Internet Engineering Task Force (IETF) S. Litkowski, Ed.
Request for Comments: 7916 B. Decraene
Category: Standards Track Orange
ISSN: 2070-1721 C. Filsfils
K. Raza
Cisco Systems
M. Horneffer
Deutsche Telekom
P. Sarkar
Individual Contributor
June 2016
Operational Management of Loop-Free Alternates
Abstract
Loop-Free Alternates (LFAs), as defined in RFC 5286, constitute an IP
Fast Reroute (IP FRR) mechanism enabling traffic protection for IP
traffic (and, by extension, MPLS LDP traffic). Following early
deployment experiences, this document provides operational feedback
on LFAs, highlights some limitations, and proposes a set of
refinements to address those limitations. It also proposes required
management specifications.
This proposal is also applicable to remote-LFA solutions.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in 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/rfc7916.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Operational Issues with Default LFA Tiebreakers . . . . . . . 4
3.1. Case 1: PE Router Protecting against Failures within Core
Network . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Case 2: PE Router Chosen to Protect against Core Failures
while P Router LFA Exists . . . . . . . . . . . . . . . . 5
3.3. Case 3: Suboptimal P Router Alternate Choice . . . . . . 6
3.4. Case 4: No-Transit LFA Computing Node . . . . . . . . . . 7
4. Need for Coverage Monitoring . . . . . . . . . . . . . . . . 8
5. Need for LFA Activation Granularity . . . . . . . . . . . . . 9
6. Configuration Requirements . . . . . . . . . . . . . . . . . 9
6.1. LFA Enabling/Disabling Scope . . . . . . . . . . . . . . 10
6.2. Policy-Based LFA Selection . . . . . . . . . . . . . . . 10
6.2.1. Connected versus Remote Alternates . . . . . . . . . 11
6.2.2. Mandatory Criteria . . . . . . . . . . . . . . . . . 12
6.2.3. Additional Criteria . . . . . . . . . . . . . . . . . 12
6.2.4. Evaluation of Criteria . . . . . . . . . . . . . . . 12
6.2.5. Retrieving Alternate Path Attributes . . . . . . . . 16
6.2.6. ECMP LFAs . . . . . . . . . . . . . . . . . . . . . . 21
7. Operational Aspects . . . . . . . . . . . . . . . . . . . . . 22
7.1. No-Transit Condition on LFA Computing Node . . . . . . . 22
7.2. Manual Triggering of FRR . . . . . . . . . . . . . . . . 23
7.3. Required Local Information . . . . . . . . . . . . . . . 24
7.4. Coverage Monitoring . . . . . . . . . . . . . . . . . . . 24
7.5. LFAs and Network Planning . . . . . . . . . . . . . . . . 25
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Normative References . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . 27
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
Following the first deployments of Loop-Free Alternates (LFAs), this
document provides feedback to the community about the management of
LFAs.
o Section 3 provides real use cases illustrating some limitations
and suboptimal behavior.
o Section 4 provides requirements for LFA simulations.
o Section 5 proposes requirements for activation granularity and
policy-based selection of the alternate.
o Section 6 expresses requirements for the operational management of
LFAs and, in particular, a policy framework to manage alternates.
o Section 7 details some operational considerations of LFAs, such as
IS-IS overload bit management and troubleshooting information.
1.1. 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 [RFC2119].
2. Definitions
o Per-prefix LFA computation: Evaluation for the best alternate is
done for each destination prefix, as opposed to the "per-next-hop"
simplification technique proposed in Section 3.8 of [RFC5286].
o PE router: Provider Edge router. These routers connect customers
to each other.
o P router: Provider router. These routers are core routers without
customer connections. They provide transit between PE routers,
and they form the core network.
o Core network: subset of the network composed of P routers and
links between them.
o Core link: network link part of the core network, i.e., a link
between P routers.
o Link-protecting LFA: alternate providing protection against link
failure.
o Node-protecting LFA: alternate providing protection against node
failure.
o Connected alternate: alternate adjacent (at the IGP level) to the
Point of Local Repair (PLR) (i.e., an IGP neighbor).
o Remote alternate: alternate that does not share an IGP adjacency
with the PLR.
3. Operational Issues with Default LFA Tiebreakers
[RFC5286] introduces the notion of tiebreakers when selecting the LFA
among multiple candidate alternate next hops. When multiple LFAs
exist, [RFC5286] has favored the selection of the LFA that provides
the best coverage against the failure cases. While this is indeed a
goal, it is one among multiple goals, and in some deployments this
leads to the selection of a suboptimal LFA. The following sections
detail real use cases related to such limitations.
Note that the use case for LFA computation per destination
(per-prefix LFA) is assumed throughout this analysis. We also assume
in the network figures that all IP prefixes are advertised with
zero cost.
3.1. Case 1: PE Router Protecting against Failures within Core Network
P1 --------- P2 ---------- P3 --------- P4
| 1 100 1 |
| |
| 100 | 100
| |
| 1 100 1 | 1 5k
P5 --------- P6 ---------- P7 --------- P8 --- P9 -- PE1
| | | | | |
5k| |5k 5k| |5k | 5k | 5k
| | | | | |
| +-- PE4 --+ | +---- PE2 ----+
| | |
+---- PE5 ----+ | 5k
|
PE3
Px routers are P routers using n * 10 Gbps links.
