Internet Engineering Task Force T. Li
Internet-Draft Arista Networks
Intended status: Standards Track February 5, 2019
Expires: August 9, 2019
Area Abstraction for IS-IS
draft-li-lsr-area-abstraction-01
Abstract
Link state routing protocols have hierarchical abstraction already
built into them. However, when lower levels are used for transit,
they must expose their internal topologies, leading to scale issues.
To avoid this, this document discusses extensions to the IS-IS
routing protocol that would allow level 1 areas to provide transit,
yet only inject an abstraction of the level 1 topology into level 2.
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This Internet-Draft will expire on August 9, 2019.
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Internet-Draft Area Abstraction February 2019
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Area Abstraction . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Area Leader Election . . . . . . . . . . . . . . . . . . 4
2.2. LSP Generation . . . . . . . . . . . . . . . . . . . . . 5
2.3. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Level 2 SPF Considerations . . . . . . . . . . . . . . . 5
3. Area Proxy System Identifier TLV . . . . . . . . . . . . . . 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The IS-IS routing protocol IS-IS [ISO10589] currently supports a two
level hierarchy of abstraction. The fundamental unit of abstraction
is the 'area', which is a (hopefully) connected set of systems
running IS-IS at the same level. Level 1, the lowest level, is
abstracted by routers that participate in both Level 1 and Level 2,
and they inject area information into Level 2. Level 2 systems
seeking to access Level 1, use this abstraction to compute the
shortest path to the Level 1 area. The full topology database of
Level 1 is not injected into Level 2, only a summary of the address
space contained within the area, so the scalability of the Level 2
link state database is protected.
This works well if the Level 1 area is tangential to the Level 2
area. This also works well if there are a number of routers in both
Level 1 and Level 2 and they are adjacent, so Level 2 traffic will
never need to transit Level 1 only routers. Level 1 will not contain
any Level 2 topology, and Level 2 will only contain area abstractions
for Level 1.
Unfortunately, this scheme does not work so well if the Level 1 area
needs to provide transit for Level 2 traffic. For Level 2 shortest
path first (SPF) computations to work correctly, the transit topology
must also appear in the Level 2 link state database. This implies
that all routers that could possibly provide transit, plus any links
that might also provide Level 2 transit must also become part of the
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