x000C |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K1 | K2 | K3 | K4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K5 | K6 | Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
K-values: The K-values associated with the EIGRP composite metric
equation. The default values for weights are:
K1 - 1
K2 - 0
K3 - 1
K4 - 0
K5 - 0
K6 - 0
Hold Time: The amount of time in seconds that a receiving router
should consider the sending neighbor valid. A valid neighbor is
one that is able to forward packets and participates in EIGRP. A
router that considers a neighbor valid will store all routing
information advertised by the neighbor.
6.7.2. 0x0002 - AUTHENTICATION_TYPE
This TLV may be used in any EIGRP packet and conveys the
authentication type and data used. Routers receiving a mismatch in
authentication shall discard the packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0002 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth Type | Auth Length | Auth Data (Variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication Type: The type of authentication used.
Authentication Length: The length, measured in octets, of the
individual authentication.
Authentication Data: Variable-length field reflected by "Auth
Length", which is dependent on the type of authentication used.
Multiple authentication types can be present in a single
AUTHENTICATION_TYPE TLV.
6.7.2.1. 0x02 - MD5 Authentication Type
MD5 Authentication will use Auth Type code 0x02, and the Auth Data
will be the MD5 Hash value.
6.7.2.2. 0x03 - SHA2 Authentication Type
SHA2-256 Authentication will use Type code 0x03, and the Auth Data
will be the 256-bit SHA2 [6] Hash value.
6.7.3. 0x0003 - SEQUENCE_TYPE
This TLV is used for a sender to tell receivers to not accept packets
with the CR-flag CR-Flag set. This is used to order multicast and unicast
addressed packets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0003 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Address Length | Protocol Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Address Length and Protocol Address will be repeated one or more
times based on the Length Field. field.
Address Length: Number of octets for the address that follows. For
IPv4, the value is 4. For IPv6, it is 16. For AppleTalk, the
value is 4; for Novell IPX, the value is 10 (both are no longer in
use).
Protocol Address: Neighbor address on interface in which the HELLO
with SEQUENCE TLV is sent. Each address listed in the HELLO
packet is a neighbor that should not enter Conditionally Received
mode.
6.7.4. 0x0004 - SOFTWARE_VERSION_TYPE
Field Length
Vender OS major version 1
Vender OS minor version 1
EIGRP major revision 1
EIGRP minor revision 1
The EIGRP TLV Version fields are used to determine TLV format
versions. Routers using Version 1.2 TLVs do not understand version Version
2.0 TLVs, therefore Version 2.0 routers must send the packet with
both TLV formats in a mixed network.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0004 | 0x000C |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Vendor Major V.|Vendor Minor V.| EIGRP Major V.| EIGRP Minor V.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.7.5. 0x0005 - MULTICAST_SEQUENCE_TYPE
The next multicast sequence SEQUENCE TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0005 | 0x0008 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.7.6. 0x0006 - PEER_INFORMATION_TYPE
This TLV is reserved, and not part of this document.
6.7.7. 0x0007 - PEER_ TERMINATION_TYPE
This TLV is used in HELLO packets to notify the list of neighbor(s)
the router has reset the adjacency. This TLV is used in HELLO
packets to notify the list of neighbors that the router has reset the
adjacency. This is used anytime a router needs to reset an
adjacency, or signal an adjacency it is going down.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0007 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address List (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Implementation note: Older Cisco routers implement this using the
"Parameters TLV" with all K-values set to 255 (except K6).
6.7.8. 0x0008 - TID_LIST_TYPE
List of sub-topology identifiers, including the Base Topology,
supported by the router.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0008 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Topology Identification List (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If this information changes from the last state, it means either a
new topology was added or an existing topology was removed. This TLV
is ignored until the three-way handshake has finished
When the TID list is received, it compares the list to the previous
list sent. If a TID is found that does not previously exist, the TID
is added to the neighbor's topology list, and the existing sub-
topology is sent to the peer.
If a TID that was in a previous list is not found, the TID is removed
from the neighbor's topology list and all routes learned though that
neighbor for that sub-topology are removed from the topology table.
6.8. Classic Route Information TLV Types
6.8.1. Classic Flag Field Encoding
EIGRP transports a number of flags with in the TLVs to indicate
addition route state information. These bits are defined as follows:
Flags Field
-----------
Source Withdraw (Bit 0) - Indicates if the router that is the
original source of the destination is withdrawing the route from the
network or if the destination is lost due as a result of a network
failure.
Candidate Default (CD) (Bit 1) - Set to indicate the destination
should be regarded as a candidate for the default route. An EIGRP
default route is selected from all the advertised candidate default
routes with the smallest metric.
ACTIVE (Bit 2) - Indicates if the route is in the ACTIVE State.
6.8.2. Classic Metric Encoding
The handling of bandwidth and delay for Classic TLVs is encoded in
the packet "scaled" form relative to how they are represented on the
physical link.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scaled Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scaled Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reliability | Load | Internal Tag | Flags Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Scaled Delay: An administrative parameter assigned statically on a
per-interface-type basis to represent the time it takes along an
unloaded path. This is expressed in units of tens of microseconds
divvied by 256. A delay of 0xFFFFFFFF indicates an unreachable
route.
Scaled Bandwidth: The path bandwidth measured in bits per second. In
units of 2,560,000,000/kbps.
MTU: The minimum MTU size for the path to the destination.
Hop Count: The number of router traversals to the destination.
Reliability: The current error rate for the path, measured as an
error percentage. A value of 255 indicates 100% reliability
Load: The load utilization of the path to the destination, measured
as a percentage. A value of 255 indicates 100% load.
Internal-Tag: A tag assigned by the network administrator that is
untouched by EIGRP. This allows a network administrator to filter
routes in other EIGRP border routers based on this value.
Flags Field: See Section 6.8.1.
6.8.3. Classic Exterior Encoding
Additional routing information so provided for destinations outside
of the EIGRP AS as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router Identifier (RID) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Autonomous System (AS) Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Administrative Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Protocol Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Extern Protocol| Flags Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Identifier (RID): A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.
External Autonomous System (AS) Number: A 32-bit number indicating
the external AS of which the sending router is a member. If the
source protocol is EIGRP, this field will be the [VRID, AS] pair.
If the external protocol does not have an AS, other information
can be used (for example, Cisco uses process-id for OSPF).
Administrative Tag: A tag assigned by the network administrator that
is untouched by EIGRP. This allows a network administrator to
filter routes in other EIGRP border routers based on this value.
External Protocol Metric: 32-bit value of the composite metric that
resides in the routing table as learned by the foreign protocol.
If the External Protocol is IGRP or another EIGRP routing process,
the value can optionally be the composite metric or 0, and the
metric information is stored in the metric section.
External Protocol: Contains an enumerated value defined in Section
6.2 to identify the routing protocol (external protocol)
redistributing the route.
Flags Field: See Section 6.8.1
6.8.4. Classic Destination Encoding
EIGRP carries destination in a compressed form, where the number of
bits significant in the variable-length address field are indicated
in a counter.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subnet Mask | Destination Address (variable length) |
| Bit Count | ((Bit Count - 1) / 8) + 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Subnet Mask Bit Count: 8-bit value used to indicate the number of
bits in the subnet mask. A value of 0 indicates the default
network, and no address is present.
Destination Address: A variable-length field used to carry the
destination address. The length is determined by the number of
consecutive bits in the destination address. The formula to
calculate the length is address-family dependent:
IPv4: ((Bit Count - 1) / 8) + 1
IPv6: (Bit Count == 128) ? 16 : ((x / 8) + 1)
6.8.5. IPv4-Specific TLVs
INTERNAL_TYPE 0x0102
EXTERNAL_TYPE 0x0103
COMMUNITY_TYPE 0x0104
6.8.5.1. IPv4 INTERNAL_TYPE
This TLV conveys IPv4 destination and associated metric information
for IPv4 networks. Routes advertised in this TLV are network
interfaces that EIGRP is configured on as well as networks that are
learned via other routers running EIGRP.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0x02 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next-Hop Forwarding Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vector Metric Section (see Section 6.8.2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| Destination Section |
| IPv4 Address (variable length) |
| (see Section 6.8.4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next-Hop Forwarding Address: IPv4 address represented by four 8-bit
values (total 4 octets). If the value is zero (0), the IPv4
address from the received IPv4 header is used as the next hop for
the route. Otherwise, the specified IPv4 address will be used.
Vector Metric Section: The vector metrics for destinations contained
in this TLV. See the description of "metric encoding" in Section
6.8.2.
Destination Section: The network/subnet/host destination address
being requested. See the description of "destination" in Section
6.8.4.
