RFC 5311
Simplified Extension of Link State PDU (LSP) Space for IS-IS
Bestie, RFC 5311 is here and it's a whole main character 💅🎀
The ID card: It's a Standards Track document from the Network Working Group, numbered RFC 5311 and titled Simplified Extension of Link State PDU (LSP) Space for IS-IS 🦋. The authors are D. McPherson (Ed.) from Arbor Networks, plus L. Ginsberg, S. Previdi, and M. Shand from Cisco Systems ✨. It's dated February 2009, and it obsoletes RFC 3786 🫶.
Status of This Memo 📜
This is an Internet standards track protocol for the Internet community, and it's open for discussion and suggestions for improvements 💌. For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1) 🔍. Distribution of this memo is unlimited 🌸.
Copyright Notice ©️
Copyright (c) 2009 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) as they're in effect on the publication date 📄. Read those carefully, because they spell out your rights and restrictions for this document 👀.
Abstract 🌈
This doc describes a simpler way to stretch the Link State PDU (LSP) space past the 256 LSP limit 🚀. It's meant to be the preferred replacement for the method in RFC 3786 👑.
Table of Contents 🗂️
1. Overview ........................................................2
2. Specification of Requirements ...................................3
3. Definition of Commonly Used Terms ...............................3
4. Utilizing Additional System IDs .................................4
4.1. Additional Information in Extended LSPs ....................4
4.2. Extended LSP Restrictions ..................................4
4.2.1. TLVs That MUST NOT Appear ...........................4
4.2.2. Leaf Advertisements in Extended LSPs ................5
4.2.3. IS Neighbor Advertisement Restrictions ..............5
4.2.4. Area Addresses ......................................6
4.2.5. Overload, Attached, Partition Repair Bits ...........6
4.3. Originating LSP Requirements ...............................6
4.4. IS Alias ID TLV (IS Alias ID) ..............................7
4.5. New TLVs in Support of IS Neighbor Attributes ..............7
5. Comparison with the RFC 3786 Solution ...........................8
6. Deployment Considerations .......................................8
6.1. Advertising New TLVs in Extended LSPs ......................9
6.2. Reachability and Non-SPF TLV Staleness .....................9
6.3. Normal LSP OL State and Use of Extended LSPs ...............9
6.4. Moving Neighbor Attribute INFO LSPs ........................9
6.5. Advertising Leaf INFO Extended LSPs .......................10
7. Security Considerations ........................................10
8. IANA Considerations ............................................10
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................111. Overview 🌟
[IS-IS] defines the set of LSPs a system can originate at each level, and that set is capped at 256 LSPs 🙃. [IS-IS] also sets the maximum size of an LSP (originatingLxLSPBufferSize) at 1492 bytes 📏. The carrying capacity of an LSP set is bounded, but so far it's been plenty for the advertisements an area or domain needs in existing deployments 🤷♀️. The problem is that new info (multi-topology support, traffic engineering info, router capabilities, and so on) could blow right past that capacity 💥.
This whole thing first got attention when traffic engineering extensions showed up 🚦. That led to the solution in [RFC3786]. Unfortunately, that solution comes with restrictions that are needed to stay interoperable with systems that don't support the extensions 😬.
This document defines extensions that let a system go past the 256 LSP limit without any interoperability issues for systems that don't support the extension 🦋. It's a simpler solution, and that's exactly why it's the preferred one ✨.
2. Specification of Requirements 📝
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 [BCP14] 🔑.
3. Definition of Commonly Used Terms 📚
This section defines the terms used throughout the doc, and the terminology matches what RFC 3786 uses 🤓.
- Originating System: A physical IS running the IS-IS protocol. This document describes a way for one physical IS to originate LSPs on behalf of multiple virtual ISs, so the Originating System stands for that single physical IS 🖥️.
- Normal system-id: The system-id of an Originating System, as defined by [IS-IS] 🪪.
- Additional system-id: A system-id other than the "Normal system-id" that the network administrator assigns to an Originating System so it can generate Extended LSPs 🎟️. Like the Normal system-id, it must be unique throughout the routing area (Level-1) or domain (Level-2) 🔒.
- Original LSP: An LSP that uses the Normal system-id in its LSP ID 🌱.
- Extended LSP: An LSP that uses an Additional system-id in its LSP ID ✨.