PEs are connected using links with lower bandwidth.
Figure 1
In Figure 1, let us consider the traffic flowing from PE1 to PE4.
The nominal path is P9-P8-P7-P6-PE4. Let us now consider the failure
of link P7-P8. As the P4 primary path to PE4 is P8-P7-P6-PE4, P4 is
not an LFA for P8 (because P4 will loop traffic back to P8), and the
only available LFA is PE2.
When the core link P8-P7 fails, P8 switches all traffic destined to
PE4/PE5 towards the node PE2. Hence, a PE node and PE links are used
to protect against the failure of a core link. Typically, PE links
have less capacity than core links, and congestion may occur on PE2
links. Note that although PE2 is not directly affected by the
failure, its links become congested, and its traffic will suffer from
the congestion.
In summary, in the case of P8-P7 link failure, the impact on customer
traffic is:
o From PE2's point of view:
* without LFA: no impact.
* with LFA: traffic is partially dropped (but possibly
prioritized by a QoS mechanism). It must be highlighted that
in such a situation, traffic not affected by the failure may be
affected by the congestion.
o From P8's point of view:
* without LFA: traffic is totally dropped until convergence
occurs.
* with LFA: traffic is partially dropped (but possibly
prioritized by a QoS mechanism).
Besides the congestion aspects of using a PE router as an alternate
to protect against a core failure, a service provider may consider
this to be a bad routing design and would want to prevent it.
3.2. Case 2: PE Router Chosen to Protect against Core Failures while
P Router LFA Exists
P1 --------- P2 ------------ P3 ------- P4
| 1 100 | 1 |
| | |
| 100 | 30 | 30
| | |
| 1 50 50 | 10 | 1 5k
P5 --------- P6 --- P10 ---- P7 ------- P8 --- P9 -- PE1
| | | | \ |
5k| |5k 5k| |5k \ 5k | 5k
| | | | \ |
| +-- PE4 --+ | +---- PE2 ----+
| | |
+---- PE5 ----+ | 5k
|
PE3
Px routers are P routers meshed with n * 10 Gbps links.
PEs are meshed using links with lower bandwidth.
Figure 2
In Figure 2, let us consider the traffic coming from PE1 to PE4. The
nominal path is P9-P8-P7-P10-P6-PE4. Let us now consider the failure
of the link P7-P8. For P8, P4 is a link-protecting LFA and PE2 is a
node-protecting LFA. PE2 is chosen as the best LFA, due to the
better type of protection that it provides. Just as in case 1, this
may lead to congestion on PE2 links upon LFA activation.
3.3. Case 3: Suboptimal P Router Alternate Choice
+--- PE3 ---+
/ \
1000 / \ 1000
/ \
+----- P1 ---------------- P2 ----+
| | 500 | |
| 10 | | | 10
| | | |
R5 | 10 | 10 R7
| | | |
| 10 | | | 10
| | 500 | |
+---- P3 ----------------- P4 ----+
\ /
1000 \ / 1000
\ /
+--- PE1 ---+
Px routers are P routers.
P1-P2 and P3-P4 links are 1 Gbps links.
All other inter-Px links are 10 Gbps links.
Figure 3
In Figure 3, let us consider the failure of link P1-P3. For
destination PE3, P3 has two possible alternates:
o P4, which is node-protecting
o R5, which is link-protecting
P4 is chosen as the best LFA, due to the better type of protection
that it provides. However, for bandwidth capacity reasons, it
may not be desirable to use P4. A service provider may prefer to use
high-bandwidth links as the preferred LFA. In this example,
preferring the shortest path over the type of protection may achieve
the expected behavior, but in cases where metrics do not reflect the
bandwidth, this technique would not work and some other criteria
would need to be involved when selecting the best LFA.
3.4. Case 4: No-Transit LFA Computing Node
P1 P2
| \ / |
50 | 50 \/ 50 | 50
| /\ |
PE1-+ +-- PE2
\ /
45 \ / 45
-PE3-
(No-transit condition set)
Figure 4
The IS-IS and OSPF protocols define some way to prevent a router from
being used for transit.
The IS-IS overload bit is defined in [ISO10589], and the OSPF R-bit
is defined in [RFC5340]. Also, the OSPF stub router is defined in
[RFC6987] as a method to prevent transit on a node by advertising
MaxLinkMetric on all non-stub links.
In Figure 4, PE3 has its no-transit condition set (permanently, for
design reasons) and wants to protect traffic using an LFA for
destination PE2.
On PE3, the loop-free condition is not satisfied: 100 !< 45 + 45.
PE1 is thus not considered as an LFA. However, thanks to the
no-transit condition on PE3, we know that PE1 will not loop the
traffic back to PE3. So, PE1 is an LFA to reach PE2.
In the case of a no-transit condition set on a node, LFA behavior
must be clarified.
4. Need for Coverage Monitoring
As per [RFC6571], LFA coverage depends strongly on the network
topology that is in use. Even if the remote-LFA mechanism [RFC7490]
significantly extends the coverage of the basic LFA specification,
there are still some cases where protection would not be available.