6.8.5.2. IPv4 EXTERNAL_TYPE
This TLV conveys IPv4 destination and metric information for routes
learned by other routing protocols that EIGRP injects into the AS.
Available with this information is the identity of the routing
protocol that created the route, the external metric, the AS number,
an indicator if it should be marked as part of the EIGRP AS, and a
network-administrator tag used for route filtering at EIGRP AS
boundaries.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0x03 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next-Hop Forwarding Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Exterior Section (see Section 6.8.3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vector Metric Section (see Section 6.8.2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| Destination Section |
| IPv4 Address (variable length) |
| (see Section 6.8.4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next-Hop Forwarding Address: IPv4 address represented by four 8-bit
values (total 4 octets). If the value is zero (0), the IPv4
address from the received IPv4 header is used as the next hop for
the route. Otherwise, the specified IPv4 address will be used.
Exterior Section: Additional routing information provided for a
destination that is outside of the AS and that has been
redistributed into the EIGRP. See the description of "exterior
encoding" in Section 6.8.3.
Vector Metric Section: Vector metrics for destinations contained in
this TLV. See the description of "metric encoding" in Section
6.8.2.
Destination Section: The network/subnet/host destination address
being requested. See the description of "destination" in Section
6.8.4.
6.8.5.3. IPv4 COMMUNITY_TYPE
This TLV is used to provide community tags for specific IPv4
destinations.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0x04 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Destination |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Community Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Community List |
| (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IPv4 Destination: The IPv4 address with which the community
information should be stored.
Community Length: A 2-octet unsigned number that indicates the length
of the Community List. The length does not include the IPv4
Address, Reserved, or Length fields.
Community List: One or more 8-octet EIGRP communities, as defined in
Section 6.4.
6.8.6. IPv6-Specific TLVs
INTERNAL_TYPE 0x0402
EXTERNAL_TYPE 0x0403
COMMUNITY_TYPE 0x0404
6.8.6.1. IPv6 INTERNAL_TYPE
This TLV conveys the IPv6 destination and associated metric
information for IPv6 networks. Routes advertised in this TLV are
network interfaces that EIGRP is configured on as well as networks
that are learned via other routers running EIGRP.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x04 | 0x02 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Next-Hop Forwarding Address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vector Metric Section (see Section 6.8.2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| Destination Section |
| IPv6 Address (variable length) |
| (see Section 6.8.4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next-Hop Forwarding Address: This IPv6 address is represented by
eight groups of 16-bit values (total 16 octets). If the value is
zero (0), the IPv6 address from the received IPv6 header is used
as the next hop for the route. Otherwise, the specified IPv6
address will be used.
Vector Metric Section: Vector metrics for destinations contained in
this TLV. See the description of "metric encoding" in Section
6.8.2.
Destination Section: The network/subnet/host destination address
being requested. See the description of "destination" in Section
6.8.4.
6.8.6.2. IPv6 EXTERNAL_TYPE
This TLV conveys IPv6 destination and metric information for routes
learned by other routing protocols that EIGRP injects into the
topology. Available with this information is the identity of the
routing protocol that created the route, the external metric, the AS
number, an indicator if it should be marked as part of the EIGRP AS,
and a network administrator tag used for route filtering at EIGRP AS
boundaries.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x04 | 0x03 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Next-Hop Forwarding Address |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Exterior Section (see Section 6.8.3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vector Metric Section (see Section 6.8.2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| Destination Section |
| IPv6 Address (variable length) |
| (see Section 6.8.4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next-Hop Forwarding Address: IPv6 address is represented by eight
groups of 16-bit values (total 16 octets). If the value is zero
(0), the IPv6 address from the received IPv6 header is used as the
next hop for the route. Otherwise, the specified IPv6 address
will be used.
Exterior Section: Additional routing information provided for a
destination that is outside of the AS and that has been
redistributed into the EIGRP. See the description of "exterior
encoding" in Section 6.8.3.
Vector Metric Section: vector metrics for destinations contained in
this TLV. See the description of "metric encoding" in Section
6.8.2.
Destination Section: The network/subnet/host destination address
being requested. See the description of "destination" in Section
6.8.4.
6.8.6.3 IPv6 COMMUNITY_TYPE
This TLV is used to provide community tags for specific IPv4
destinations.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x04 | 0x04 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Destination |
| (16 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Community Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Community List |
| (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination: The IPv6 address with which the community information
should be stored.
Community Length: A 2-octet unsigned number that indicates the length
of the Community List. The length does not include the IPv6
Address, Reserved, or Length fields.
Community List: One or more 8-octet EIGRP communities, as defined in
Section 6.4.
6.9. Multiprotocol Route Information TLV Types
This TLV conveys topology and associated metric information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Header Version | Opcode | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Virtual Router ID | Autonomous System Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Header Encoding |
| (see Section 6.9.1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Wide Metric Encoding |
| (see Section 6.9.2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Descriptor |
| (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.9.1. TLV Header Encoding
There has been a long-standing requirement for EIGRP to support
routing technologies, such as multi-topologies, and to provide the
ability to carry destination information independent of the
transport. To accomplish this, a Vector has been extended to have a
new "Header Extension Header" section. This is a variable-length
field and, at a minimum, it will support the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type High | Type Low | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AFI | TID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router Identifier (RID) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The available fields are:
TYPE - Topology TLVs have the following TYPE codes:
Type High: 0x06
Type Low:
REQUEST_TYPE 0x01
INTERNAL_TYPE 0x02
EXTERNAL_TYPE 0x03
Router Identifier (RID): A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.
6.9.2. Wide Metric Encoding
Multiprotocol TLVs will provide an extendable section of metric
information, which is not used for the primary routing compilation.
Additional per-path information is included to enable per-path cost
calculations in the future. Use of the per-path costing along with
the VID/TID will prove a complete solution for multidimensional
routing.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset | Priority | Reliability | Load |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Opaque Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Attributes |
| (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
Offset: Number of 16-bit words in the Extended Attribute section that
are used to determine the start of the destination information. A
value of zero indicates no Extended Attributes are attached.
Priority: Priority of the prefix when processing a route. In an AS
using priority values, a destination with a higher priority
receives preferential treatment and is serviced before a
destination with a lower priority. A value of zero indicates no
priority is set.
Reliability: The current error rate for the path. Measured as an
error percentage. A value of 255 indicates 100% reliability
Load: The load utilization of the path to the destination, measured
as a percentage. A value of 255 indicates 100% load.
MTU: The minimum MTU size for the path to the destination. Not used
in metric calculation but available to underlying protocols
Hop Count: The number of router traversals to the destination.
Delay: The one-way latency along an unloaded path to the destination
expressed in units of picoseconds per kilobit. This number is not
scaled; a value of 0xFFFFFFFFFFFF indicates an unreachable route.
Bandwidth: The path bandwidth measured in kilobit per second as
presented by the interface. This number is not scaled; a value of
0xFFFFFFFFFFFF indicates an unreachable route.
Reserved: Transmitted as 0x0000.
Opaque Flags: 16-bit protocol-specific flags. Values currently
defined by Cisco are:
OPAQUE_SRCWD 0x01 Route Source Withdraw
OPAQUE_CD 0x02 Candidate default route
OPAQUE_ACTIVE 0x04 Route is currently in active state
OPAQUE_REPL 0x08 Route is replicated from another VRF
Extended Attributes (Optional): When present, defines extendable per-
destination attributes. This field is not normally transmitted.
6.9.3. Extended Metrics
Extended metrics allow for extensibility of the vector metrics in a
manner similar to RFC 6390 [11]. Each Extended metric shall consist
of a header identifying the type (Opcode) and the length (Offset)
followed by application-specific information. Extended metric values
not understood must be treated as opaque and passed along with the
associated route.
The general formats for the Extended Metric fields are:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode | Offset | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Opcode: Indicates the type of Extended Metric.
Offset: Number of 16-bit words in the application-specific
information. Offset does not include the length of the Opcode or
Offset.
Data: Zero or more octets of data as defined by Opcode.
6.9.3.1. 0x00 - NoOp
This is used to pad the attribute section to ensure 32-bit alignment
of the metric encoding section.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are:
Opcode: Transmitted as zero (0).
Offset: Transmitted as zero (0) indicating no data is present.
Data: No data is present with this attribute.
6.9.3.2. 0x01 - Scaled Metric
If a route is received from a back-rev neighbor, and the route is
selected as the best path, the scaled metric received in the older
UPDATE may be attached to the packet. If received, the value is for
informational purposes and is not affected by K6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0x04 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scaled Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scaled Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved: Transmitted as 0x0000
Scaled Bandwidth: The minimum bandwidth along a path expressed in
units of 2,560,000,000/kbps. A bandwidth of 0xFFFFFFFF indicates
an unreachable route.