- LSP set: All LSPs of a given level that share the same system ID and Pseudonode ID. (The LSPID field then only changes in the LSP number octet.) This is the complete set of link state info at a given level, originated using that system ID/Pseudonode ID 🗃️. The term exists to clear up confusion between a logical LSP and a single Link State PDU, which is sometimes called an LSP fragment. The fragment is the unit of info the update process handles 🧩.
- Extended LSP set: An LSP set made up of LSPs that use an Additional system-id 🌈.
- Extension-capable IS: An IS that implements the mechanisms described in this document 🦾.
- Virtual IS: The system, identified by an Additional system-id, that's advertised as originating the Extended LSPs. These LSPs put the Additional system-id in their LSP IDs 👻.
4. Utilizing Additional System IDs 🧪
This extension lets an Originating System be given additional system-ids that can be used to generate extra LSP sets 🎁. Those additional system-ids follow the same restrictions as normal system-ids. So at Level-1, the additional system-id MUST be unique within the Level-1 area, and at Level-2, it MUST be unique within the domain 🔐.
Extended LSPs go through the IS-IS Update Process the same way normal LSPs do, meaning the same rules for generation, flooding, purging, and so on all apply 🌊. Specifically, if the Extended LSP with LSP number zero and remaining lifetime > 0 isn't present for a particular additional system-id, then none of the Extended LSPs in that Extended LSP set shall be processed 🚫.
4.1. Additional Information in Extended LSPs 🪪
The LSP number zero of an Extended LSP set MUST include the new IS alias ID TLV defined in Section 4.4 💖. That's what ties the Extended LSP set to the Originating System that generated the LSP(s) 🔗.
4.2. Extended LSP Restrictions 🚧
These restrictions on what can appear in an Extended LSP exist to avoid interoperability issues with systems that don't support the extensions in this document 🫶. All TLV references are based on the current definitions in the IANA IS-IS TLV Codepoints Registry 📋.
4.2.1. TLVs That MUST NOT Appear 🙅♀️
The following TLVs MUST NOT appear in an Extended LSP:
TLV Name (#)
-----------
ES Neighbors (3)
Part. DIS (4)
Prefix Neighbors (5)If any of those TLVs do show up in an Extended LSP, an Extension Capable IS MUST ignore them on receipt and SHOULD report an error 🚨. All other TLVs in that Extended LSP set MUST be processed normally ✅.
4.2.2. Leaf Advertisements in Extended LSPs 🍃
Leaf info advertisements in Extended LSPs are allowed 🌿. To include that info, there has to be an advertised neighbor between the Originating System and the Virtual IS for that Extended LSP set, where the leaf advertisements live. See Section 4.2.3 👀.
When leaf advertisements for multiple topologies (see [RFC5120]) are included in an Extended LSP set, the multi-topology TLV (229) MUST include all topologies that have a leaf advertisement 🗺️.
These TLVs count as leaf advertisements:
TLV Name (#)
-----------
IP Int. Reach (128)
IP Ext. Address (130)
The extended IP reachability TLV (135)
MT IP Reach (235)
IPv6 IP Reach (236)
MT IPv6 IP Reach (237)4.2.3. IS Neighbor Advertisement Restrictions 🤝
In an Extended LSP, IS Neighbor Reachability can only be advertised for reachability to the Originating System 💞. A neighbor to the Originating System MUST be advertised in Extended LSPs 📣. If multi-topology capability [RFC5120] is supported, an MT IS Neighbor advertisement to the Originating System IS MUST be included for every topology advertised in the Extended LSP set 🌍. Neighbor advertisement(s) to the Originating System in an Extended LSP MUST use a non-zero metric and SHOULD use a metric of MaxLinkMetric-1 🔢.
These restrictions cover every TLV used to advertise neighbor reachability, including these:
TLV Name (#)
-----------
IIS Neighbors (2)
The extended IS reachability TLV (22)
MT-ISN (222)4.2.4. Area Addresses 🏡
LSP number zero of an Extended LSP set MUST include an Area Address TLV 🏷️. The set of area addresses advertised MUST be a subset of the set of Area Addresses advertised in the normal LSP number zero at the corresponding level 🧁. Preferably, the advertisement SHOULD be syntactically identical to the one in the normal LSP number zero at the corresponding level 💅.
4.2.5. Overload, Attached, Partition Repair Bits ⚠️
The Overload (OL), Attached (ATT), and Partition Repair (P) bits MUST be set to 0 in all Extended LSPs 🙅♀️.
Heads up, though: systems that DON'T support these extensions will read these bits the normal way in the Extended LSPs they receive 👀. If the ATT bit were set in an Extended LSP, it could suggest the Virtual IS is attached to other areas when the Originating System isn't. That might make legacy systems treat the Virtual IS as a default exit point from the area 😱🔥.
4.3. Originating LSP Requirements ✨
okay so the Original LSP set MUST include a neighbor to the Virtual IS associated with each Extended LSP set generated 💖 If multi-topology capability [RFC5120] is supported, an MT IS Neighbor advertisement to the Virtual IS MUST be included for every topology advertised in the Extended LSP set 🗺️ The neighbor advertisement(s) in the Original LSP MUST specify a metric of zero. This is the lowkey genius part: it guarantees that the two-way connectivity check between Originating System and Virtual IS will succeed, and that the cost of reaching the Virtual IS is the same as the cost to reach the Originating System ✨ iconic 💅
4.4. IS Alias ID TLV (IS Alias ID) 🎀
The IS-Alias TLV lets extension-capable ISs recognize the Originating System of an Extended LSP set 🫶 It identifies the Normal system-id of the Originating System.
Type 24
Length # of octets in the value field (7 to 255)
Value
No. of octets
+-----------------------+
| Normal System-id | 6
+-----------------------+
| Sub-TLV length | 1
+-----------------------+
| Sub-TLVs (optional) | 0 to 248
+-----------------------+Normal system-id
- The Normal system-id of the Originating System.
Sub-TLVs length
- Total length of all sub-TLVs.
Sub-TLVs
- No sub-TLVs are defined in this document. Should future extensions define sub-TLVs, the sub-TLVs MUST be formatted as described in [RFC5305] 📚
4.5. New TLVs in Support of IS Neighbor Attributes 🧚
okay this one is a whole vibe 🌸 One of the major sources of additional information in LSPs is the sub-TLV information associated with the extended IS reachability TLV (22) and MT-ISN TLV (222). This includes (but is not limited to) information required in support of Traffic Engineering (TE) as defined in [RFC5305] and [RFC5307] 🔥 The restrictions defined in this document prohibit the presence of TLV 22 and/or TLV 222 in Extended LSPs, except to advertise the neighbor relationship to the Originating System. So if there's a need to advertise information about the Originating System's neighbors in Extended LSPs, we need new TLVs to carry that sub-TLV info 🙈
Two new TLVs are therefore defined. Bestie, these are the new ones:
- IS Neighbor Attribute TLV (23). It is identical in format to the extended IS reachability TLV (22).
- MT IS Neighbor Attribute TLV (223). It is identical in format to the MT-ISN TLV (222).
These new TLVs MAY be included in Original LSPs or Extended LSPs. Regardless of which type of LSP the TLVs appear in, the information pertains to the neighbor relationship between the Originating System and the IS identified in the TLV 🦋
These TLVs MUST NOT be used to infer that a neighbor relationship exists in the absence of TLV 22 or TLV 222 (whichever applies) in the Originating LSP set for the specified neighbor. This restriction is necessary in order to maintain compatibility with systems that do not support these extensions. Not gonna lie, that's a very responsible rule 🫶
5. Comparison with the RFC 3786 Solution ✨
This document uses the same basic mechanism (additional system-ids) as RFC 3786 to let an originating system generate more than 256 LSPs. The difference is that it restricts the content of Extended LSPs to information that does NOT impact the building of a Shortest Path Tree (SPT) 🌳
Legacy IS-IS implementations that don't support the extensions defined in this document see the Extended LSPs as information associated with a system that is reachable only via the Originating System. Since no other systems are reachable via the Virtual ISs, the Shortest Path First (SPF) calculation in legacy ISs is consistent with the one performed by extension-capable ISs 💅 So there's no need for the two different operating modes defined in RFC 3786.
There's also no need for the special handling of the original LSP set and the Extended LSP set(s) as a single Logical LSP during the SPF, which is specified in Section 5 of RFC 3786 🎀
6. Deployment Considerations 🌈
There are a number of deployment considerations that limit the usefulness of Extended LSPs unless all systems are extension-capable ISs 😭
6.1. Advertising New TLVs in Extended LSPs 🦋
As Extended LSPs MAY be used to advertise TLVs associated with other protocol extensions (defining those is outside the scope of this document) and/or the extensions defined in Section 4.4 of this document, it's obvious that legacy IS-IS implementations will have limited use of the information in Extended LSPs 💀 The implication is that as implementations are revised to support the protocol extensions that define new TLVs/sub-TLVs that MAY be advertised in Extended LSPs, the implementation SHOULD also be revised to support the extensions defined in this document. That way it can process the new information whether it appears in normal or Extended LSPs ✨
6.2. Reachability and Non-SPF TLV Staleness 🔍
In cases where non-SPF information is advertised in LSPs, it's necessary to determine whether the system that originated the advertisement is reachable. This guarantees that a receiving IS does not use or leak stale information 🙅♀️ As long as the OL bit is NOT set by the Originating System in normal LSPs, reachability to the Virtual IS will be consistent with reachability to the Originating System. So no special rules are required in this case 💖
6.3. Normal LSP OL State and Use of Extended LSPs 🚫
If the Originating System sets the OL bit in a normal LSP, legacy systems will see the Virtual ISs associated with that Originating System as unreachable, and therefore will not use the information in the corresponding Extended LSPs. Under these circumstances, Extension-capable ISs MUST also see the Virtual ISs as unreachable. This avoids potential routing loops in cases where leaf information is advertised in Extended LSPs 🔁
6.4. Moving Neighbor Attribute INFO LSPs 📦
Section 4.4 defines new TLVs that MAY be used to advertise neighbor attribute information in Extended LSPs. When neighbor attribute information associated with the same context (e.g., the same link) appears in both an Original LSP and in one or more Extended LSP sets, the following rules apply for each attribute:
- If the attribute information does not conflict, it MUST be considered additive. ➕
- If the attribute information conflicts, then the information in the Original LSP, if present, MUST be used. If no information is in the Original LSP, then the information from the Extended LSP with the lowest system-id SHALL be preferred. 🥇
- In cases where information about the same neighbor/link/attribute appears in both TLV 22 and TLV 23 (or TLV 222 and TLV 223 for the same MTID), then the information in TLV 22 (or TLV 222) MUST be used and the information in TLV 23 (or TLV 223) MUST be ignored. 🙃
Using the new TLVs for neighbor attribute information gives you some extra perks too 🎁:
- Elimination of the need for redundant IS neighbor TLVs to be processed as part of the SPF. 🧹
- Easier support for a set of TE information associated with a single link that exceeds the 255-byte TLV limit, by letting the interpretation of multiple TLVs be considered additive rather than mutually exclusive. 💪
6.5. Advertising Leaf INFO Extended LSPs 🍃
The need to advertise leaf information in Extended LSPs may come up because of extensive leaking of inter-level information, or because of support for multiple topologies as described in [RFC5120]. When leaf information is advertised in Extended LSPs, those LSPs now contain information that MUST be processed in order to correctly update the forwarding plane of an IS. This may increase how often events trigger forwarding plane updates by ISs in the network 📈 It is therefore recommended that, when possible, leaf information be restricted to the normal LSP set 🫶
7. Security Considerations 🔐
This document raises no new security issues for IS-IS. For general security considerations for IS-IS, see [RFC5304]. Security is always a main character moment, so go read that one too 💜
8. IANA Considerations 📋
This document defines the following new ISIS TLVs, which are reflected in the ISIS TLV codepoint registry 🎀
Type Description IIH LSP SNP
---- ----------------------------------- --- --- ---
23 IS Neighbor Attribute n y n
24 IS Alias ID n y n
223 MT IS Neighbor Attribute n y n9. References 📚
9.1. Normative References
[IS-IS] ISO, "Intermediate system to Intermediate system 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.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5307] Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, October 2008.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.9.2. Informative References
[RFC3786] Hermelin, A., Previdi, S., and M. Shand, "Extending the
Number of Intermediate System to Intermediate System (IS-
IS) Link State PDU (LSP) Fragments Beyond the 256 Limit",
RFC 3786, May 2004.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.Authors' Addresses 💌
Danny McPherson (editor)
Arbor Networks, Inc.
EMail: danny@arbor.net
Les Ginsberg
Cisco Systems
EMail: ginsberg@cisco.com
Stefano Previdi
Cisco Systems
EMail: sprevidi@cisco.com
Mike Shand
Cisco Systems
EMail: mshand@cisco.com📦 where this page comes from
- ietf: rfc5311 2009-02
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms