RFC 7775 PROPOSED STANDARD IS-IS · ROUTING · ROUTE PREFERENCE
IS-IS Route Preference for Extended IP and IPv6 Reachability
A clear ranking of IS-IS routes learned from IPv4 and IPv6 extended reachability, and a fix for how the up/down bit is treated in Level 2.
RFC EDITOR
Ginsberg, L., Litkowski, S., and S. Previdi, "IS-IS Route Preference for Extended IP and IPv6 Reachability", RFC 7775, DOI 10.17487/RFC7775, February 2016, <https://www.rfc-editor.org/info/rfc7775>.
BIBTEX
@misc{rfc7775,
series = {Request for Comments},
number = 7775,
howpublished = {RFC 7775},
publisher = {RFC Editor},
doi = {10.17487/RFC7775},
url = {https://www.rfc-editor.org/info/rfc7775},
author = {L. Ginsberg and S. Litkowski and S. Previdi},
title = {{IS-IS Route Preference for Extended IP and IPv6 Reachability}},
pagetotal = 11,
year = 2016,
month = feb,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 7775 Cisco Systems
Updates: 5308 S. Litkowski
Category: Standards Track Orange Business Service
ISSN: 2070-1721 S. Previdi
Cisco Systems
February 2016
IS-IS Route Preference for Extended IP and IPv6 Reachability
Abstract
In existing specifications, the route preferences for IPv4/IPv6
Extended Reachability TLVs are not explicitly stated. There are also
inconsistencies in the definition of how the up/down bit applies to
route preference when the prefix advertisement appears in Level 2
Link State Protocol Data Units (LSPs). This document addresses these
issues.
This document updates RFC 5308.
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/rfc7775.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use of the Up/Down Bit in Level 2 LSPs . . . . . . . . . . . 3
3. Types of Routes in IS-IS Supported by Extended Reachability
TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Types of Routes Supported by TLVs 135 and 235 . . . . . . 4
3.2. Types of Routes Supported by TLVs 236 and 237 . . . . . . 6
3.3. Order of Preference for All Types of Routes Supported by
TLVs 135 and 235 . . . . . . . . . . . . . . . . . . . . 7
3.4. Order of Preference for All Types of Routes Supported by
TLVs 236 and 237 . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Example Interoperability Issue . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[RFC5302] defines the route preference rules as they apply to TLVs
128 and 130. [RFC5305] introduced the IP Extended Reachability TLV
135 but did not explicitly adapt the route preference rules defined
in [RFC5302] for the new TLV. [RFC5308] defines the IPv6
Reachability TLV 236 and does include an explicit statement regarding
route preference -- but the statement introduces use of the up/down
bit in advertisements that appear in Level 2 LSPs, which is
inconsistent with statements made in [RFC5302] and [RFC5305]. This
document defines explicit route preference rules for TLV 135, revises
the route preference rules for TLV 236, and clarifies the usage of
the up/down bit when it appears in TLVs in Level 2 LSPs. This
document is a clarification (NOT a correction) of [RFC5302] and
[RFC5305]; it is a correction of the route preference rules defined
in [RFC5308] to be consistent with the rules for IPv4. It also makes
explicit that the same rules apply to the Multi-Topology (MT)
equivalent TLVs 235 and 237.
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 RFC 2119 [RFC2119].
2. Use of the Up/Down Bit in Level 2 LSPs
The up/down bit was introduced in support of leaking prefixes
downwards in the IS-IS level hierarchy. Routes that are leaked
downwards have the bit set to 1. Such prefixes MUST NOT be leaked
upwards in the hierarchy. So long as we confine ourselves to a
single IS-IS instance and the current number of supported levels
(two), it is impossible to have a prefix advertised in a Level 2 LSP
and have the up/down bit set to 1. However, because [RFC5302]
anticipated a future extension to IS-IS that might support additional
levels, it allowed for the possibility that the up/down bit might be
set in a Level 2 LSP and supported easy migration in the event such
an extension was introduced. Section 3.3 of [RFC5302] states:
...it is RECOMMENDED that implementations ignore the up/down bit
in L2 LSPs, and accept the prefixes in L2 LSPs regardless of
whether the up/down bit is set.