Scaled Delay: An administrative parameter assigned statically on a
per-interface-type basis to represent the time it takes along an
unloaded path. This is expressed in units of tens of microseconds
divvied by 256. A delay of 0xFFFFFFFF indicates an unreachable
route.
6.9.3.3. 0x02 - Administrator Tag
This is used to provide
EIGRP administrative tags for specific topology
entries. It is tag does not affected by K6. alter the path decision-making
process. Routers can set a tag value on a route and use the flags to
apply specific routing polices within their network.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | 0x02 | Administrator Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Administrator Tag (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Administrator Tag: A tag assigned by the network administrator that
is untouched by EIGRP. This allows a network administrator to
filter routes in other EIGRP border routers based on this value.
6.9.3.4. 0x03 - Community List
This is
EIGRP communities themselves do not alter the path decision-making
process, communities can be used as flags in order to provide communities for mark a set of
routes. Upstream routers can then use these flags to apply specific topology entries.
It is not affected by K6
routing polices within their network.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x03 | Offset | Community List |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Offset: Number of 16-bit words in the sub-field.
Community List: One or more 8-octet EIGRP communities, as defined in
Section 6.4.
6.9.3.5. 0x04 - Jitter
(Optional) EIGRP can carry one-way Jitter in networks that carry UDP
traffic if the node is capable of measuring UDP Jitter. The Jitter
reported to will be averaged with any existing Jitter data and
include in the route updates. If no Jitter value is reported by the
peer for a given destination, EIGRP will use the locally collected
value.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x04 | 0x03 | Jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Jitter: The measure of the variability over time of the latency
across a network measured in measured in microseconds.
6.9.3.6. 0x05 - Quiescent Energy
(Optional) EIGRP can carry energy usage by nodes in networks if the
node is capable of measuring energy. The Quiescent Energy reported
will be added to any existing energy data and include in the route
updates. If no energy data is reported by the peer for a given
destination, EIGRP will use the locally collected value.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x05 | 0x02 | Q-Energy (high) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Q-Energy (low) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Q-Energy: Paths with higher idle (standby) energy usage will be
reflected in a higher aggregate metric than those having lower
energy usage. If present, this number will represent the idle
power consumption expressed in milliwatts per kilobit.
6.9.3.7. 0x06 - Energy
(Optional) EIGRP can carry energy usage by nodes in networks if the
node is capable of measuring energy. The active Energy reported will
be added to any existing energy data and include in the route
updates. If no energy data is reported by the peer for a given
destination, EIGRP will use the locally collected value.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x06 | 0x02 | Energy (high) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Energy (low) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Energy: Paths with higher active energy usage will be reflected in a
higher aggregate metric than those having lower energy usage. If
present, this number will represent the power consumption
expressed in milliwatts per kilobit.
6.9.3.8. 0x07 - AddPath
The Add Path enables EIGRP to advertise multiple best paths to
adjacencies. There will be up to a maximum of four AddPaths
supported, where the format of the field will be as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x07 | Offset | AddPath (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Offset: Number of 16-bit words in the sub-field.
AddPath: Length of this field will vary in length based on whether it
contains IPv4 or IPv6 data.
6.9.3.8.1. AddPath with IPv4 Next Hop
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x07 | Offset | Next-Hop Addr. (Upper 2 bytes)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address (Lower 2 bytes) | RID (Upper 2 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RID (Upper 2 bytes) | Admin Tag (Upper 2 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Admin Tag (Upper 2 bytes) |Extern Protocol| Flags Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next-Hop Address: An IPv4 address represented by four 8-bit values
(total 4 octets). If the value is zero (0), the IPv6 address from
the received IPv4 header is used as the next hop for the route.
Otherwise, the specified IPv4 address will be used.
Router Identifier (RID): A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.
Admin Tag: A 32-bit administrative tag assigned by the network. This
allows a network administrator to filter routes based on this
value.
If the route is of type external, then two additional bytes will be
added as follows:
External Protocol: Contains an enumerated value defined in Section
6.2 to identify the routing protocol (external protocol)
redistributing the route.
Flag
Flags Field: See Section 6.8.1.
