Internet Engineering Task Force T. Li
Internet-Draft Arista Networks
Intended status: Standards Track August 28, 2019
Expires: February 29, 2020

Area Abstraction for IS-IS


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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Table of Contents

1. Introduction

The IS-IS routing protocol IS-IS 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 Level 2 topology. If this is a relatively tiny portion of the Level 1 area, this is not onerous.

However, with today's data center topologies, this is problematic. A common application is to use a Layer 3 Leaf-Spine (L3LS) topology, which is a folded 3-stage Clos fabric. It can also be thought of as a complete bipartite graph. In such a topology, the desire is to use Level 1 to contain the routing of the entire L3LS topology and then to use Level 2 for the remainder of the network. Leaves in the L3LS topology are appropriate for connection outside of the data center itself, so they would provide connectivity for Level 2. If there are multiple connections to Level 2 for redundancy, or to other areas, these too would also be made to the leaves in the topology. This creates a difficulty because there are now multiple Level 2 leaves in the topology, with connectivity between the leaves provide by the spines.

Following the current rules of IS-IS, all spine routers would necessarily be part of the Level 2 topology, plus all links between a Level 2 leaf and the spines. In the limit, where all leaves need to support Level 2, it implies that the entire L3LS topology becomes part of Level 2. This is seriously problematic as it more than doubles the link state database held in the L3LS topology and eliminates any benefits of the hierarchy.

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.

2. Area Abstraction

To address this, we propose to completely abstract away the details of the Level 1 area topology within Level 2, making the entire area look like a single system directly connected to all of the area's Level 2 neighbors. By only providing an abstraction of the topology, Level 2's requirement for connectivity can be satisfied without the full overhead of the area's internal topology. It then becomes the responsibility of the Level 1 area to ensure the forwarding connectivity that's advertised.

For the purposes of this discussion, we'll consider a single Level 1 IS-IS area as the Target Area. All routers within this area speak Level 1 IS-IS on all of the links within this topology. We assume that the Target Area is always connected. We propose to implement Area Abstraction by having a Level 2 Proxy LSP that represents the entire Target Area. This is the only LSP from the area that will be injected into the overall Level 2 link state database.

There are four classes of routers that we need to be concerned with in this discussion:

Target Area Router
A router within the Target Area that runs Level 1 IS-IS. Some Target Area Routers may also run Level 2.
Area Leader
The Area Leader is a Target Area Router that is elected to represent the Level 1 area by injecting the Proxy LSP into the Level 2 link state database. The Area Leader runs Level 2 as well as Level 1. There may be multiple candidates for Area Leader, but only one is elected at a given time.
Area Edge Router
An Area Edge Router is a Target Area Router that also runs Level 2 and has at least one Level 2 interface outside of the Target Area.
Area Neighbor
An Area Neighbor is a Level 2 router that is outside of the Target Area that has an adjacency with an Area Edge Router.

The Area Leader has several responsibilities. First, it must inject Area Proxy System Identifier into the Level 1 link state database. Second, the Area Leader must generate the Proxy LSP for the Target Area.

All Area Edge Routers learn the Area Proxy System Identifier from the Level 1 link state database and use that as the system identifier in their Level 2 IS-IS Hello PDUs on interfaces outside the Target Area. Area Neighbors should then advertise an adjacency to the Area Proxy System Identifier. The Area Edge Routers MUST also maintain a Level 2 adjacency with the Area Leader, either via a direct link or via a tunnel.

Area Edge Routers MUST be able to provide transit to Level 2 traffic. We propose that the Area Edge Routers use Segment Routing (SR) and, during Level 2 SPF computation, use the SR forwarding path to reach the exit Area Edge Routers. To support SR, Area Edge Routers SHOULD advertise Adjacency Segment Identifiers for their adjacency to the Area Leader. Other mechanisms are possible and are a local decision.

2.1. Area Leader Election

The Area Leader is selected using the election mechanisms described in Dynamic Flooding for IS-IS.

