Internet Engineering Task Force T. Li
Internet-Draft Arista Networks
Intended status: Standards Track December 7, 2018
Expires: June 10, 2019
Level 1 Area Abstraction for IS-IS
draft-li-lsr-isis-area-abstraction-00
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 topology into level 2.
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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 . . . . . . . . . . . . . . . . . . . . . 4
2.3. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 5
3. Area Pseudonode TLV . . . . . . . . . . . . . . . . . . . . . 5
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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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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 [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 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 [RFC2119].
2. Area Abstraction
We propose to completely abstract away the Level 2 topology of the
Level 1 area, 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.
We propose to implement Area Abstraction by having a Level 2
pseudonode that represents the entire Level 1 area. This is the only
LSP from the area that will be injected into the overall Level 2 link
state database.
There are three classes of routers that we need to be concerned with
in this discussion:
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Area Leader The Area Leader is a router in the Level 1 area that is
elected to represent the Level 1 area by injecting an LSP into the
Level 2 link state database.
Area Edge Router An Area Edge Router is a router that is part of the
Level 1 area as well as Level 2 and has at least one Level 2
interface outside of the Area.
Area Neighbor An Area Neighbor is a Level 2 router that is outside
of the Level 1 Area.
The Area Leader has several responsibilities. First, it must inject
a pseudonode identifier into the Level 1 link state database. This
is the Area Pseudonode Identifier. Second, the Area Leader must
generate the pseudonode LSP for the Area.
All Area Edge Routers learn the Area Pseudonode Identifier from the
Level 1 link state database and use that as the identifier in their
Level 2 IS-IS Hello PDUs on interfaces outside the Level 1 area.
Area Neighbors should then advertise an adjacency to the pseudonode.
The Area Edge Routers MUST also maintain an 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)
[I-D.ietf-spring-segment-routing] 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.
2.1. Area Leader Election
The Area Leader is selected using the election mechanisms described
in Dynamic Flooding for IS-IS [I-D.li-lsr-dynamic-flooding].
2.2. LSP Generation
For each of its Level 1 areas, Area Edge Routers generate 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 area and must be filtered by all Area Edge Routers to
not be flooded outside of the Level 1 Area.
The Area Leader uses the Level 2 LSPs generated by the Area Edge
Routers to generate the Area Pseudonode LSP. This LSP is originated
using the Area Pseudonode 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
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pseudonode, this will result in a bi-directional adjacency. The Area
Pseudonode LSP is the only LSP that is injected into the overall
Level 2 link state database, with all other Level 2 LSPs from the
area being filtered out at the area boundary.
2.3. Redundancy
If the Area Leader fails, another candidate may become Area Leader
and MUST regenerate the Area Pseudonode LSP. The failure of the Area
Leader is not visible outside of the area and appears to simply be an
update of the Area Pseudonode LSP.
3. Area Pseudonode TLV
The Area Pseudonode TLV allows the Area Leader to advertise the
existence of an Area Pseudonode Identifier. This TLV is injected
into one of the Area Leader's Level 1 LSPs.
The format of the Area Pseudonode 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 | Pseudonode ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudonode ID continued ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Type: XXX
TLV Length: 2 + (length of a system ID + 1)
Pseudonode ID: A pseudonode ID, which is the length of a system ID
plus one octet. field.
4. Acknowledgements
The author would like to thank Bruno Decraene for his many helpful
comments.
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.
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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-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[I-D.li-lsr-dynamic-flooding]
Li, T., Psenak, P., Ginsberg, L., Przygienda, T., Cooper,
D., Jalil, L., and S. Dontula, "Dynamic Flooding on Dense
Graphs", draft-li-lsr-dynamic-flooding-02 (work in
progress), December 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,
<https://www.rfc-editor.org/info/rfc2119>.
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,
<http://dx.doi.org/10.1002/j.1538-7305.1953.tb01433.x>.
Author's Address
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Tony Li
Arista Networks
5453 Great America Parkway
Santa Clara, California 95054
USA
Email: tony.li@tony.li
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