As network topologies are constantly evolving (network extension,
additional capacity, latency optimization, etc.), the protection
coverage may change. Fast Reroute (FRR) functionality may be
critical for some services supported by the network; a service
provider must always know what type of protection coverage is
currently available on the network. Moreover, predicting protection
coverage in the event of network topology changes is mandatory.
Today, network simulation tools associated with "what if" scenarios
are often used by service providers for the overall network design
(capacity, path optimization, etc.). Sections 7.3, 7.4, and 7.5 of
this document propose the addition of LFA information into such tools
and within routers, so that a service provider may be able to:
o evaluate protection coverage after a topology change.
o adjust the topology change to cover the primary need (e.g.,
latency optimization, bandwidth increase) as well as LFA
protection.
o constantly monitor the LFA coverage in the live network and
receive alerts.
Documentation of LFA selection algorithms by implementers (default
and tuning options) is important in order to make it possible for
third-party modules to model these policy-based LFA selection
algorithms.
5. Need for LFA Activation Granularity
As in all FRR mechanisms, an LFA installs backup paths in the
Forwarding Information Base (FIB). Depending on the hardware used by
a service provider, FIB resources may be critical. Activating LFAs
by default on all available components (IGP topologies, interfaces,
address families, etc.) may lead to a waste of FIB resources, as
generally only a few destinations in a network should be protected
(e.g., loopback addresses supporting MPLS services) compared to the
number of destinations in the Routing Information Base (RIB).
Moreover, a service provider may implement multiple different FRR
mechanisms in its networks for different applications (e.g.,
Maximally Redundant Trees (MRTs), TE FRR). In this scenario, an
implementation MAY allow the computation of alternates for a specific
destination even if the destination is already protected by another
mechanism. This will provide redundancy and permit the operator to
select the best option for FRR, using a policy language.
Section 6 provides some implementation guidelines.
6. Configuration Requirements
Controlling the selection of the best alternate and the granularity
of LFA activation is a requirement for service providers. This
section defines configuration requirements for LFAs.
6.1. LFA Enabling/Disabling Scope
The granularity of LFA activation SHOULD be controlled (as alternate
next hops consume memory in the forwarding plane).
An implementation of an LFA SHOULD allow its activation, with the
following granularities:
o Per routing context: Virtual Routing and Forwarding (VRF),
virtual/logical router, global routing table, etc.
o Per interface.
o Per protocol instance, topology, area.
o Per prefix: Prefix protection SHOULD have a higher priority
compared to interface protection. This means that if a specific
prefix must be protected due to a configuration request, an LFA
MUST be computed and installed for that prefix even if the primary
outgoing interface is not configured for protection.
An implementation of an LFA MAY allow its activation, with the
following criteria:
o Per address family: IPv4 unicast, IPv6 unicast.
o Per MPLS control plane: For MPLS control planes that inherit
routing decisions from the IGP routing protocol, the MPLS
data plane may be protected by an LFA. The implementation may
allow an operator to control this inheritance of protection from
the IP prefix to the MPLS label bound to this prefix. The
inheritance of protection will concern IP-to-MPLS, MPLS-to-MPLS,
and MPLS-to-IP entries. As an example, LDP and Segment Routing
extensions [SEG-RTG-ARCH] for IS-IS and OSPF are control-plane
eligible for this inheritance of protection.
6.2. Policy-Based LFA Selection
When multiple alternates exist, the LFA selection algorithm is based
on tiebreakers. Current tiebreakers do not provide sufficient
control regarding how the best alternate is chosen. This document
proposes an enhanced tiebreaker allowing service providers to manage
all specific cases:
1. An LFA implementation SHOULD support policy-based decisions for
determining the best LFA.
2. Policy-based decisions SHOULD be based on multiple criteria, with
each criterion having a level of preference.
3. If the defined policy does not allow the determination of a
unique best LFA, an implementation SHOULD pick only one based on
its own decision. For load-balancing purposes, an implementation
SHOULD also support the election of multiple LFAs.
4. The policy SHOULD be applicable to a protected interface or a
specific set of destinations. In the case of applicability to
the protected interface, all destinations primarily routed on
that interface SHOULD use the policy for that interface.
5. The choice of whether or not to dynamically re-evaluate policy
(in the event of a policy change) is left to the implementation.
If a dynamic approach is chosen, the implementation SHOULD
recompute the best LFAs and reinstall them in the FIB without
service disruption. If a non-dynamic approach is chosen, the
policy would be taken into account upon the next IGP event. In
this case, the implementation SHOULD support a command to
manually force the recomputation/reinstallation of LFAs.
6.2.1. Connected versus Remote Alternates
In addition to connected LFAs, tunnels (e.g., IP, LDP, RSVP-TE,
Segment Routing) to distant routers may be used to complement LFA
coverage (tunnel tail used as virtual neighbor). When a router has
multiple alternate candidates for a specific destination, it may have
connected alternates and remote alternates (reachable via a tunnel).