6.9.3.8.2. AddPath with IPv6 Next Hop
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x07 | Offset | Next-Hop Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| (16 octets) |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | RID (Upper 2 byes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RID (Upper 2 byes) | Admin Tag (Upper 2 byes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Admin Tag (Upper 2 byes) | Extern Protocol | Flags Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next-Hop Address: An IPv6 address represented by eight groups of
16-bit values (total 16 octets). If the value is zero (0), the
IPv6 address from the received IPv6 header is used as the next hop
for the route. Otherwise, the specified IPv6 address will be
used.
Router Identifier (RID): A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.
Admin Tag: A 32-bit administrative tag assigned by the network. This
allows a network administrator to filter routes based on this
value. If the route is of type external, then two addition bytes
will be added as follows:
External Protocol: Contains an enumerated value defined in Section
6.2 to identify the routing protocol (external protocol)
redistributing the route.
Flag
Flags Field: See Section 6.8.1.
6.9.4. Exterior Encoding
Additional routing information provided for destinations outside of
the EIGRP AS as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router Identifier (RID) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Autonomous System (AS) Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Protocol Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Extern Protocol| Flags Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Identifier (RID): A 32-bit number provided by the router
sourcing the information to uniquely identify it as the source.
External Autonomous System (AS) Number: A 32-bit number indicating
the external AS of which the sending router is a member. If the
source protocol is EIGRP, this field will be the [VRID, AS] pair.
If the external protocol does not have an AS, other information
can be used (for example, Cisco uses process-id for OSPF).
External Protocol Metric: A 32-bit value of the metric used by the
routing table as learned by the foreign protocol. If the External
Protocol is IGRP or EIGRP, the value can (optionally) be 0, and
the metric information is stored in the metric section.
External Protocol: Contains an enumerated value defined in Section
6.2 to identify the routing protocol (external protocol)
redistributing the route.
Flag
Flags Field: See Section 6.8.1.
6.9.5. Destination Encoding
Destination information is encoded in Multiprotocol packets in the
same manner used by Classic TLVs. This is accomplished by using a
counter to indicate how many significant bits are present in the
variable-length address field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subnet Mask | Destination Address (variable length |
| Bit Count | ((Bit Count - 1) / 8) + 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Subnet Mask Bit Count: 8-bit value used to indicate the number of
bits in the subnet mask. A value of 0 indicates the default
network and no address is present.
Destination Address: A variable-length field used to carry the
destination address. The length is determined by the number of
consecutive bits in the destination address. The formula to
calculate the length is address-family dependent:
IPv4: ((Bit Count - 1) / 8) + 1
IPv6: (Bit Count == 128) ? 16 : ((x / 8) + 1)
6.9.6. Route Information
6.9.6.1. INTERNAL TYPE
This TLV conveys destination information based on the IANA AFI
defined in the TLV Header (see Section 6.9.1), and associated metric
information. Routes advertised in this TLV are network interfaces
that EIGRP is configured on as well as networks that are learned via
other routers running EIGRP.
6.9.6.2. EXTERNAL TYPE
This TLV conveys destination information based on the IANA AFI
defined in the TLV Header (see Section 6.9.1), and metric information
for routes learned by other routing protocols that EIGRP injects into
the AS. Available with this information is the identity of the
routing protocol that created the route, the external metric, the AS
number, an indicator if it should be marked as part of the EIGRP AS,
and a network administrator tag used for route filtering at EIGRP AS
boundaries.
7. Security Considerations
Being promiscuous, EIGRP will neighbor with any router that sends a
valid HELLO packet. Due to security considerations, this
"completely" open aspect requires policy capabilities to limit
peering to valid routers.
EIGRP does not rely on a PKI or a heavyweight authentication system.
These systems challenge the scalability of EIGRP, which was a primary
design goal.
Instead, Denial-of-Service (DoS) attack prevention will depend on
implementations rate-limiting packets to the control plane as well as
authentication of the neighbor through the use of MD5 or SHA2-256
[6].
8. IANA Considerations
This document serves as the sole reference for two multicast
addresses: 224.0.0.10 for IPv4 "EIGRP Routers" [13] and
FF02:0:0:0:0:0:0:A for IPv6 "EIGRP Routers" [14]. It also serves as
assignment for protocol number 88 (EIGRP) [15].
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[2] Garcia-Luna-Aceves, J.J., "A Unified Approach to Loop-Free
Routing Using Distance Vectors or Link States", SIGCOMM '89,
Symposium proceedings on Communications architectures &
protocols, Volume 19, pages 212-223, ACM
089791-332-9/89/0009/0212, DOI 10.1145/75247.75268, 1989.
