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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   the Trust Legal Provisions and are provided without warranty as
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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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   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 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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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

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   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

   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

   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.  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 [I-D.ietf-lsr-dynamic-flooding].

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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

   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,

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   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 ...

      TLV Type: XXX

      TLV Length: 2 + (length of a system ID)

      Proxy System Identier: The area's Proty System Identifier, which
      is the length of a system identifier.  field.

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

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7.  References

7.1.  Normative References

              Li, T., Psenak, P., Ginsberg, L., Chen, H., Przygienda,
              T., Cooper, D., Jalil, L., and S. Dontula, "Dynamic
              Flooding on Dense Graphs", draft-ietf-lsr-dynamic-
              flooding-03 (work in progress), June 2019.

              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.

              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


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