Connected alternates may not always provide an optimal routing path,
and it may be preferable to select a remote alternate over a
connected alternate. Some uses of tunnels to extend LFA [RFC5286]
coverage are described in [RFC7490] and [TI-LFA]. [RFC7490] and
[TI-LFA] present some use cases for LDP tunnels and Segment Routing
tunnels, respectively. This document considers any type of tunneling
techniques to reach remote alternates (IP, Generic Routing
Encapsulation (GRE), LDP, RSVP-TE, the Layer 2 Tunneling Protocol
(L2TP), Segment Routing, etc.) and does not restrict the remote
alternates to the uses presented in these other documents.
In Figure 1, there is no P router alternate for P8 to reach PE4 or
PE5, so P8 is using PE2 as an alternate; this may generate congestion
when FRR is activated. Instead, we could have a remote alternate for
P8 to protect traffic to PE4 and PE5. For example, a tunnel from P8
to P3 (following the shortest path) can be set up, and P8 would be
able to use P3 as a remote alternate to protect traffic to PE4 and
PE5. In this scenario, traffic will not use a PE link during FRR
activation.
When selecting the best alternate, the selection algorithm MUST
consider all available alternates (connected or tunnel). For
example, with remote LFAs, computation of PQ sets [RFC7490] SHOULD be
performed before the selection of the best alternate.
6.2.2. Mandatory Criteria
An LFA implementation MUST support the following criteria:
o Non-candidate link: A link marked as "non-candidate" will never be
used as an LFA.
o A primary next hop being protected by another primary next hop of
the same prefix (ECMP case).
o Type of protection provided by the alternate: link protection or
node protection. In the case of preference for node protection,
an implementation SHOULD support fallback to link protection if
node protection is not available.
o Shortest path: lowest IGP metric used to reach the destination.
o Shared Risk Link Groups (SRLGs) (as defined in Section 3 of
[RFC5286]; see also Section 6.2.4.1 for more details).
6.2.3. Additional Criteria
An LFA implementation SHOULD support the following criteria:
o A downstream alternate: Preference for a downstream path over a
non-downstream path SHOULD be configurable.
o Link coloring with "include", "exclude", and preference-based
systems (see Section 6.2.4.2).
o Link bandwidth (see Section 6.2.4.3).
o Alternate preference / node coloring (see Section 6.2.4.4).
6.2.4. Evaluation of Criteria
6.2.4.1. SRLGs
Section 3 of [RFC5286] proposes the reuse of GMPLS IGP extensions to
encode SRLGs [RFC5307] [RFC4203]. Section 3 of [RFC5286] also
describes the algorithm to compute SRLG protection.
When SRLG protection is computed, an implementation SHOULD allow the
following:
o Exclusion of alternates in violation of SRLGs.
o Maintenance of a preference system between alternates based on
SRLG violations. How the preference system is implemented is out
of scope for this document, but here are two examples:
* Preference based on the number of violations. In this case,
more violations = less preferred.
* Preference based on violation cost. In this case, each SRLG
violation has an associated cost. The lower violation costs
are preferred.
When applying SRLG criteria, the SRLG violation check SHOULD be
performed on sources to alternates as well as alternates to
destination paths, based on the SRLG set of the primary path. In the
case of remote LFAs, PQ-to-destination path attributes would be
retrieved from the Shortest Path Tree (SPT) rooted at the PQ.
6.2.4.2. Link Coloring
Link coloring is a powerful system to control the choice of
alternates. Link colors are markers that will allow the encoding of
properties of a particular link. Protecting interfaces are tagged
with colors. Protected interfaces are configured to include some
colors with a preference level and exclude others.
Link color information SHOULD be signaled in the IGP, and
administrative-group IGP extensions [RFC5305] [RFC3630] that are
already standardized, implemented, and widely used SHOULD be used for
encoding and signaling link colors.
PE2
| +---- P4
| /
PE1 ---- P1 --------- P2
| 10 Gbps
1 Gbps |
|
P3
Figure 5
In the example in Figure 5, the P1 router is connected to three P
routers and two PEs. P1 is configured to protect the P1-P4 link. We
assume that, given the topology, all neighbors are candidate LFAs.
We would like to enforce a policy in the network where only a core
router may protect against the failure of a core link and where
high-capacity links are preferred.
In this example, we can use the proposed link coloring by:
o Marking the PE links with the color RED.
o Marking the 10 Gbps core link with the color BLUE.
o Marking the 1 Gbps core link with the color YELLOW.
o Configuring the protected interface P1->P4 as follows:
* Include BLUE, preference 200.
* Include YELLOW, preference 100.
* Exclude RED.
Using this, PE links will never be used to protect against P1-P4 link
failure, and the 10 Gbps link will be preferred.
The main advantage of this solution is that it can easily be
duplicated on other interfaces and other nodes without change. A
service provider has only to define the color system (associate a
color with a level of significance), as it is done already for TE
affinities or BGP communities.
An implementation of link coloring:
o SHOULD support multiple "include" and "exclude" colors on a single
protected interface.
o SHOULD provide a level of preference between included colors.
o SHOULD support the configuration of multiple colors on a single
protecting interface.
6.2.4.3. Bandwidth
As mentioned in previous sections, not taking into account the
bandwidth of an alternate could lead to congestion during FRR
activation. We propose that the bandwidth criteria be based on the
link speed information, for the following reasons:
o If a router S has a set of X destinations primarily forwarded to
N, using per-prefix LFAs may lead to having a subset of X
protected by a neighbor N1, another subset by N2, another subset
by Nx, etc.
o S is not aware of traffic flows to each destination, so in the
case of FRR activation, S is not able to evaluate how much traffic
will be sent to N1, N2, Nx, etc.
Based on this, it is not useful to gather available bandwidth on
alternate paths, as the router does not know how much bandwidth it
requires for protection. The proposed link speed approach provides a
good approximation at low cost, as information is easily available.
The bandwidth criteria of the policy framework SHOULD work in at
least the following two ways:
o Prune: Exclude an LFA if the link speed to reach it is lower than
the link speed of the primary next-hop interface.
o Prefer: Prefer an LFA based on its bandwidth to reach it compared
to the link speed of the primary next-hop interface.
6.2.4.4. Alternate Preference / Node Coloring
Rather than tagging interfaces on each node (using link colors) to
identify the types of alternate nodes (as an example), it would be
helpful if routers could be identified in the IGP. This would allow
grouped processing on multiple nodes. As an implementation needs to
exclude some specific alternates (see Section 6.2.3), an
implementation SHOULD be able to:
o give preference to a specific alternate.
o give preference to a group of alternates.
o exclude a specific alternate.
o exclude a group of alternates.
A specific alternate may be identified by its interface, IP address,
or router ID, and a group of alternates may be identified by a marker
(tag) advertised in IGP. The IGP encoding and signaling for marking
groups of alternates SHOULD be done according to [RFC7917] and
[RFC7777]. Using a tag/marker is referred to as "node coloring", as
compared to the link coloring option presented in Section 6.2.4.2.
Consider the following network:
PE3
|
|
PE2
| +---- P4
| /
PE1 ---- P1 -------- P2
| 10 Gbps
1 Gbps |
|
P3
Figure 6
In the example above, each node is configured with a specific tag
flooded through the IGP.
o PE1,PE3: 200 (non-candidate).
o PE2: 100 (edge/core).
o P1,P2,P3: 50 (core).
A simple policy could be configured on P1 to choose the best
alternate for P1->P4 based on the function or role of the router,
as follows:
o criterion 1 -> alternate preference: exclude tags 100 and 200.
o criterion 2 -> bandwidth.
6.2.5. Retrieving Alternate Path Attributes
6.2.5.1. Alternate Path
The alternate path is composed of two distinct parts: PLR to
alternate and alternate to destination.
N1 -- R1 ---- R2
/50 \ \
/ R3 --- R4
/ \
S -------- E ------- D
\\ //
\\ //
N2 ---- PQ ---- R5
Figure 7
In Figure 7, we consider a primary path from S to D, with S using E
as the primary next hop. All metrics are 1, except that {S,N1} = 50.
Two alternate paths are available:
o {S,N1,R1,R2|R3,R4,D}, where N1 is a connected alternate. This
consists of two sub-paths:
* {S,N1}: path from the PLR to the alternate.
* {N1,R1,R2|R3,R4,D}: path from the alternate to the destination.
o {S,N2,PQ,R5,D}, where the PQ is a remote alternate. Again, the
path consists of two sub-paths:
* {S,N2,PQ}: path from the PLR to the alternate.
* {PQ,R5,D}: path from the alternate to the destination.
As displayed in Figure 7, some parts of the alternate path may fan
out to multiple paths due to ECMP.
6.2.5.2. Alternate Path Attributes
Some criteria listed in the previous sections require the retrieval
of some characteristics of the alternate path (SRLG, bandwidth,
color, tag, etc.). We call these characteristics "path attributes".
A path attribute can record a list of node properties (e.g., node
tag) or link properties (e.g., link color).
This document defines two types of path attributes:
o Cumulative attribute: When a path attribute is cumulative, the
implementation SHOULD record the value of the attribute on each
element (link and node) along the alternate path. SRLG, link
color, and node color are cumulative attributes.
o Unitary attribute: When a path attribute is unitary, the
implementation SHOULD record the value of the attribute only on
the first element along the alternate path (first node, or first
link). Bandwidth is a unitary attribute.
N1 -- R1 ---- R2
/ \
/ 50 R4
/ \
S -------- E ------- D
Figure 8
In Figure 8, N1 is a connected alternate to reach D from S. We
consider that all links have a RED color except {R1,R2}, which is
BLUE. We consider all links to be 10 Gbps except {N1,R1}, which is
2.5 Gbps. The bandwidth attribute collected for the alternate path
will be 10 Gbps. As the attribute is unitary, only the link speed of
the first link {S,N1} is recorded. The link color attribute
collected for the alternate path will be {RED,RED,BLUE,RED,RED}. As
the attribute is cumulative, the value of the attribute on each link
along the path is recorded.
6.2.5.3. Connected Alternate
For an alternate path using a connected alternate:
o Attributes from the PLR to the alternate are retrieved from the
interface connected to the alternate. If the alternate is
connected through multiple interfaces, the evaluation of
attributes SHOULD be done once per interface (each interface is
considered as a separate alternate) and once per ECMP group of
interfaces (Layer 3 bundle).
o Path attributes from the alternate to the destination are
retrieved from the SPT rooted at the alternate. As the alternate
is a connected alternate, the SPT has already been computed to
find the alternate, so there is no need for additional
computation.
