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IETF RFC 5130
A Policy Control Mechanism in IS-IS Using Administrative Tags
Last modified on Saturday, February 9th, 2008
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Network Working Group S. Previdi
Request for Comments: 5130 M. Shand, Ed.
Category: Standards Track Cisco Systems
C. Martin
iPath Services
February 2008
A Policy Control Mechanism in IS-IS Using Administrative Tags
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes an extension to the IS-IS protocol to add
operational capabilities that allow for ease of management and
control over IP prefix distribution within an IS-IS domain. This
document enhances the IS-IS protocol by extending the information
that an Intermediate System (IS) router can place in Link State
Protocol (LSP) Data Units for policy use. This extension will
provide operators with a mechanism to control IP prefix distribution
throughout multi-level IS-IS domains.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . . 2
3. Sub-TLV Additions . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. 32-bit Administrative Tag Sub-TLV 1 . . . . . . . . . . . . 3
3.2. 64-bit Administrative Tag Sub-TLV 2 . . . . . . . . . . . . 3
4. Ordering of Tags . . . . . . . . . . . . . . . . . . . . . . . 3
5. Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
9. Manageability Considerations . . . . . . . . . . . . . . . . . 5
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 6
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
12.1. Normative References . . . . . . . . . . . . . . . . . . . 6
12.2. Informative References . . . . . . . . . . . . . . . . . . 6
Previdi, et al. Standards Track PAGE 1 
RFC 5130 IS-IS Admin Tags February 2008
1. Introduction
As defined in [RFC 1195] and extended in [RFC 3784], the IS-IS protocol
[ISO10589] may be used to distribute IPv4 prefix reachability
information throughout an IS-IS domain. In addition, thanks to
extensions made in [RFC 5120] and [ISIS-IPv6], IS-IS may be used to
distribute IPv6 reachability information.
The IPv4 prefix information is encoded as TLV type 128 and 130 in
[RFC 1195], with additional information carried in TLV 135 as
specified in [RFC 3784] and TLV 235 as defined in [RFC 5120]. In
particular, the extended IP Reachability TLV (TLV 135) contains
support for a larger metric space, an up/down bit to indicate
redistribution between different levels in the hierarchy, an IP
prefix, and one or more sub-TLVs that can be used to carry specific
information about the prefix. TLV 235 is a derivative of TLV 135,
with the addition of Multi-Topology membership information [RFC 5120].
The IPv6 prefix information is encoded as TLV 236 in [ISIS-IPv6], and
TLV 237 in [RFC 5120].
This document defines 2 new sub-TLVs for TLV 135, TLV 235, TLV 236
and TLV 237 that may be used to carry administrative information
about an IP prefix.
2. Conventions Used in This Document
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 BCP 14, [RFC 2119].
3. Sub-TLV Additions
This document creates 2 new "Administrative Tag" sub-TLVs to be added
to TLV 135, TLV 235, TLV 236 and TLV 237. These TLVs specify one or
more 32- or 64-bit unsigned integers that may be associated with an
IP prefix. Example uses of these tags include carrying BGP standard
(or extended) communities and controlling redistribution between
levels and areas, different routing protocols, or multiple instances
of IS-IS running on the same router.
The methods for which their use is employed is beyond the scope of
this document and left to the implementer and/or operator.
The encoding of the sub-TLV(s) is discussed in the following
subsections.
Previdi, et al. Standards Track PAGE 2
RFC 5130 IS-IS Admin Tags February 2008
3.1. 32-bit Administrative Tag Sub-TLV 1
The Administrative Tag SHALL be encoded as one or more 4-octet
unsigned integers using Sub-TLV 1 in TLV 135 [RFC 3784], TLV 235
[RFC 5120], TLV 236 [ISIS-IPv6], and TLV 237 [RFC 5120]. The
Administrative Tag Sub-TLV has following structure:
o 1 octet of type (value: 1)
o 1 octet of length (value: multiple of 4)
o one or more instances of 4 octets of administrative tag
On receipt, an implementation MAY consider only one encoded tag, in
which case, the first encoded tag MUST be considered and any
additional tags ignored. A tag value of zero is reserved and SHOULD
be treated as "no tag".
3.2. 64-bit Administrative Tag Sub-TLV 2
The Administrative Tag SHALL be encoded as one or more 8-octet
unsigned integers using Sub-TLV 2 in TLV 135 [RFC 3784], TLV 235
[RFC 5120], TLV 236 [ISIS-IPv6], and TLV 237 [RFC 5120]. The 64-bit
Administrative Tag Sub-TLV has following structure:
o 1 octet of type (value: 2)
o 1 octet of length (value: multiple of 8)
o one or more instances of 8 octets of administrative tag
On receipt, an implementation MAY consider only one encoded tag; in
which case, the first encoded tag MUST be considered and any
additional tags ignored. A tag value of zero is reserved and SHOULD
be treated as "no tag".
4. Ordering of Tags
The semantics of the tag order are implementation-dependent. That
is, there is no implied meaning to the ordering of the tags that
indicates a certain operation or set of operations need be performed
based on the order of the tags. Each tag SHOULD be treated as an
autonomous identifier that MAY be used in policy to perform a policy
action. Whether or not tag A precedes or succeeds tag B SHOULD not
change the meaning of the tag set. However, when propagating TLVs
that contain multiple tags between levels, an implementation SHOULD
Previdi, et al. Standards Track PAGE 3
RFC 5130 IS-IS Admin Tags February 2008
preserve the ordering such that the first tag remains the first tag,
so that implementations that only recognize a single tag will have a
consistent view across levels.
Each IS that receives an LSP with TLV(s) 135 and/or 235 and/or 236
and/or 237, that have associated sub-TLV(s) 1 and/or 2, MAY operate
on the tag values as warranted by the implementation. If an
implementation needs to change tag values, for example, when
propagating TLVs between levels at an area boundary, then the TLV(s)
SHOULD be copied to the newly generated Level-1 or Level-2 LSP. At
that point, the contents of the sub-TLV(s) MAY change as dictated by
the policy action. In the event that no change is required, the sub-
TLV(s) SHOULD be copied in order into the new LSP, such that ordering
is preserved.
5. Compliance
A compliant IS-IS implementation MUST be able to assign one tag to
any IP prefix in any of the following TLVs: TLV 135, TLV 235, TLV
236, TLV 237. It MUST be able to interpret a single tag present in
the sub-TLV, or the first tag where there is more than one tag
present in the sub-TLV.
A compliant IS-IS implementation MAY be able to assign more than one
tag to any IP prefix in any of the following TLVs: TLV 135, TLV 235,
TLV 236, TLV 237. It MAY be able to interpret the second and
subsequent tags where more than one tag is present in the sub-TLV.
When propagating TLVs between levels, a compliant IS-IS
implementation MAY be able to rewrite or remove one or more tags
associated with a prefix in any of the following TLVs: TLV 135, TLV
235, TLV 236, TLV 237.
6. Operations
An administrator associates an Administrative Tag value with some
interesting property. When IS-IS advertises reachability for some IP
prefix that has that property, it adds the Administrative Tag to the
IP reachability information TLV for that prefix, and the tag "sticks"
to the prefix as it is flooded throughout the routing domain.
Consider the network in Figure 1. We wish to "leak" L1 prefixes
[RFC 2966] with some property, A, from L2 to the L1 router R1.
Without policy groups, there is no way for R2 to know property A
prefixes from property B prefixes.
Previdi, et al. Standards Track PAGE 4
RFC 5130 IS-IS Admin Tags February 2008
R2--------R3--------R4
L2 / \
- - - /- - - - - - - - - - - - - -
L1 / \
R1----1.1.1.0/24 (A) R5
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1.1.2.0/24 (B)
Figure 1: Example of usage
We associate Administrative Tag 100 with property A, and have R5
attach that value to the IP extended reachability information TLV for
prefix 1.1.2.0/24. R2 has a policy in place to "match prefixes with
Administrative Tag 100, and leak to L1".
The previous example is rather simplistic; it seems that it would be
just as easy for R2 simply to match the prefix 1.1.2.0/24. However,
if there are a large number of routers that need to apply some policy
according to property A and a large number of "A" prefixes, this
mechanism can be quite helpful.
Implementations that support only a single tag and those that support
multiple tags may coexist in the same IS-IS domain. An
implementation supporting multiple tags SHOULD therefore assign any
tag that is required to be interpreted by all systems as the first
tag in any set of multiple tags.
7. Security Considerations
This document raises no new security issues for IS-IS, as any
annotations to IP prefixes should not pass outside the administrative
control of the network operator of the IS-IS domain. Such an
allowance would violate the spirit of Interior Gateway Protocols in
general and IS-IS in particular.
8. IANA Considerations
IANA has assigned "1" as the type code of the 32-bit Administrative
Tag Sub-TLV and "2" as the type code of the 64-bit Administrative Tag
Sub-TLV.
9. Manageability Considerations
These extensions have been designed, developed, and deployed for many
years and do not have any new impact on management and operation of
the IS-IS protocol via this standardization process.
Previdi, et al. Standards Track PAGE 5
RFC 5130 IS-IS Admin Tags February 2008
10. Acknowledgements
The authors would like to thank Henk Smit for clarifying the best
place to describe this new information, Tony Li and Tony Przygienda
for useful comments on this document, and Danny McPherson for some
much needed formatting assistance.
11. Contributors
Brad Neal contributed portions of this document.
12. References
12.1. Normative References
[ISO10589] International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routing 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, Nov 2002.
[RFC 1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[ISIS-IPv6] Hopps, C., "Routing IPv6 with IS-IS", Work in Progress,
October 2007.
[RFC 2966] Li, T., Przygienda, T., and H. Smit, "Domain-wide Prefix
Distribution with Two-Level IS-IS", RFC 2966,
October 2000.
[RFC 3784] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE)",
RFC 3784, June 2004.
[RFC 5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in IS-IS", RFC 5120,
February 2008.
Previdi, et al. Standards Track PAGE 6
RFC 5130 IS-IS Admin Tags February 2008
Authors' Addresses
Stefano Previdi
Cisco Systems
Via Del Serafico, 200
00142 Rome,
Italy
EMail: sprevidi@cisco.com
Mike Shand (editor)
Cisco Systems
250, Longwater Avenue.
Reading, Berks RG2 6GB
UK
Phone: +44 208 824 8690
EMail: mshand@cisco.com
Christian Martin
iPath Services
EMail: chris@ipath.net
Previdi, et al. Standards Track PAGE 7
RFC 5130 IS-IS Admin Tags February 2008
Full Copyright Statement
Copyright © The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Previdi, et al. Standards Track PAGE 8
A Policy Control Mechanism in IS-IS Using Administrative Tags
RFC TOTAL SIZE: 16284 bytes
PUBLICATION DATE: Saturday, February 9th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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