[3] Garcia-Luna-Aceves, J.J., "Loop-Free Routing using Diffusing
Computations", Network Information Systems Center, SRI
International, appeared in IEEE/ACM Transactions on Networking,
Vol. 1, No. 1, DOI 10.1109/90.222913, 1993.
[4] Rosen, E. and Y. Rekhter, "IANA Registries for BGP Extended
Communities", RFC 7153, DOI 10.17487/RFC7153, March 2014,
<http://www.rfc-editor.org/info/rfc7153>.
[5] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, DOI 10.17487/RFC3692,
January 2004, <http://www.rfc-editor.org/info/rfc3692>.
[6] Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-384, and
HMAC-SHA-512 with IPsec", RFC 4868, DOI 10.17487/RFC4868, May
2007, <http://www.rfc-editor.org/info/rfc4868>.
[7] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, DOI 10.17487/RFC1112, August 1989,
<http://www.rfc-editor.org/info/rfc1112>.
[8] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.
[9] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998,
<http://www.rfc-editor.org/info/rfc2460>.
9.2. Informative References
[10] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
[11] Clark, A. and B. Claise, "Guidelines for Considering New
Performance Metric Development", BCP 170, RFC 6390,
DOI 10.17487/RFC6390, October 2011,
<http://www.rfc-editor.org/info/rfc6390>.
[12] IANA, "Address Family Numbers",
<http://www.iana.org/assignments/address-family-numbers>.
[13] IANA, "IPv4 Multicast Address Space Registry",
<http://www.iana.org/assignments/multicast-addresses>.
[14] IANA, "IPv6 Multicast Address Space Registry",
<http://www.iana.org/assignments/ipv6-multicast-addresses>.
[15] IANA, "Protocol Numbers",
<http://www.iana.org/assignments/protocol-numbers>.
Acknowledgments
Thank you goes to Dino Farinacci, Bob Albrightson, and Dave Katz.
Their significant accomplishments towards the design and development
of the EIGRP provided the bases for this document.
A special and appreciative thank you goes to the core group of Cisco
engineers whose dedication, long hours, and hard work led the
evolution of EIGRP over the past decade. They are Donnie Savage,
Mickel Ravizza, Heidi Ou, Dawn Li, Thuan Tran, Catherine Tran, Don
Slice, Claude Cartee, Donald Sharp, Steven Moore, Richard Wellum, Ray
Romney, Jim Mollmann, Dennis Wind, Chris Van Heuveln, Gerald Redwine,
Glen Matthews, Michael Wiebe, and others.
The authors would like to gratefully acknowledge many people who have
contributed to the discussions that lead to the making of this
proposal. They include Chris Le, Saul Adler, Scott Van de Houten,
Lalit Kumar, Yi Yang, Kumar Reddy, David Lapier, Scott Kirby, David
Prall, Jason Frazier, Eric Voit, Dana Blair, Jim Guichard, and Alvaro
Retana.
In addition to the tireless work provided by the Cisco engineers over
the years, we would like to personally recognize the teams that
created open source versions of EIGRP:
o Linux implementation developed by the Quagga team: Jan Janovic,
Matej Perina, Peter Orsag, and Peter Paluch.
o BSD implementation developed and released by Renato Westphal.
Authors' Addresses
Donnie V. Savage
Cisco Systems, Inc.
7025 Kit Creek Rd., RTP,
Morrisville, NC 27560
United States
Phone: 919-392-2379
Email: dsavage@cisco.com
Donald Slice
Cumulus Networks
Apex, NC
United States
Email: dslice@cumulusnetworks.com
James Ng
Cisco Systems, Inc.
7025 Kit Creek Rd., RTP,
Morrisville, NC 27560
United States
Phone: 919-392-2582
Email: jamng@cisco.com
Peter Paluch
University of Zilina
Univerzitna 8215/1, Zilina 01026
Slovakia
Phone: 421-905-164432
Email: Peter.Paluch@fri.uniza.sk
Steven Moore
Cisco Systems, Inc.
7025 Kit Creek Rd., RTP,
Morrisville, NC 27560
United States
Phone: 408-895-2031
Email: smoore@cisco.com
Russ White
LinkedIn
Apex, NC
United States
Phone: 1-877-308-0993
Email: russw@riw.us