N1 -- R1 ---- R2
50//50 \
// \
i1//i2 \
S -------- E -------- D
Figure 9
In Figure 9, we consider a primary path from S to D, with S using E
as the primary next hop. All metrics are considered as 1 except
{S,N1} links, which are using a metric of 50. We consider the
following SRLGs on links:
o {S,N1} using i1: SRLG1,SRLG10.
o {S,N1} using i2: SRLG2,SRLG20.
o {N1,R1}: SRLG3.
o {R1,R2}: SRLG4.
o {R2,D}: SRLG5.
o {S,E}: SRLG10.
o {E,D}: SRLG6.
S is connected to the alternate using two interfaces: i1 and i2.
If i1 and i2 are not part of an ECMP group, the evaluation of
attributes is done once per interface, and each interface is
considered as a separate alternate path. Two alternate paths will be
available with the associated SRLG attributes:
o Alternate path #1: {S,N1 using if1,R1,R2,D}:
SRLG1,SRLG10,SRLG3,SRLG4,SRLG5.
o Alternate path #2: {S,N1 using if2,R1,R2,D}:
SRLG2,SRLG20,SRLG3,SRLG4,SRLG5.
Alternate path #1 is sharing risks with the primary path and may be
pruned, or its preference may be revoked, per user-defined policy.
If i1 and i2 are part of an ECMP group, the evaluation of attributes
is done once per ECMP group, and the implementation considers a
single alternate path {S,N1 using if1|if2,R1,R2,D} with the following
SRLG attributes: SRLG1,SRLG10,SRLG2,SRLG20,SRLG3,SRLG4,SRLG5. The
alternate path is sharing risks with the primary path and may be
pruned, or its preference may be revoked, per user-defined policy.
6.2.5.4. Remote Alternate
For alternate path using a remote alternate (tunnel):
o Attributes on the path from the PLR to the alternate are retrieved
using the PLR's primary SPT (when using a PQ node from the
P-space) or the immediate neighbor's SPT (when using a PQ from the
extended P-space). These are then combined with the attributes of
the link(s) to reach the immediate neighbor. In both cases, no
additional SPT is required.
o Attributes from the remote alternate to the destination path may
be retrieved from the SPT rooted at the remote alternate. An
additional forward SPT is required for each remote alternate
(PQ node), as indicated in Section 2.3.2 of [REMOTE-LFA-NODE]. In
some remote-alternate scenarios, like [TI-LFA], alternate-to-
destination path attributes may be obtained using a different
technique.
The number of remote alternates may be very high. In the case of
remote LFAs, simulations of real-world network topologies have shown
that as many as hundreds of PQs are possible. The computational
overhead of collecting all path attributes of all such PQs to
destination paths could grow beyond reasonable levels.
To handle this situation, implementations need to limit the number of
remote alternates to be evaluated to a finite number before
collecting alternate path attributes and running the policy
evaluation. Section 2.3.3 of [REMOTE-LFA-NODE] provides a way to
reduce the number of PQs to be evaluated.
Some other remote alternate techniques using static or dynamic
tunnels may not require this pruning.
Link Remote Remote
alternate alternate alternate
------------- ------------------ -------------
Alternates | LFA | | rLFA (PQs) | | Static/ |
| | | | | Dynamic |
sources | | | | | tunnels |
------------- ------------------ -------------
| | |
| | |
| -------------------------- |
| | Prune some alternates | |
| | (sorting strategy) | |
| -------------------------- |
| | |
| | |
------------------------------------------------
| Collect alternate attributes |
------------------------------------------------
|
|
-------------------------
| Evaluate policy |
-------------------------
|
|
Best alternates
Figure 10
6.2.5.5. Collecting Attributes in the Case of Multiple Paths
As described in Section 6.2.5, there may be some situations where an
alternate path or part of an alternate path fans out to multiple
paths (e.g., ECMP). When collecting path attributes in such a case,
an implementation SHOULD consider the union of attributes of each
sub-path.
In Figure 7 (in Section 6.2.5.1), S has two alternate paths to
reach D. Each alternate path fans out to multiple paths due to ECMP.
Consider the following link color attributes: all links are RED
except {R1,R3}, which is BLUE. The user wants to use an alternate
path with only RED links. The first alternate path
{S,N1,R1,R2|R3,R4,D} does not fit the constraint, as {R1,R3} is BLUE.
The second alternate path {S,N2,PQ,R5,D} fits the constraint and will
be preferred, as it uses only RED links.
6.2.6. ECMP LFAs
10
PE2 - PE3
| |
50 | 5 | 50
P1----P2
\\ //
50 \\ // 50
PE1
Links between P1 and PE1 are L1 and L2.
Links between P2 and PE1 are L3 and L4.
Figure 11
In Figure 11, the primary path from PE1 to PE2 is through P1, using
ECMP on two parallel links -- L1 and L2. In the case of standard
ECMP behavior, if L1 is failing, the post-convergence next hop would
become L2 and ECMP would no longer be in use. If an LFA is
activated, as stated in Section 3.4 of [RFC5286], "alternate
next-hops may themselves also be primary next-hops, but need not be"
and "alternate next-hops should maximize the coverage of the failure
cases." In this scenario, there is no alternate providing node
protection, so PE1 will prefer L2 as the alternate to protect L1;
this makes sense compared to post-convergence behavior.
