Internet Engineering Task Force (IETF) L. Ginsberg
Request for Comments: 7981 S. Previdi
Obsoletes: 4971 Cisco Systems
Category: Standards Track M. Chen
ISSN: 2070-1721 Huawei Technologies Co., Ltd
October 2016 IS-IS Extensions for Advertising Router Information
This document defines a new optional Intermediate System to
Intermediate System (IS-IS) TLV named CAPABILITY, formed of multiple
sub-TLVs, which allows a router to announce its capabilities within
an IS-IS level or the entire routing domain. This document obsoletes
Status of This Memo
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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.
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Table of Contents
1. Introduction ....................................................21.1. Requirements Language ......................................32. IS-IS Router CAPABILITY TLV .....................................33. Elements of Procedure ...........................................44. Interoperability with Routers Not Supporting the IS-IS Router
CAPABILITY TLV ..................................................65. Security Considerations .........................................66. IANA Considerations .............................................77. References ......................................................77.1. Normative References .......................................77.2. Informative References .....................................8Appendix A. Changes to RFC 4971 ...................................9
Authors' Addresses ................................................101. Introduction
There are several situations where it is useful for the IS-IS
[ISO10589] [RFC1195] routers to learn the capabilities of the other
routers of their IS-IS level, area, or routing domain. For the sake
of illustration, three examples related to MPLS Traffic Engineering
(TE) are described here:
1. Mesh-group: The setting up of a mesh of TE Label Switched Paths
(LSPs) [RFC5305] requires some significant configuration effort.
[RFC4972] proposes an auto-discovery mechanism whereby every
Label Switching Router (LSR) of a mesh advertises its mesh-group
membership by means of IS-IS extensions.
2. Point-to-Multipoint TE LSP (RFC4875): A specific sub-TLV
[RFC5073] allows an LSR to advertise its Point-to-Multipoint
capabilities ([RFC4875] and [RFC4461]).
3. Inter-area traffic engineering: Advertisement of the IPv4 and/or
the IPv6 Traffic Engineering Router IDs.
The use of IS-IS for Path Computation Element (PCE) discovery may
also be considered and will be discussed in the PCE WG.
The capabilities mentioned above require the specification of new
sub-TLVs carried within the IS-IS Router CAPABILITY TLV defined in
Note that the examples above are provided for the sake of
illustration. This document proposes a generic capability
advertising mechanism that is not limited to MPLS Traffic
This document defines a new optional IS-IS TLV named CAPABILITY,
formed of multiple sub-TLVs, which allows a router to announce its
capabilities within an IS-IS level or the entire routing domain. The
applications mentioned above require the specification of new sub-
TLVs carried within the IS-IS Router CAPABILITY TLV defined in this
Definition of these sub-TLVs is outside the scope of this document.
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. IS-IS Router CAPABILITY TLV
The IS-IS Router CAPABILITY TLV is composed of 1 octet for the type,
1 octet that specifies the number of bytes in the value field, and a
variable length value field that starts with 4 octets of Router ID,
indicating the source of the TLV, followed by 1 octet of flags.
A set of optional sub-TLVs may follow the flag field. Sub-TLVs are
formatted as described in [RFC5305].
LENGTH: from 5 to 255
Router ID (4 octets)
Flags (1 octet)
Set of optional sub-TLVs (0-250 octets)
0 1 2 3 4 5 6 7
| Reserved |D|S|
Currently, two bit flags are defined.
S bit (0x01): If the S bit is set(1), the IS-IS Router CAPABILITY TLV
MUST be flooded across the entire routing domain. If the S bit is
not set(0), the TLV MUST NOT be leaked between levels. This bit MUST
NOT be altered during the TLV leaking.
D bit (0x02): When the IS-IS Router CAPABILITY TLV is leaked from
Level 2 (L2) to Level 1 (L1), the D bit MUST be set. Otherwise, this
bit MUST be clear. IS-IS Router CAPABILITY TLVs with the D bit set
MUST NOT be leaked from Level 1 to Level 2. This is to prevent TLV
The IS-IS Router CAPABILITY TLV is OPTIONAL. As specified in
Section 3, more than one IS-IS Router CAPABILITY TLV from the same
source MAY be present.
This document does not specify how an application may use the IS-IS
Router CAPABILITY TLV, and such specification is outside the scope of
3. Elements of Procedure
The Router ID SHOULD be identical to the value advertised in the
Traffic Engineering Router ID TLV [RFC5305]. If no Traffic
Engineering Router ID is assigned, the Router ID SHOULD be identical
to an IP Interface Address [RFC1195] advertised by the originating
IS. If the originating node does not support IPv4, then the reserved
value 0.0.0.0 MUST be used in the Router ID field, and the IPv6 TE
Router ID sub-TLV [RFC5316] MUST be present in the TLV. IS-IS Router
CAPABILITY TLVs that have a Router ID of 0.0.0.0 and do NOT have the
IPv6 TE Router ID sub-TLV present MUST NOT be used.
When advertising capabilities with different flooding scopes, a
router MUST originate a minimum of two IS-IS Router CAPABILITY TLVs,
each TLV carrying the set of sub-TLVs with the same flooding scope.
For instance, if a router advertises two sets of capabilities, C1 and
C2, with an area/level scope and routing domain scope respectively,
C1 and C2 being specified by their respective sub-TLV(s), the router
will originate two IS-IS Router CAPABILITY TLVs:
o One IS-IS Router CAPABILITY TLV with the S flag cleared, carrying
the sub-TLV(s) relative to C1. This IS-IS Router CAPABILITY TLV
will not be leaked into another level.
o One IS-IS Router CAPABILITY TLV with the S flag set, carrying the
sub-TLV(s) relative to C2. This IS-IS Router CAPABILITY TLV will
be leaked into other IS-IS levels. When the TLV is leaked from
Level 2 to Level 1, the D bit will be set in the Level 1 LSP
In order to prevent the use of stale IS-IS Router CAPABILITY TLVs, a
system MUST NOT use an IS-IS Router CAPABILITY TLV present in an LSP
of a system that is not currently reachable via Level x paths, where
"x" is the level (1 or 2) in which the sending system advertised the
TLV. This requirement applies regardless of whether or not the
sending system is the originator of the IS-IS Router CAPABILITY TLV.
When an IS-IS Router CAPABILITY TLV is not used, either due to a lack
of reachability to the originating router or due to an unusable
Router ID, note that leaking the IS-IS Router CAPABILITY TLV is one
of the uses that is prohibited under these conditions.
Example: If Level 1 router A generates an IS-IS Router CAPABILITY
TLV and floods it to two L1/L2 routers, S and T, they will flood
it into the Level 2 domain. Now suppose the Level 1 area
partitions, such that A and S are in one partition and T is in
another. IP routing will still continue to work, but if A now
issues a revised version of the CAP TLV, or decides to stop
advertising it, S will follow suit, but without the above
prohibition, T will continue to advertise the old version until
the LSP times out.
Routers in other areas have to choose whether to trust T's copy of
A's IS-IS Router CAPABILITY TLV or S's copy of A's IS-IS Router
CAPABILITY TLV, and they have no reliable way to choose. By
making sure that T stops leaking A's information, the possibility
that other routers will use stale information from A is
In IS-IS, the atomic unit of the update process is a TLV -- or more
precisely, in the case of TLVs that allow multiple entries to appear
in the value field (e.g., IS-neighbors), the atomic unit is an entry
in the value field of a TLV. If an update to an entry in a TLV is
advertised in an LSP fragment different from the LSP fragment
associated with the old advertisement, the possibility exists that
other systems can temporarily have either 0 copies of a particular
advertisement or 2 copies of a particular advertisement, depending on
the order in which new copies of the LSP fragment that had the old
advertisement and the fragment that has the new advertisement arrive
at other systems.
Wherever possible, an implementation SHOULD advertise the update to
an IS-IS Router CAPABILITY TLV in the same LSP fragment as the
advertisement that it replaces. Where this is not possible, the two
affected LSP fragments should be flooded as an atomic action.
Systems that receive an update to an existing IS-IS Router CAPABILITY
TLV can minimize the potential disruption associated with the update
by employing a holddown time prior to processing the update so as to
allow for the receipt of multiple LSP fragments associated with the
same update prior to beginning processing.
Where a receiving system has two copies of an IS-IS Router CAPABILITY
TLV from the same system that have conflicting information for a
given sub-TLV, the procedure used to choose which copy shall be used
4. Interoperability with Routers Not Supporting the IS-IS Router
Routers that do not support the IS-IS Router CAPABILITY TLV MUST
silently ignore the TLV(s) and continue processing other TLVs in the
same LSP. Routers that do not support specific sub-TLVs carried
within an IS-IS Router CAPABILITY TLV MUST silently ignore the
unsupported sub-TLVs and continue processing those sub-TLVs that are
supported in the IS-IS Router CAPABILITY TLV. How partial support
may impact the operation of the capabilities advertised within the
IS-IS Router CAPABILITY TLV is outside the scope of this document.
In order for IS-IS Router CAPABILITY TLVs with domain-wide scope
originated by L1 routers to be flooded across the entire domain, at
least one L1/L2 router in every area of the domain MUST support the
Router CAPABILITY TLV.
If leaking of the IS-IS Router CAPABILITY TLV is required, the entire
CAPABILITY TLV MUST be leaked into another level without change
(except for changes to the TLV flags as noted in Section 2) even
though it may contain some sub-TLVs that are unsupported by the
router doing the leaking.
5. Security Considerations
Any new security issues raised by the procedures in this document
depend upon the opportunity for LSPs to be snooped and modified, the
ease/difficulty of which has not been altered. As the LSPs may now
contain additional information regarding router capabilities, this
new information would also become available to an attacker.
Specifications based on this mechanism need to describe the security
considerations around the disclosure and modification of their
Appendix A. Changes to RFC 4971
This document makes the following changes to RFC 4971.
RFC 4971 only allowed a 32-bit Router ID in the fixed header of TLV
242. This is problematic in an IPv6-only deployment where an IPv4
address may not be available. This document specifies:
1. The Router ID SHOULD be identical to the value advertised in the
Traffic Engineering Router ID TLV (134) if available.
2. If no Traffic Engineering Router ID is assigned, the Router ID
SHOULD be identical to an IP Interface Address [RFC1195]
advertised by the originating IS.
3. If the originating node does not support IPv4, then the reserved
value 0.0.0.0 MUST be used in the Router ID field, and the IPv6
TE Router ID sub-TLV [RFC5316] MUST be present in the TLV.
In addition, some clarifying editorial changes have been made.
The authors of RFC 4971 thanked Jean-Louis Le Roux, Paul Mabey,
Andrew Partan, and Adrian Farrel for their useful comments.
The authors of this document would like to thank Kris Michielsen for
calling attention to the problem associated with an IPv6-only router.
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