[RFC5305] addressed an additional case wherein an implementation
included support for multiple virtual routers running IS-IS in
different areas. In such a case, it is possible to redistribute
prefixes between two IS-IS instances in the same manner that prefixes
are redistributed from other protocols into IS-IS. This introduced
the possibility that a prefix could be redistributed from Level 1 to
Level 1 (as well as between Level 2 and Level 2), and in the event
the redistributed route was leaked from Level 1 to Level 2, two
different routers in different areas would be advertising the same
prefix into the Level 2 sub-domain. To prevent this, Section 4.1 of
[RFC5305] specifies:
If a prefix is advertised from one area to another at the same
level, then the up/down bit SHALL be set to 1.
However, the statement in [RFC5302] that the up/down bit is ignored
in Level 2 LSPs is not altered by [RFC5305].
The conclusion then is that there is no "L2 inter-area route";
indeed, no such route type is defined by [RFC5302]. However,
[RFC5308] ignored this fact and introduced such a route type in
Section 5 when it specified a preference for "Level 2 down prefix".
This is an error that this document corrects. As changing the use of
the up/down bit in TLVs 236 and 237 may introduce interoperability
issues, implementors may wish to support transition mechanisms from
the behavior described in [RFC5308] to the behavior described in this
document.
3. Types of Routes in IS-IS Supported by Extended Reachability TLVs
[RFC5302] is the authoritative reference for the types of routes
supported by TLVs 128 and 130. However, a number of attributes
supported by those TLVs are NOT supported by TLVs 135, 235, 236, and
237. Distinction between internal/external metrics is not supported.
In the case of IPv4 TLVs (135 and 235), the distinction between
internal and external route types is not supported. However, the
Prefix Attribute Flags sub-TLV defined in [PFXATTR] reintroduces the
distinction between internal and external route types. The
definitions below include references to the relevant attribute bits
from [PFXATTR].
3.1. Types of Routes Supported by TLVs 135 and 235
This section defines the types of route supported for IPv4 when using
TLV 135 [RFC5305] and/or TLV 235 [RFC5120]. The text follows as
closely as possible the original text from [RFC5302].
L1 intra-area routes: These are advertised in L1 LSPs, in TLV 135 or
TLV 235. The up/down bit is set to 0. These IP prefixes are
directly connected to the advertising router. If the Prefix
Attribute Flags sub-TLV is included, both the X-Flag and the
R-Flag are set to 0.
L1 external routes: These are advertised in L1 LSPs, in TLV 135 or
TLV 235. The up/down bit is set to 0. These IP prefixes are
learned from other protocols and are usually not directly
connected to the advertising router. If the Prefix Attribute
Flags sub-TLV is included, the X-Flag is set to 1, and the R-Flag
is set to 0.
L2 intra-area routes: These are advertised in L2 LSPs, in TLV 135 or
TLV 235. The up/down bit is set to 0. These IP prefixes are
directly connected to the advertising router. If the Prefix
Attribute Flags sub-TLV is included, both the X-Flag and the
R-Flag are set to 0.
L1->L2 inter-area routes: These are advertised in L2 LSPs, in TLV
135 or TLV 235. The up/down bit is set to 0. These IP prefixes
are learned via L1 routing and were derived during the L1 Shortest
Path First (SPF) computation from prefixes advertised in L1 LSPs
in TLV 135 or TLV 235. If the Prefix Attribute Flags sub-TLV is
included, the R-Flag is set to 1.
L2->L2 inter-area routes: These are advertised in L2 LSPs, in TLV
135 or TLV 235. The up/down bit is set to 1 but is ignored and
treated as if it were set to 0. These IP prefixes are learned
from another IS-IS instance usually operating in another area. If
the Prefix Attribute Flags sub-TLV is included, the X-Flag is set
to 1, and the R-Flag is set to 0.
L2 external routes: These are advertised in L2 LSPs, in TLV 135 or
TLV 235. The up/down bit is set to 0. These IP prefixes are
learned from other protocols and are usually not directly
connected to the advertising router. If the Prefix Attribute
Flags sub-TLV is included, the X-Flag is set to 1, and the R-Flag
is set to 0.
L2->L1 inter-area routes: These are advertised in L1 LSPs, in TLV
135 or TLV 235. The up/down bit is set to 1. These IP prefixes
are learned via L2 routing and were derived during the L2 SPF
computation from prefixes advertised in TLV 135 or TLV 235. If
the Prefix Attribute Flags sub-TLV is included, the R-Flag is set
to 1.
L1->L1 inter-area routes: These are advertised in L1 LSPs, in TLV
135 or TLV 235. The up/down bit is set to 1. These IP prefixes
are learned from another IS-IS instance usually operating in
another area. If the Prefix Attribute Flags sub-TLV is included,
the X-Flag is set to 1, and the R-Flag is set to 0.
3.2. Types of Routes Supported by TLVs 236 and 237
This section defines the types of route supported for IPv6 when using
TLV 236 [RFC5308] and/or TLV 237 [RFC5120].
L1 intra-area routes: These are advertised in L1 LSPs, in TLV 236 or
TLV 237. The up/down bit is set to 0. The external bit is set to
0. These IPv6 prefixes are directly connected to the advertising
router. If the Prefix Attribute Flags sub-TLV is included, the
R-Flag is set to 0.
L1 external routes: These are advertised in L1 LSPs, in TLV 236 or
TLV 237. The up/down bit is set to 0. The external bit is set to
1. These IPv6 prefixes are learned from other protocols and are
usually not directly connected to the advertising router. If the
Prefix Attribute Flags sub-TLV is included, the R-Flag is set to
0.
L2 intra-area routes: These are advertised in L2 LSPs, in TLV 236 or
TLV 237. The up/down bit is set to 0. The external bit is set to
0. These IPv6 prefixes are directly connected to the advertising
router. If the Prefix Attribute Flags sub-TLV is included, the
R-Flag is set to 0.
L1->L2 inter-area routes: These are advertised in L2 LSPs, in TLV
236 or TLV 237. The up/down bit is set to 0. The external bit is
set to 0. These IPv6 prefixes are learned via L1 routing and were
derived during the L1 Shortest Path First (SPF) computation from
prefixes advertised in L1 LSPs in TLV 236 or TLV 237. If the
Prefix Attribute Flags sub-TLV is included, the R-Flag is set to
1.
L2 external routes: These are advertised in L2 LSPs, in TLV 236 or
TLV 237. The up/down bit is set to 0. The external bit is set to
1. These IPv6 prefixes are learned from other protocols and are
usually not directly connected to the advertising router. If the
Prefix Attribute Flags sub-TLV is included, the R-Flag is set to
0.
L1->L2 external routes: These are advertised in L2 LSPs, in TLV 236
or TLV 237. The up/down bit is set to 0. The external bit is set
to 1. These IPv6 prefixes are learned via L1 routing and were
derived during the L1 Shortest Path First (SPF) computation from
L1 external routes advertised in L1 LSPs in TLV 236 or TLV 237.
If the Prefix Attribute Flags sub-TLV is included, the R-Flag is
set to 1.
L2->L2 inter-area routes: These are advertised in L2 LSPs, in TLV
236 or TLV 237. The up/down bit is set to 1 but is ignored and
treated as if it were set to 0. The external bit is set to 1.
These IP prefixes are learned from another IS-IS instance usually
operating in another area. If the Prefix Attribute Flags sub-TLV
is included, the R-Flag is set to 0.
L2->L1 inter-area routes: These are advertised in L1 LSPs, in TLV
236 or TLV 237. The up/down bit is set to 1. The external bit is
set to 0. These IPv6 prefixes are learned via L2 routing and were
derived during the L2 SPF computation from prefixes advertised in
TLV 236 or TLV 237. If the Prefix Attribute Flags sub-TLV is
included, the R-Flag is set to 1.
L2->L1 external routes: These are advertised in L1 LSPs, in TLV 236
or TLV 237. The up/down bit is set to 1. The external bit is set
to 1. These IPv6 prefixes are learned via L2 routing and were
derived during the L2 SPF computation from prefixes advertised in
TLV 236 or TLV 237. If the Prefix Attribute Flags sub-TLV is
included, the R-Flag is set to 1.
L1->L1 inter-area routes: These are advertised in L1 LSPs, in TLV
236 or TLV 237. The up/down bit is set to 1. The external bit is
set to 1. These IP prefixes are learned from another IS-IS
instance usually operating in another area. If the Prefix
Attribute Flags sub-TLV is included, the R-Flag is set to 0.
3.3. Order of Preference for All Types of Routes Supported by TLVs 135
and 235
This document defines the following route preferences for IPv4 routes
advertised in TLVs 135 or 235. Note that all types of routes listed
for a given preference are treated equally.
1. L1 intra-area routes; L1 external routes
2. L2 intra-area routes; L2 external routes; L1->L2 inter-area
routes; L2-L2 inter-area routes
3. L2->L1 inter-area routes; L1->L1 inter-area routes
3.4. Order of Preference for All Types of Routes Supported by TLVs 236
and 237
This document defines the following route preferences for IPv6 routes
advertised in TLVs 236 or 237. Note that all types of routes listed
for a given preference are treated equally.
1. L1 intra-area routes; L1 external routes
2. L2 intra-area routes; L2 external routes; L1->L2 inter-area
routes; L1-L2 external routes; L2-L2 inter-area routes
3. L2->L1 inter-area routes; L2->L1 external routes; L1->L1 inter-
area routes
4. Security Considerations
This document raises no new security considerations. Security
considerations for the IS-IS protocol are covered in [ISO10589],
[RFC5304], and [RFC5310].
5. References
5.1. 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>.
[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,
<http://www.rfc-editor.org/info/rfc5120>.
[RFC5302] Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix
Distribution with Two-Level IS-IS", RFC 5302,
DOI 10.17487/RFC5302, October 2008,
<http://www.rfc-editor.org/info/rfc5302>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <http://www.rfc-editor.org/info/rfc5304>.
[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>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<http://www.rfc-editor.org/info/rfc5308>.
[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, <http://www.rfc-editor.org/info/rfc5310>.
5.2. Informative References
[PFXATTR] Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
U. Chunduri, "IS-IS Prefix Attributes for Extended IP and
IPv6 Reachability", Work in Progress, draft-ietf-isis-
prefix-attributes-04, January 2016.
Appendix A. Example Interoperability Issue
This example documents a real-world interoperability issue that
occurs because implementations from different vendors have
interpreted the use of the up/down bit in Level 2 LSPs
inconsistently.
L2 L2 L2 L2|L2 L2
10/8 - R0 ----- R1 ----- R2 ----- R3 ----- R4 ---- 10/8
|
Figure 1
In Figure 1, both R0 and R4 are advertising the prefix 10/8. Two IS-
IS Level 2 instances are running on R3 to separate the network into
two areas. R3 is performing route leaking and advertises prefixes
from R4 to the other Level 2 process. The network is using extended
metrics (TLV 135 defined in [RFC5305]). R0 advertises 10/8 with
metric 2000, and R3 advertises 10/8 with metric 100. All links have
a metric of 1. When advertising 10/8 in its Level 2 LSP, R3 sets the
down bit as specified in [RFC5305].