Consider a different scenario, again referring to Figure 11, where L1
and L2 are configured as a Layer 3 bundle using a local feature and
L3/L4 comprise a second Layer 3 bundle. Layer 3 bundles are
configured as if a link in the bundle is failing; the traffic must be
rerouted out of the bundle. Layer 3 bundles are generally introduced
to increase bandwidth between nodes. In a nominal situation, ECMP is
still available from PE1 to PE2, but if L1 is failing, the
post-convergence next hop would become the ECMP on L3 and L4. In
this case, LFA behavior SHOULD be adapted in order to reflect the
bandwidth requirement.
We would expect the following FIB entry on PE1:
On PE1: PE2 +--> ECMP -> L1
| |
| +----> L2
|
+--> LFA (ECMP) -> L3
|
+----------> L4
Figure 12
If L1 or L2 is failing, traffic must be switched on the LFA ECMP
bundle rather than using the other primary next hop.
As mentioned in Section 3.4 of [RFC5286], protecting a link within an
ECMP by another primary next hop is not a MUST. Moreover, as already
discussed in this document, maximizing coverage against the failure
cases may not be the right approach, and a policy-based choice of an
alternate may be preferred.
An implementation SHOULD allow setting a preference to protect a
primary next hop with another primary next hop. An implementation
SHOULD also allow setting a preference to protect a primary next hop
with a NON-primary next hop. An implementation SHOULD allow the use
of an ECMP bundle as an LFA.
7. Operational Aspects
7.1. No-Transit Condition on LFA Computing Node
In Section 3.5 of [RFC5286], the setting of the no-transit condition
(through the IS-IS overload bit or the OSPF R-bit) in an LFA
computation is only taken into account for the case where a neighbor
has the no-transit condition set.
In addition to Inequality 1 (Loop-Free Criterion)
(Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D))
[RFC5286], the IS-IS overload bit or the OSPF R-bit of the LFA
calculating neighbor (S) SHOULD be taken into account. Indeed, if it
has the IS-IS overload bit set or the OSPF R-bit clear, no neighbor
will loop traffic back to itself.
An OSPF router acting as a stub router [RFC6987] SHOULD behave as if
the R-bit was clear regarding the LFA computation.
7.2. Manual Triggering of FRR
Service providers often perform manual link shutdown (using a
router's command-line interface (CLI)) to perform network
changes/tests. A manual link shutdown may be done at multiple
levels: physical interface, logical interface, IGP interface,
Bidirectional Forwarding Detection (BFD) session, etc. In
particular, testing or troubleshooting FRR requires that manual
shutdown be performed on the remote end of the link, as a local
shutdown would not generally trigger FRR.
To permit such a situation, an implementation SHOULD support
triggering/activating LFA FRR for a given link when a manual shutdown
is done on a component that currently supports FRR activation.
An implementation MAY also support FRR activation for a specific
interface or a specific prefix on a primary next-hop interface and
revert without any action on any running component of the node (links
or protocols). In this use case, the FRR activation time needs to be
controlled by a timer in case the operator forgot to revert the
traffic to the primary path. When the timer expires, the traffic is
automatically reverted to the primary path. This will simplify the
testing of the FRR path; traffic can then be reverted back to the
primary path without causing a global network convergence.
For example:
o If an implementation supports FRR activation upon a BFD
session-down event, that implementation SHOULD support FRR
activation when a manual shutdown is done on the BFD session. But
if an implementation does not support FRR activation upon a BFD
session-down event, there is no need for that implementation to
support FRR activation upon manual shutdown of a BFD session.
o If an implementation supports FRR activation upon a physical
link-down event (e.g., Rx laser "off" detection, error threshold
raised), that implementation SHOULD support FRR activation when a
manual shutdown of a physical interface is done. But if an
implementation does not support FRR activation upon a physical
link-down event, there is no need for that implementation to
support FRR activation upon manual shutdown of a physical link.
o A CLI command may allow switching from the primary path to the FRR
path to test the FRR path for a specific interface or prefix.
There is no impact on the control plane; only the data plane of
the local node may be changed. A similar command may allow
switching traffic back from the FRR path to the primary path.
7.3. Required Local Information
The introduction of LFAs in a network requires some enhancements to
standard routing information provided by implementations. Moreover,
due to "non-100%" coverage, coverage information is also required.
Hence, an implementation:
o MUST be able to display, for every prefix, the primary next hop as
well as the alternate next-hop information.
o MUST provide coverage information per LFA activation domain (area,
level, topology, instance, virtual router, address family, etc.).
o MUST provide the number of protected prefixes as well as
non-protected prefixes globally.
o SHOULD provide the number of protected prefixes as well as
non-protected prefixes per link.
o MAY provide the number of protected prefixes as well as
non-protected prefixes per priority if the implementation supports
prefix-priority insertion in the RIB/FIB.
o SHOULD provide a reason for choosing an alternate (policy and
criteria) and for excluding an alternate.
o SHOULD provide the list of non-protected prefixes and the reason
why they are not protected (e.g., no protection required, no
alternate available).
7.4. Coverage Monitoring
It is pretty easy to evaluate the coverage of a network in a nominal
situation, but topology changes may change the level of coverage. In
some situations, the network may no longer be able to provide the
required level of protection. Hence, it becomes very important for
service providers to receive alerts regarding changes in coverage.
An implementation SHOULD:
o provide an alert system if total coverage (for a node) is below a
defined threshold or when coverage returns to normal.
o provide an alert system if coverage for a specific link is below a
defined threshold or when coverage returns to normal.
An implementation MAY:
o trigger an alert if a specific destination is not protected
anymore or when protection comes back up for this destination.
Although the procedures for providing alerts are beyond the scope of
this document, we recommend that implementations consider standard
and well-used mechanisms like syslog or SNMP traps.
7.5. LFAs and Network Planning
The operator may choose to run simulations in order to ensure a
certain type of full coverage for the whole network or a given subset
of the network. This is particularly likely if he operates the
network in the sense of the third backbone profile described in
Section 4 of [RFC6571]; that is, he seeks to design and engineer the
network topology in such a way that a certain level of coverage is
always achieved. Obviously, a complete and exact simulation of the
IP FRR coverage can only be achieved if the behavior is deterministic
and the algorithm used is available to the simulation tool. Thus, an
implementation SHOULD:
o Behave deterministically in its LFA selection process. That is,
in the same topology and with the same policy configuration, the
implementation MUST always choose the same alternate for a given
prefix.
o Document its behavior. The implementation SHOULD provide enough
documentation regarding its behavior to allow an implementer of a
simulation tool to foresee the exact choice of the LFA
implementation for every prefix in a given topology. This SHOULD
take into account all possible policy configuration options. One
possible way to document this behavior is to disclose the
algorithm used to choose alternates.
8. Security Considerations
The policy mechanism introduced in this document allows the tuning of
the selection of the alternate. This is not seen as a security
threat, because:
o all candidates are already eligible as per [RFC5286] and
considered usable.
o the policy is based on information from the router's own
configuration and from the IGP, both of which are considered
trusted.
Hence, this document does not introduce any new security
considerations as compared to [RFC5286].
As noted above, the policy mechanism introduced in this document
allows the tuning of the selection of the best alternate but does not
change the list of alternates that are eligible. As described in
Section 7 of [RFC5286], this best alternate "can be used anyway when
a different topological change occurs, and hence this can't be viewed
as a new security threat."
9. References
9.1. Normative References
[ISO10589]
International Organization for Standardization,
"Intermediate System to Intermediate System intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)",
ISO Standard 10589, 2002.
[RFC2119] 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>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<http://www.rfc-editor.org/info/rfc3630>.
[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<http://www.rfc-editor.org/info/rfc4203>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<http://www.rfc-editor.org/info/rfc5286>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <http://www.rfc-editor.org/info/rfc5305>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<http://www.rfc-editor.org/info/rfc5307>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<http://www.rfc-editor.org/info/rfc5340>.
[RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<http://www.rfc-editor.org/info/rfc6571>.
[RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC6987, September 2013,
<http://www.rfc-editor.org/info/rfc6987>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<http://www.rfc-editor.org/info/rfc7490>.
[RFC7777] Hegde, S., Shakir, R., Smirnov, A., Li, Z., and B.
Decraene, "Advertising Node Administrative Tags in OSPF",
RFC 7777, DOI 10.17487/RFC7777, March 2016,
<http://www.rfc-editor.org/info/rfc7777>.
[RFC7917] Sarkar, P., Ed., Gredler, H., Hegde, S., Litkowski, S.,
and B. Decraene, "Advertising Node Administrative Tags in
IS-IS", RFC 7917, DOI 10.17487/RFC7917, June 2016,
<http://www.rfc-editor.org/info/rfc7917>.
9.2. Informative References
[REMOTE-LFA-NODE]
Sarkar, P., Ed., Hegde, S., Bowers, C., Gredler, H., and
S. Litkowski, "Remote-LFA Node Protection and
Manageability", Work in Progress, draft-ietf-rtgwg-rlfa-
node-protection-05, December 2015.
[SEG-RTG-ARCH]
Filsfils, C., Ed., Previdi, S., Ed., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", Work in Progress, draft-ietf-spring-
segment-routing-08, May 2016.
[TI-LFA] Francois, P., Filsfils, C., Bashandy, A., Decraene, B.,
and S. Litkowski, "Topology Independent Fast Reroute using
Segment Routing", Work in Progress, draft-francois-
segment-routing-ti-lfa-00, November 2013.
Contributors
Significant contributions were made by Pierre Francois, Hannes
Gredler, Chris Bowers, Jeff Tantsura, Uma Chunduri, Acee Lindem, and
Mustapha Aissaoui, whom the authors would like to acknowledge.
Authors' Addresses
Stephane Litkowski (editor)
Orange
Email: stephane.litkowski@orange.com
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
Clarence Filsfils
Cisco Systems
Email: cfilsfil@cisco.com
Kamran Raza
Cisco Systems
Email: skraza@cisco.com
Martin Horneffer
Deutsche Telekom
Email: Martin.Horneffer@telekom.de
Pushpasis Sarkar
Individual Contributor
Email: pushpasis.ietf@gmail.com