Internet Engineering Task Force (IETF) N. Shen
Request for Comments: 8500 Cisco Systems
Category: Standards Track S. Amante
ISSN: 2070-1721 Apple Inc.
M. Abrahamsson
T-Systems Nordic
February 2019
IS-IS Routing with Reverse Metric
Abstract
This document describes a mechanism to allow IS-IS routing to quickly
and accurately shift traffic away from either a point-to-point or
multi-access LAN interface during network maintenance or other
operational events. This is accomplished by signaling adjacent IS-IS
neighbors with a higher reverse metric, i.e., the metric towards the
signaling IS-IS router.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8500.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Node and Link Isolation . . . . . . . . . . . . . . . . . 2
1.2. Distributed Forwarding Planes . . . . . . . . . . . . . . 3
1.3. Spine-Leaf Applications . . . . . . . . . . . . . . . . . 3
1.4. LDP IGP Synchronization . . . . . . . . . . . . . . . . . 3
1.5. IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . 3
1.6. Specification of Requirements . . . . . . . . . . . . . . 4
2. IS-IS Reverse Metric TLV . . . . . . . . . . . . . . . . . . 4
3. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 6
3.1. Processing Changes to Default Metric . . . . . . . . . . 6
3.2. Multi-Topology IS-IS Support on Point-to-Point Links . . 7
3.3. Multi-access LAN Procedures . . . . . . . . . . . . . . . 7
3.4. LDP/IGP Synchronization on LANs . . . . . . . . . . . . . 8
3.5. Operational Guidelines . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Node Isolation Challenges . . . . . . . . . . . . . 12 13
Appendix B. Link Isolation Challenges . . . . . . . . . . . . . 13
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14 15
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 14 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 15
1. Introduction
The IS-IS [ISO10589] routing protocol has been widely used in
Internet Service Provider IP/MPLS networks. Operational experience
with the protocol combined with ever increasing requirements for
lossless operations have demonstrated some operational issues. This
document describes the issues and a mechanism for mitigating them.
This document defines the IS-IS "Reverse Metric" mechanism that
allows an IS-IS node to send a Reverse Metric TLV through the IS-IS
Hello (IIH) PDU to the neighbor or pseudonode to adjust the routing
metric on the inbound direction.
1.1. Node and Link Isolation
The IS-IS routing mechanism has the overload bit, which can be used
by operators to perform disruptive maintenance on the router. But in
many operational maintenance cases, it is not necessary to divert all
the traffic away from this node. It is necessary to avoid only a
single link during the maintenance. More detailed descriptions of
the challenges can be found in Appendix Appendices A and Appendix B of this document.
1.2. Distributed Forwarding Planes
In a distributed forwarding platform, different forwarding line cards
may have interfaces and IS-IS connections to neighbor routers. If
one of the line card's software resets, it may take some time for the
forwarding entries to be fully populated on the line card, in
particular if the router is a PE (Provider Edge) router in an ISP's
MPLS VPN. An IS-IS adjacency may be established with a neighbor
router long before the entire BGP VPN prefixes are downloaded to the
forwarding table. It is important to signal to the adjacent IS-IS
routers to raise metric values and not to use the corresponding IS-IS
adjacency inbound to this router if possible. Temporarily signaling
the 'Reverse Metric' over this link to discourage the traffic via the
corresponding line card will help to reduce the traffic loss in the
network. In the meantime, the remote PE routers will select a
different set of PE routers for the BGP best path calculation or use
a different link towards the same PE router on which a line card is
resetting.
1.3. Spine-Leaf Applications
In the IS-IS Spine-Leaf extension [IS-IS-SL-EXT], the leaf nodes will
perform equal-cost or unequal-cost load sharing towards all the spine
nodes. In certain operational cases, for instance, when one of the
backbone links on a spine node is congested, a spine node can push a
higher metric towards the connected leaf nodes to reduce the transit
traffic through the corresponding spine node or link.
1.4. LDP IGP Synchronization
In [RFC5443], a mechanism is described to achieve LDP IGP
synchronization by using the maximum link metric value on the
interface. But in the case of a new IS-IS node joining the broadcast
network (LAN), it is not optimal to change all the nodes on the LAN
to the maximum link metric value, as described in [RFC6138]. In this
case, the Reverse Metric can be used to discourage both outbound and
inbound traffic without affecting the traffic of other IS-IS nodes on
the LAN.
1.5. IS-IS Reverse Metric
This document uses the routing protocol itself as the transport
mechanism to allow one IS-IS router to advertise a "reverse metric"
in an IS-IS Hello (IIH) PDU to an adjacent node on a point-to-point
or multi-access LAN link. This would allow the provisioning to be
performed only on a single node, setting a "reverse metric" on a link
and having traffic bidirectionally shift away from that link
gracefully to alternate viable paths.
This Reverse Metric mechanism is used for both point-to-point and
multi-access LAN links. Unlike the point-to-point links, the IS-IS
protocol currently does not have a way to influence the traffic
towards a particular node on LAN links. This mechanism provides
IS-IS routing with the capability of altering traffic in both
directions on either a point-to-point link or a multi-access link of
an IS-IS node.
The metric value in the Reverse Metric TLV and the Traffic
Engineering metric in the sub-TLV being advertised are offsets or
relative metrics to be added to the existing local link and Traffic
Engineering metric values of the receiver; the accumulated metric
value is bounded as described in Section 2.
1.6. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. IS-IS Reverse Metric TLV
The Reverse Metric TLV is a new TLV to be used inside an IS-IS Hello
PDU. This TLV is used to support the IS-IS Reverse Metric mechanism
that allows a "reverse metric" to be sent to the IS-IS neighbor.
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 | Length | Flags | Metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metric (Continued) | sub-TLV Len |Optional sub-TLV
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Reverse Metric TLV
The Value part of the Reverse Metric TLV is composed of a 3 octet
field containing an IS-IS Metric value, a 1 octet field of Flags, and
a 1 octet Reverse Metric sub-TLV length field representing the length
of a variable number of sub-TLVs. If the "sub-TLV Len" is non-zero,
then the Value field MUST also contain one or more sub-TLVs.
The Reverse Metric TLV MAY be present in any IS-IS Hello PDU. A
sender MUST only transmit a single Reverse Metric TLV in an IS-IS
Hello PDU. If a received IS-IS Hello PDU contains more than one
Reverse Metric TLV, an implementation MUST ignore all the Reverse
Metric TLVs.
TYPE: 16
LENGTH: variable (5 - 255 octets)
VALUE:
Flags (1 octet)
Metric (3 octets)
sub-TLV length (1 octet)
sub-TLV data (0 - 250 octets)
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |U|W|
+-+-+-+-+-+-+-+-+
Figure 2: Flags
The Metric field contains a 24-bit unsigned integer. This value is a
metric offset that a neighbor SHOULD add to the existing configured
Default Metric for the IS-IS link [ISO10589]. Refer to "Elements of
Procedure" in Section 3 of this document for details on how an IS-IS
router should process the Metric field in a Reverse Metric TLV.
The Metric field, in the Reverse Metric TLV, is a "reverse offset
metric" that will either be in the range of 0 - 63 when a "narrow"
IS-IS metric is used (IS Neighbors TLV / Pseudonode LSP) [RFC1195] or
in the range of 0 - (2^24 - 2) when a "wide" Traffic Engineering
metric value is used (Extended IS Reachability TLV) [RFC5305]
[RFC5817]. As described below, when the U bit is set, the
accumulated value of the wide metric is in the range of
0 - (2^24 - 1), with the (2^24 - 1) metric value as non-reachable in
IS-IS routing. The IS-IS metric value of (2^24 - 2) serves as the
link of last resort.
There are currently only two Flag bits defined.
W bit (0x01): The "Whole LAN" bit is only used in the context of
multi-access LANs. When a Reverse Metric TLV is transmitted from a
node to the Designated Intermediate System (DIS), if the "Whole LAN"
bit is set (1), then a DIS SHOULD add the received Metric value in
the Reverse Metric TLV to each node's existing Default Metric in the
Pseudonode LSP. If the "Whole LAN" bit is not set (0), then a DIS
SHOULD add the received Metric value in the Reverse Metric TLV to the
existing "default metric" in the Pseudonode LSP for the single node
from whom the Reverse Metric TLV was received. Please refer to
"Multi-access LAN Procedures", in Section 3.3, for additional
details. The W bit MUST be clear when a Reverse Metric TLV is
transmitted in an IIH PDU on a point-to-point link and MUST be
ignored when received on a point-to-point link.
U bit (0x02): The "Unreachable" bit specifies that the metric
calculated by the addition of the reverse metric to the "default
metric" is limited to the maximum value of (2^24-1). This "U" bit
applies to both the default metric in the Extended IS Reachability
TLV and the Traffic Engineering Default Metric sub-TLV of the link.
This is only relevant to the IS-IS "wide" metric mode.
The Reserved bits of Flags field MUST be set to zero and MUST be
ignored when received.
The Reverse Metric TLV MAY include sub-TLVs when an IS-IS router
wishes to signal additional information to its neighbor. In this
document, the Reverse Metric Traffic Engineering Metric sub-TLV, with
Type 18, is defined. This Traffic Engineering Metric contains a
24-bit unsigned integer. This sub-TLV is optional; if it appears
more than once, then the entire Reverse Metric TLV MUST be ignored.
Upon receiving this Traffic Engineering METRIC sub-TLV in a Reverse
Metric TLV, a node SHOULD add the received Traffic Engineering Metric
offset value to its existing configured Traffic Engineering Default
Metric within its Extended IS Reachability TLV. The use of other
sub-TLVs is outside the scope of this document. The "sub-TLV Len"
value MUST be set to zero when an IS-IS router does not have Traffic
Engineering sub-TLVs that it wishes to send to its IS-IS neighbor.
3. Elements of Procedure
3.1. Processing Changes to Default Metric
It is important to use the same IS-IS metric type on both ends of the
link and in the entire IS-IS area or level. On the receiving side of
the 'reverse-metric' TLV, the accumulated value of the configured
metric and the reverse-metric needs to be limited to 63 in "narrow"
metric mode and to (2^24 - 2) in "wide" metric mode. This applies to
both the Default Metric of Extended IS Reachability TLV and the
Traffic Engineering Default Metric sub-TLV in LSP or Pseudonode LSP
for the "wide" metric mode case. If the "U" bit is present in the
flags, the accumulated metric value is to be limited to (2^24 - 1)
for both the normal link metric and Traffic Engineering metric in
IS-IS "wide" metric mode.
If an IS-IS router is configured to originate a Traffic Engineering
Default Metric sub-TLV for a link but receives a Reverse Metric TLV
from its neighbor that does not contain a Traffic Engineering Default
Metric sub-TLV, then the IS-IS router MUST NOT change the value of
its Traffic Engineering Default Metric sub-TLV for that link.
3.2. Multi-Topology IS-IS Support on Point-to-Point Links
The Reverse Metric TLV is applicable to Multi-topology IS-IS (M-ISIS)
[RFC5120]. On point-to-point links, if an IS-IS router is configured
for M-ISIS, it MUST send only a single Reverse Metric TLV in IIH PDUs
toward its neighbor(s) on the designated link. When an M-ISIS router
receives a Reverse Metric TLV, it MUST add the received Metric value
to its Default Metric of the link in all Extended IS Reachability
TLVs for all topologies. If an M-ISIS router receives a Reverse
Metric TLV with a Traffic Engineering Default Metric sub-TLV, then
the M-ISIS router MUST add the received Traffic Engineering Default
Metric value to each of its Default Metric sub-TLVs in all of its MT
Intermediate Systems TLVs. If an M-ISIS router is configured to
advertise Traffic Engineering Default Metric sub-TLVs for one or more
topologies but does not receive a Traffic Engineering Default Metric
sub-TLV in a Reverse Metric TLV, then the M-ISIS router MUST NOT
change the value in each of the Traffic Engineering Default Metric
sub-TLVs for all topologies.
3.3. Multi-access LAN Procedures
On a Multi-access LAN, only the DIS SHOULD act upon information
contained in a received Reverse Metric TLV. All non-DIS nodes MUST
silently ignore a received Reverse Metric TLV. The decision process
of the routers on the LAN MUST follow the procedure in
Section 7.2.8.2 of [ISO10589], and use the "Two-way connectivity
check" during the topology and route calculation.
The Reverse Metric Traffic Engineering sub-TLV also applies to the
DIS. If a DIS is configured to apply Traffic Engineering over a link
and it receives Traffic Engineering Metric sub-TLV in a Reverse
Metric TLV, it should update the Traffic Engineering Default Metric
sub-TLV value of the corresponding Extended IS Reachability TLV or
insert a new one if not present.
In the case of multi-access LANs, the "W" Flags bit is used to signal
from a non-DIS to the DIS whether or not to change the metric and,
optionally, Traffic Engineering parameters for all nodes in the
Pseudonode LSP or solely the node on the LAN originating the Reverse
Metric TLV.
A non-DIS node, e.g., Router B, attached to a multi-access LAN will
send the DIS a Reverse Metric TLV with the W bit clear when Router B
wishes the DIS to add the Metric value to the Default Metric
contained in the Pseudonode LSP specific to just Router B. Other
non-DIS nodes, e.g., Routers C and D, may simultaneously send a
Reverse Metric TLV with the W bit clear to request the DIS to add
their own Metric value to their Default Metric contained in the
Pseudonode LSP.
As long as at least one IS-IS node on the LAN sending the signal to
DIS with the W bit set, the DIS would add the metric value in the
Reverse Metric TLV to all neighbor adjacencies in the Pseudonode LSP,
regardless if some of the nodes on the LAN advertise the Reverse
Metric TLV without the W bit set. The DIS MUST use the reverse
metric of the highest source MAC address Non-DIS advertising the
Reverse Metric TLV with the W bit set.
Local provisioning on the DIS to adjust the Default Metric(s) is
another way to insert Reverse Metric in the Pseudonode LSP towards an
IS-IS node on a LAN. In the case where a Reverse Metric TLV is also
used in the IS-IS Hello PDU of the node, the local provisioning MUST
take precedence over received Reverse Metric TLVs. For instance,
local policy on the DIS may be provisioned to ignore the W bit
signaling on a LAN.
Multi-topology IS-IS [RFC5120] specifies there is no change to
construction of the Pseudonode LSP regardless of the Multi-topology
(MT) capabilities of a multi-access LAN. If any MT capable node on
the LAN advertises the Reverse Metric TLV to the DIS, the DIS should
update, as appropriate, the Default Metric contained in the
Pseudonode LSP. If the DIS updates the Default Metric and floods a
new Pseudonode LSP, those default metric values will be applied to
all topologies during Multi-topology Shortest Path First
calculations.
3.4. LDP/IGP Synchronization on LANs
As described in [RFC6138], when a new IS-IS node joins a broadcast
network, it is unnecessary and sometimes even harmful for all IS-IS
nodes on the LAN to advertise the maximum link metric. [RFC6138]
proposes a solution to have the new node not advertise its adjacency
towards the pseudonode when it is not in a "cut-edge" position.
With the introduction of Reverse Metric in this document, a simpler
alternative solution to the above mentioned problem can be used. The
Reverse Metric allows the new node on the LAN to advertise its
inbound metric value to be the maximum, and this puts the link of
this new node in the last resort position without impacting the other
IS-IS nodes on the same LAN.
Specifically, when IS-IS adjacencies are being established by the new
node on the LAN, besides setting the maximum link metric value
(2^24 - 2) on the interface of the LAN for LDP IGP synchronization as
described in [RFC5443], it SHOULD advertise the maximum metric offset
value in the Reverse Metric TLV in its IIH PDU sent on the LAN. It
SHOULD continue this advertisement until it completes all the LDP
label binding exchanges with all the neighbors over this LAN, either
by receiving the LDP End-of-LIB [RFC5919] for all the sessions or by
exceeding the provisioned timeout value for the node LDP/IGP
synchronization.
3.5. Operational Guidelines
For the use case in Section 1.1, a router SHOULD limit the period of
advertising a Reverse Metric TLV towards a neighbor only for the
duration of a network maintenance window.
The use of a Reverse Metric does not alter IS-IS metric parameters
stored in a router's persistent provisioning database.
If routers that receive a Reverse Metric TLV send a syslog message or
SNMP trap, this will assist in rapidly identifying the node in the
network that is advertising an IS-IS metric or Traffic Engineering
parameters different from that which is configured locally on the
device.
When the link Traffic Engineering metric is raised to (2^24 - 1)
[RFC5817], either due to the Reverse Metric mechanism or by explicit
user configuration, this SHOULD immediately trigger the CSPF
(Constrained Shortest Path First) recalculation to move the Traffic
Engineering traffic away from that link. It is RECOMMENDED also that
the CSPF does the immediate CSPF recalculation when the Traffic
Engineering metric is raised to (2^24 - 2) to be the last resort
link.
It is advisable that implementations provide a configuration
capability to disable any IS-IS metric changes by a Reverse Metric
mechanism through neighbors' Hello PDUs.
If an implementation enables this mechanism by default, it is
RECOMMENDED that it be disabled by the operators when not explicitly
using it.
4. Security Considerations
Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
[RFC5310], and with various deployment and operational security
considerations in [RFC7645]. The enhancement in this document makes
it possible for one IS-IS router to manipulate the IS-IS Default
Metric and, optionally, Traffic Engineering parameters of adjacent
IS-IS neighbors on point-to-point or LAN interfaces. Although IS-IS
routers within a single Autonomous System nearly always are under the
control of a single administrative authority, it is highly
recommended that operators configure authentication of IS-IS PDUs to
mitigate use of the Reverse Metric TLV as a potential attack vector.
5. IANA Considerations
IANA has allocated IS-IS TLV Codepoint 16 for the Reverse Metric TLV.
This new TLV has the following attributes: IIH = y, LSP = n, SNP = n,
Purge = n.
This document also introduces a new registry for sub-TLVs of the
Reverse Metric TLV. The registration policy is Expert Review as
defined in [RFC8126]. This registry is part of the "IS-IS TLV
Codepoints" registry. The name of the registry is "Sub-TLVs for TLV
16 (Reverse Metric TLV)". The defined values are:
0: Reserved
1-17: Unassigned
18: Traffic Engineering Metric as specified in this document
(Section 2)
19-255: Unassigned
6. References
6.1. Normative References
[ISO10589]
ISO, "Information technology -- Telecommunications and
information exchange between systems -- 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/IEC 10589:2002, Second Edition,
November 2002.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
2009, <https://www.rfc-editor.org/info/rfc5443>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[IS-IS-SL-EXT]
Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
Routing for Spine-Leaf Topology", Work in Progress, draft-
ietf-lsr-isis-spine-leaf-ext-00,
draft-ietf-lsr-isis-spine-leaf-ext-00, December 2018.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5817] Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
"Graceful Shutdown in MPLS and Generalized MPLS Traffic
Engineering Networks", RFC 5817, DOI 10.17487/RFC5817,
April 2010, <https://www.rfc-editor.org/info/rfc5817>.
[RFC5919] Asati, R., Mohapatra, P., Chen, E., and B. Thomas,
"Signaling LDP Label Advertisement Completion", RFC 5919,
DOI 10.17487/RFC5919, August 2010,
<https://www.rfc-editor.org/info/rfc5919>.
[RFC6138] Kini, S., Ed. and W. Lu, Ed., "LDP IGP Synchronization for
Broadcast Networks", RFC 6138, DOI 10.17487/RFC6138,
February 2011, <https://www.rfc-editor.org/info/rfc6138>.
[RFC7645] Chunduri, U., Tian, A., and W. Lu, "The Keying and
Authentication for Routing Protocol (KARP) IS-IS Security
Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015,
<https://www.rfc-editor.org/info/rfc7645>.
Appendix A. Node Isolation Challenges
On rare occasions, it is necessary for an operator to perform
disruptive network maintenance on an entire IS-IS router node, i.e.,
major software upgrades, power/cooling augments, etc. In these
cases, an operator will set the IS-IS Overload Bit (OL bit) within
the Link State Protocol Data Units (LSPs) of the IS-IS router about
to undergo maintenance. The IS-IS router immediately floods its
updated LSPs to all IS-IS routers in the IS-IS domain. Upon receipt
of the updated LSPs, all IS-IS routers recalculate their Shortest
Path First (SPF) tree excluding IS-IS routers whose LSPs have the OL
bit set. This effectively removes the IS-IS router about to undergo
maintenance from the topology, thus preventing it from receiving any
transit traffic during the maintenance period.
After the maintenance activity has completed, the operator resets the
IS-IS Overload Bit within the LSPs of the original IS-IS router
causing it to flood updated IS-IS LSPs throughout the IS-IS domain.
All IS-IS routers recalculate their SPF tree and now include the
original IS-IS router in their topology calculations, allowing it to
be used for transit traffic again.
Isolating an entire IS-IS router from the topology can be especially
disruptive due to the displacement of a large volume of traffic
through an entire IS-IS router to other suboptimal paths (e.g., those
with significantly larger delay). Thus, in the majority of network
maintenance scenarios, where only a single link or LAN needs to be
augmented to increase its physical capacity, or is experiencing an
intermittent failure, it is much more common and desirable to
gracefully remove just the targeted link or LAN from service
temporarily, so that the least amount of user-data traffic is
affected during the link-specific network maintenance.
Appendix B. Link Isolation Challenges
Before network maintenance events are performed on individual
physical links or LANs, operators substantially increase the IS-IS
metric simultaneously on both devices attached to the same link or
LAN. In doing so, the devices generate new Link State Protocol Data
Units (LSPs) that are flooded throughout the network and cause all
routers to gradually shift traffic onto alternate paths with very
little or no disruption to in-flight communications by applications
or end users. When performed successfully, this allows the operator
to confidently perform disruptive augmentation, fault diagnosis, or
repairs on a link without disturbing ongoing communications in the
network.
There are a number of challenges with the above solution. First, it
is quite common to have routers with several hundred interfaces and
individual interfaces that move anywhere from several hundred
gigabits/second to terabits/second of traffic. Thus, it is
imperative that operators accurately identify the same point-to-point
link on two separate devices in order to increase (and afterward
decrease) the IS-IS metric appropriately. Second, the aforementioned
solution is very time-consuming and even more error-prone to perform
when it's necessary to temporarily remove a multi-access LAN from the
network topology. Specifically, the operator needs to configure ALL
devices that have interfaces attached to the multi-access LAN with an
appropriately high IS-IS metric (and then decrease the IS-IS metric
to its original value afterward). Finally, with respect to multi-
access LANs, there is currently no method to bidirectionally isolate
only a single node's interface on the LAN when performing more fine-
grained diagnoses and repairs to the multi-access LAN.
In theory, use of a Network Management System (NMS) could improve the
accuracy of identifying the appropriate subset of routers attached to
either a point-to-point link or a multi-access LAN. It could also
signal to those devices, using a network management protocol, to
adjust the IS-IS metrics on the pertinent set of interfaces. The
reality is that NMSs are, to a very large extent, not used within
Service Provider's networks for a variety of reasons. In particular,
NMSs do not interoperate very well across different vendors or even
separate platform families within the same vendor.
Acknowledgments
The authors would like to thank Mike Shand, Dave Katz, Guan Deng,
Ilya Varlashkin, Jay Chen, Les Ginsberg, Peter Ashwood-Smith, Uma
Chunduri, Alexander Okonnikov, Jonathan Harrison, Dave Ward, Himanshu
Shah, Wes George, Danny McPherson, Ed Crabbe, Russ White, Robert
Raszuk, Tom Petch, Stewart Bryant, and Acee Lindem for their comments
and contributions.
Contributors
Tony Li
Email: tony.li@tony.li
Authors' Addresses
Naiming Shen
Cisco Systems
560 McCarthy Blvd.
Milpitas, CA 95035
United States of America
Email: naiming@cisco.com
Shane Amante
Apple Inc.
One Apple Park Way
Cupertino, CA 95014
United States of America
Email: amante@apple.com
Mikael Abrahamsson
T-Systems Nordic
Kistagangen 26
Stockholm
Sweden
Email: Mikael.Abrahamsson@t-systems.se