R1, R2, and R3 are from three different vendors (R1->Vendor1,
R2->Vendor2, R3->Vendor3). During interoperability testing, routing
loops are observed in this scenario.
o R2 has two possible paths to reach 10/8: Level 2 route with metric
2002 and up/down bit set to 0 (from R0) and Level 2 route with
metric 101 and up/down bit set to 1 (from R3). R2 selects R1 as
the next hop to 10/8 because it prefers the route that does NOT
have the up/down bit set.
o R3 has two possible paths to reach 10/8: Level 2 route with metric
2003 and up/down bit set to 0 (from R0) and Level 2 route with
metric 101 and up/down bit set to 0 (from R4). R3 selects R4 as
the next hop due to lowest metric.
o R1 has two possible paths to reach 10/8: Level 2 route with metric
2001 and up/down bit set to 0 (from R0) and Level 2 route with
metric 102 and up/down bit set to 1 (from R3). R1 selects R2 as
the next hop due to lowest metric.
When R1 or R2 try to send traffic to 10/8, packets loop due to
inconsistent routing decisions between R1 and R2.
Acknowledgements
The authors wish to thank Ahmed Bashandy for his insightful review.
Authors' Addresses
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95035
United States
Email: ginsberg@cisco.com
Stephane Litkowski
Orange Business Service
Email: stephane.litkowski@orange.com
Stefano Previdi
Cisco Systems
Via Del Serafico 200
Rome 0144
Italy
Email: sprevidi@cisco.com
AI ELI5
On this page
PROBLEMThe route preferences for the IPv4 and IPv6 Extended Reachability TLVs were never stated explicitly, and the use of the up/down bit in Level 2 LSPs was inconsistent between RFCs, so different vendors chose different routes.
MECHANISMThe RFC lists the route types each TLV can carry, says the up/down bit is ignored (treated as 0) in Level 2 LSPs, and defines three preference groups: Level 1 internal and external routes first, then Level 2 and L1->L2 routes, then routes leaked downward or between instances at Level 1.
WHY IT MATTERSImplementations can choose routes the same way, which avoids the routing loop shown in the appendix. It updates RFC 5308 and corrects its IPv6 rule to match IPv4.
§1Why Route Preference Needed Fixing
RFC 5302 defines route preference rules for TLVs 128 and 130. RFC 5305 introduced the IP Extended Reachability TLV 135 but did not explicitly adapt those rules. RFC 5308 defined the IPv6 Reachability TLV 236 and did state a preference, but in a way that uses the up/down bit in Level 2 LSPs, which is inconsistent with RFC 5302 and RFC 5305.
This RFC defines explicit rules for TLV 135, revises the rules for TLV 236 and clarifies the up/down bit in Level 2 TLVs. It is a clarification (not a correction) of RFC 5302 and RFC 5305, and a correction of RFC 5308 so that IPv6 matches IPv4. The same rules apply to the Multi-Topology (MT) TLVs 235 and 237.
Older rulebooks left a gap and one of them said something different from the others. This new page fills the gap and makes them all agree.
§2The Up/Down Bit in Level 2 LSPs
The up/down bit exists so that prefixes can be leaked downwards in the IS-IS level hierarchy. A route leaked downwards has the bit set to 1, and such prefixes MUST NOT be leaked upwards. With a single IS-IS instance and the current two levels, a prefix in a Level 2 LSP cannot have the bit set to 1. RFC 5302 still allowed for it in case a future extension added more levels, and recommended that implementations ignore the bit in L2 LSPs and accept the prefixes either way.
RFC 5305 added the case of several virtual routers in different areas redistributing prefixes between IS-IS instances. For a prefix advertised from one area to another at the same level, the bit SHALL be set to 1. It did not change the rule that the bit is ignored in Level 2 LSPs.
The conclusion is that there is no "L2 inter-area route". RFC 5308 nevertheless introduced one by giving a preference to a "Level 2 down prefix". This RFC calls that an error and corrects it. Because the change may cause interoperability issues, implementors may wish to support transition mechanisms from the RFC 5308 behavior.
A little flag on a route says which way it came down the ladder. At the top rung the flag should be ignored, but one rulebook forgot that, and this page fixes it.
§3Route Types Carried by the Extended Reachability TLVs
RFC 5302 remains the authority for TLVs 128 and 130. TLVs 135, 235, 236 and 237 do not support the internal/external metric distinction, and the IPv4 TLVs (135 and 235) do not distinguish internal from external routes. The Prefix Attribute Flags sub-TLV from a separate work-in-progress document brings that distinction back, using an X-Flag and an R-Flag.
For IPv4 (TLVs 135 and 235) the RFC defines these route types: L1 intra-area, L1 external, L2 intra-area, L1->L2 inter-area, L2->L2 inter-area, L2 external, L2->L1 inter-area and L1->L1 inter-area. For example, L1->L2 inter-area routes are advertised in L2 LSPs with the up/down bit 0, while L2->L1 inter-area routes are advertised in L1 LSPs with the bit set to 1. An L2->L2 inter-area route has the bit set to 1, but it is ignored and treated as if it were 0.
For IPv6 (TLVs 236 and 237) the same kinds of routes appear, with an external bit set to 0 or 1 and two extra types: L1->L2 external and L2->L1 external. Again, the L2->L2 inter-area route carries up/down 1 but it is ignored and treated as 0.
Every route comes with a label saying where it started and where it has been, such as local, from far away, or passed along from another network. The rules list every kind of label.
§4The Order of Preference
Routes of the same kind listed together are treated equally. For IPv4 routes in TLVs 135 or 235 the order is:
- L1 intra-area routes and L1 external routes.
- L2 intra-area routes, L2 external routes, L1->L2 inter-area routes and L2->L2 inter-area routes.
- L2->L1 inter-area routes and L1->L1 inter-area routes.
For IPv6 routes in TLVs 236 or 237 the order is the same, with the L1->L2 external routes added to the second group and the L2->L1 external routes added to the third.
Since L2->L2 inter-area routes are treated as if the up/down bit were 0, they sit in the same group as ordinary Level 2 routes, not in a separate lower group.
There are three piles of routes. Always choose from the first pile that has something in it, then the second, then the third.
| Group | Route types |
|---|---|
| 1 | L1 intra-area; L1 external |
| 2 | L2 intra-area; L2 external; L1->L2 inter-area; L2->L2 inter-area |
| 3 | L2->L1 inter-area; L1->L1 inter-area |
§5A Real Routing Loop Example
Appendix A documents an interoperability issue caused by vendors reading the up/down bit in Level 2 LSPs differently. Two routers, R0 and R4, advertise the prefix 10/8. Router R3 runs two Level 2 IS-IS instances to split the network into two areas, and leaks prefixes from R4 into the other instance, setting the down bit as RFC 5305 specifies. R1, R2 and R3 are from three different vendors.
R0 advertises 10/8 with metric 2000 and R3 with metric 100, and all links have metric 1. R2 prefers the route that does not have the up/down bit set, so it picks R1 as next hop. R1 compares metrics (2001 versus 102) and picks R2. Each sends traffic to the other, and packets loop.
Two friends use different rules to choose a path. One thinks the path goes through the other, and the other thinks the same back. They keep handing the ball back and forth.
§6Security Considerations
The RFC states that it raises no new security considerations. Security considerations for the IS-IS protocol are covered in ISO 10589, RFC 5304 and RFC 5310.
Nothing new to worry about here. The safety rules from the older documents still apply.
§7References
[RFC5120]RFC 5120Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)", RFC 5120, February 2008.February 2008[RFC5302]RFC 5302Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix Distribution with Two-Level IS-IS", RFC 5302, October 2008.October 2008[RFC5305]RFC 5305Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, October 2008.October 2008[RFC5308]RFC 5308Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October 2008.October 2008[RFC5304]RFC 5304Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, October 2008.October 2008[RFC5310]RFC 5310Bhatia, M., et al., "IS-IS Generic Cryptographic Authentication", RFC 5310, February 2009.February 2009
[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.