2.2. LSP Generation

Each Area Edge Routers generates a Level 2 LSP that includes adjacencies to any Area Neighbors and the Area Leader. Unlike normal Level 2 operations, this LSP is not advertised outside of the Target Area and must be filtered by all Area Edge Routers to not be flooded outside of the Target Area.

The Area Leader uses the Level 2 LSPs generated by the Area Edge Routers to generate the Area Proxy LSP. This LSP is originated using the Area Proxy System Identifier and includes adjacencies for all of the Area Neighbors that have been advertised by the Area Edge Routers. Since the Area Neighbors also advertise an adjacency to the system identifier, this will result in a bi-directional adjacency. The Area Proxy LSP is the only LSP that is injected into the overall Level 2 link state database, with all other Level 2 LSPs from the Target Area being filtered out at the Target Area boundary.

2.3. Redundancy

If the Area Leader fails, another candidate may become Area Leader and MUST regenerate the Area Proxy LSP. The failure of the Area Leader is not visible outside of the area and appears to simply be an update of the Area Proxy LSP.

2.4. Level 2 SPF Considerations

When Level 2 systems outside of the Target Area perform an Level 2 SPF computation, they will use the Area Proxy LSP for computing a path transiting the Target Area. Because the Level 1 topology has been abstracted away, the cost for transiting the Target Area will be zero.

When Level 2 sytems inside of the Target Area perform a Level 2 computation, they must ignore the Area Proxy LSP. Further, because these systems do see the topology inside of the Target Area, the costs internal to the area are visible. This could lead to different and possibly inconsistent SPF results, potentially leading to forwarding loops.

To prevent this, the Level 2 systems within the Target Area must consider the metrics of links outside of the Target Area (inter-area metrics) separately from the metrics of links inside of the Target Area (intra-area metrics). Intra-area metrics as being less than any inter-area metric. Thus, if two paths have different total inter-area metrics, the path with the lower inter-area metric would be preferred, regardless of any intra-area metrics involved. However, if two paths have equal inter-area metrics, then the intra-area metrics would be used to compare the paths.

3. Area Proxy System Identifier TLV

The Area Proxy System Identifier TLV allows the Area Leader to advertise the existence of an Area Proxy System Identifier. This TLV is injected into the Area Leader's Level 1 LSP.

The format of the Area Proxy System Identifier TLV is:

    0                   1                   2       
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 
   | TLV Type      | TLV Length    |  Proxy SysID  |
   | Proxy System Identifier continued ...

4. Acknowledgements

The author would like to thank Bruno Decraene for his many helpful comments. The author would also like to thank a small group that wishes to remain anonymous for their valuable contributions.

5. IANA Considerations

This memo requests that IANA allocate and assign one code point from the IS-IS TLV Codepoints registry for the Area Pseudonode TLV.

6. Security Considerations

This document introduces no new security issues. Security of routing within a domain is already addressed as part of the routing protocols themselves. This document proposes no changes to those security architectures.

7. References

7.1. Normative References

[I-D.ietf-lsr-dynamic-flooding] Li, T., Psenak, P., Ginsberg, L., Chen, H., Przygienda, T., Cooper, D., Jalil, L. and S. Dontula, "Dynamic Flooding on Dense Graphs", Internet-Draft draft-ietf-lsr-dynamic-flooding-03, June 2019.
[I-D.ietf-spring-segment-routing] Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., Litkowski, S. and R. Shakir, "Segment Routing Architecture", Internet-Draft draft-ietf-spring-segment-routing-15, January 2018.
[ISO10589] International Organization for Standardization, "Intermediate System to Intermediate System Intra-Domain Routing Exchange Protocol for use in Conjunction with the Protocol for Providing the Connectionless-mode Network Service (ISO 8473)", ISO/IEC 10589:2002, Nov. 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

7.2. Informative References

[Clos] Clos, C., "A Study of Non-Blocking Switching Networks", The Bell System Technical Journal Vol. 32(2), DOI 10.1002/j.1538-7305.1953.tb01433.x, March 1953.

Author's Address

Tony Li Arista Networks 5453 Great America Parkway Santa Clara, California 95054 USA EMail: