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Network Working Group                                      A. Przygienda
Internet-Draft                                                 C. Bowers
Intended status: Standards Track                                 Juniper
Expires: January 7, 2021                                          Y. Lee
                                                               A. Sharma
                                                                 Comcast
                                                                R. White
                                                                 Juniper
                                                            July 6, 2020


                         IS-IS Flood Reflection
                draft-ietf-lsr-isis-flood-reflection-00

Abstract

   This document describes an optional ISIS extension that allows the
   creation of IS-IS flood reflection topologies.  Flood reflection
   allows the creation of topologies where L1 areas provide transit
   forwarding for L2 destinations within an L2 topology.  It
   accomplishes this by creating L2 flood reflection adjacencies within
   each L1 area.  The L2 flood reflection adjacencies are used to flood
   L2 LSPDUs, and they are used in the L2 SPF computation.  However,
   they are not used for forwarding.  This arrangement gives the L2
   topology better scaling properties.  In addition, only those routers
   directly participating in flood reflection have to support the
   feature.  This allows for the incremental deployment of scalable L1
   transit areas in an existing network, without the necessity of
   upgrading other routers in the network.

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].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 7, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Description . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Further Details . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Flood Reflection TLV  . . . . . . . . . . . . . . . . . . . .   8
   4.  Flood Reflection Discovery Sub-TLV  . . . . . . . . . . . . .  10
   5.  Flood Reflection Adjacency Sub-TLV  . . . . . . . . . . . . .  10
   6.  Flood Reflection Discovery  . . . . . . . . . . . . . . . . .  11
   7.  Flood Reflection Adjacency Formation  . . . . . . . . . . . .  12
   8.  Redistribution of Prefixes  . . . . . . . . . . . . . . . . .  12
   9.  Route Computation . . . . . . . . . . . . . . . . . . . . . .  13
   10. Special Considerations  . . . . . . . . . . . . . . . . . . .  13
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  New IS-IS TLV Codepoint  . . . . . . . . . . . . . . . .  14
     11.2.  Sub TLVs for TLV 242 . . . . . . . . . . . . . . . . . .  14
     11.3.  Sub TLVs for TLV 22, 23, 25, 141, 222, and 223 . . . . .  15
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     14.1.  Informative References . . . . . . . . . . . . . . . . .  15
     14.2.  Normative References . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Description

   Due to the inherent properties of link-state protocols the number of
   IS-IS routers within a flooding domain is limited by processing and
   flooding overhead on each node.  While that number can be maximized



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   by well written implementations and techniques such as exponential
   back-offs, IS-IS will still reach a saturation point where no further
   routers can be added to a single flooding domain.  In some L2
   backbone deployment scenarios, this limit presents a significant
   challenge.

   The traditional approach to increasing the scale of an IS-IS
   deployement is to break it up into multiple L1 flooding domains and a
   single L2 backbone.  This works well for designs where an L2 backbone
   connects L1 access topologies, but it is limiting where a large L2 is
   supposed to span large number of routers.  In such scenarios, an
   alternative approach is to consider multiple L2 flooding domains
   connected together via L1 flooding domains.  In other words, L2
   flooding domains are connected by "L1/L2 lanes" through the L1 areas
   to form a single L2 backbone again.  Unfortunately, in its simplest
   implementation, this requires the inclusion of most, or all, of the
   transit L1 routers as L1/L2 to allow traffic to flow along optimal
   paths through such transit areas.  Consequently, this approach fails
   to reduce the number of L2 routers involved, so it fails to increase
   the scalability of the L2 backbone.































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  +----+  +-------+            +-------+               +-------+  +----+
  | R1 |  |  R10  +------------+  R20  +---------------+  R30  |  | R4 |
  | L2 +--+ L1/L2 |            |  L1   |               | L1/L2 +--+ L2 |
  |    |  |       +--------+ +-+       |  +------------+       |  |    |
  +----+  ++-+--+-+        | | +---+---+----------+    +-+--+-++  +----+
           | |  |          | | |   |   |  |       |      |  | |
           | |  |          | | |   |   |  |  +-----------+  | |
           | |  +-------+  | | |   |   |  |  |    |         | |
           | |          |  | | |   |   |  |  |    |  +------+ |
           | +------+ +--------+   |   +-------+  |  |        |
           |        | | |  | |     |      |  | |  |  |        |
  +----+  ++------+---+ |  +---+---+---+--+  | +-------+------++  +----+
  | R2 |  |  R11  | |   |    | |  R21  |     |    |  | |  R31  |  | R5 |
  | L2 +--+ L1/L2 +------------+  L1   +---------------+ L1/L2 +--+ L2 |
  |    |  |       | |   |    | |       |     |    |  | |       |  |    |
  +----+  ++------+---+ |    | +---+--++     | +-------+------++  +----+
           |        | | |    | |   |  |      | |  |  |        |
           | +---------------+ |   |  |      | |  |  |        |
           | |      | | |      |   |  |      | |  |  |        |
           | |  +--------------+   |  +-----------------+     |
           | |  |   | | |          |         | |  |  |  |     |
  +----+  ++-+--+-+ | | +------+---+---+-----+ |  |  | ++-----++  +----+
  | R3 |  |  R12  | +----------|  R22  |       |  +----+  R32  |  | R6 |
  | L2 +--+ L1/L2 |   +--------|  L1   +-------+     | | L1/L2 +--+ L2 |
  |    |  |       +------------+       |---------------+       |  |    |
  +----+  +-------+            +-------+-------------+ +-------+  +----+


                        Figure 1: Example topology

   Figure 1 is an example of a network where a topologically rich L1
   area is used to provide transit between six different L2-only routers
   (R1-R6).  Note that the six L2-only routers do not have connectivity
   to one another over L2 links.  To take advantage of the abundance of
   paths in the L1 transit area, all the intermediate systems could be
   placed into both L1 and L2, but this essentially combines the
   separate L2 flooding domains into a single one, triggering again
   maximum L2 scale limitation we try to address in first place.

   A more effective solution would allow to reduce the number of links
   and routers exposed in L2, while still utilizing the full L1 topology
   when forwarding through the network.

   [RFC8099] describes Topology Transparent Zones (TTZ) for OSPF.  The
   TTZ mechanism represents a group of OSPF routers as a full mesh of
   adjacencies between the routers at the edge of the group.  A similar
   mechanism could be applied to ISIS as well.  However, a full mesh of
   adjacencies between edge routers (or L1/L2 nodes) significantly



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   limits the scale of the topology.  The topology in Figure 1 has 6 L1/
   L2 nodes.  Figure 2 illustrates a full mesh of L2 adjacencies between
   the 6 L1/L2 nodes, resulting in (5 * 6)/2 = 15 L2 adjacencies.  In a
   somewhat larger topology containing 20 L1/L2 nodes, the number of L2
   adjacencies in a full mesh rises to 190.


  +----+  +-------+    +-------------------------------+-------+  +----+
  | R1 |  |  R10  |    |                               |  R30  |  | R4 |
  | L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 |
  |    |  |       |    |                               |       |  |    |
  +----+  ++-+-+--+-+  |                             +-+--+---++  +----+
           | | |    |  |                             |    |   |
           | +----------------------------------------------+ |
           |   |    |  |                             |    | | |
           |   +-----------------------------------+ |    | | |
           |        |  |                           | |    | | |
           |     +----------------------------------------+ | |
           |     |  |  |                           | |      | |
  +----+  ++-----+- |  |                           | | -----+-++  +----+
  | R2 |  |  R11  | |  |                           | | |  R31  |  | R5 |
  | L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 |
  |    |  |       | |  |                           | | |       |  |    |
  +----+  ++------+------------------------------+ | | +----+-++  +----+
           |        |  |                         | | |      | |
           |        |  |                         | | |      | |
           |    +-------------------------------------------+ |
           |    |   |  |                         | | |        |
           |    |   |  |                         +----------+ |
           |    |   |  |                           | |      | |
           |    |   |  |                           +-----+  | |
           |    |   |  |                             |   |  | |
  +----+  ++----+-+-+  |                             +-+-+--+-++  +----+
  | R3 |  |  R12  |    |      L2 adjacency             |  R32  |  | R6 |
  | L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 |
  |    |  |       |    |                               |       |  |    |
  +----+  +-------+----+                               +-------+  +----+


    Figure 2: Example topology represented in L2 with a full mesh of L2
                      adjacencies between L1/L2 nodes

   BGP, as specified in [RFC4271], faced a similar scaling problem,
   which has been solved in many networks by deploying BGP route
   reflectors [RFC4456].  We note that BGP route reflectors do not
   necessarily have to be in the forwarding path of the traffic.  This
   incongruity of forwarding and control path for BGP route reflectors




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   allows the control plane to scale independently of the forwarding
   plane.

   We propose here a similar solution for IS-IS.  A simple example of
   what a flood reflector control plane approach would look like is
   shown in Figure 3, where router R21 plays the role of a flood
   reflector.  Each L1/L2 ingress/egress router builds a tunnel to the
   flood reflector, and an L2 adjacency is built over each tunnel.  In
   this solution, we need only 6 L2 adjacencies, instead of the 15
   needed for a full mesh.  In a somewhat larger topology containing 20
   L1/L2 nodes, this solution requires only 20 L2 adjacencies, instead
   of the 190 need for a full mesh.  Multiple flood reflectors can be
   used, allowing the network operator to balance between resilience,
   path utilization, and state in the control plane.  The resulting L2
   adjacency scale is R*n, where R is the number of flood reflectors
   used and n is the number of L1/L2 nodes.  This compares quite
   favorably with n*(n-1)/2 L2 adjacencies required in a fully meshed L2
   solution.


  +----+  +-------+                                    +-------+  +----+
  | R1 |  |  R10  |                                    |  R30  |  | R4 |
  | L2 +--+ L1/L2 +--------------+   +-----------------+ L1/L2 +--+ L2 |
  |    |  |       |  L2 adj      |   |      L2 adj     |       |  |    |
  +----+  +-------+  over        |   |      over       +-------+  +----+
                     tunnel      |   |      tunnel
  +----+  +-------+           +--+---+--+              +-------+  +----+
  | R2 |  |  R11  |           |   R21   |              |  R31  |  | R5 |
  | L2 +--+ L1/L2 +-----------+  L1/L2  +--------------+ L1/L2 +--+ L2 |
  |    |  |       |  L2 adj   |  flood  |   L2 adj     |       |  |    |
  +----+  +-------+  over     |reflector|   over       +-------+  +----+
                     tunnel   +--+---+--+   tunnel
  +----+  +-------+              |   |                 +-------+  +----+
  | R3 |  |  R12  +--------------+   +-----------------+  R32  |  | R6 |
  | L2 +--+ L1/L2 |  L2 adj                 L2 adj     | L1/L2 +--+ L2 |
  |    |  |       |  over                   over       |       |  |    |
  +----+  +-------+  tunnel                 tunnel     +-------+  +----+


   Figure 3: Example topology represented in L2 with L2 adjacencies from
                each L1/L2 node to a single flood reflector

   As illustrated in Figure 3, when R21 plays the role of flood
   reflector, it provides L2 connectivity among all of the previously
   disconnected L2 islands by reflooding all L2 LSPDUs.  At the same
   time, R20 and R22 remain L1-only routers.  L1-only routers and
   L1-only links are not visible in L2.  In this manner, the flood




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   reflector allows us provide L2 control plane connectivity in a
   scalable manner.

   As described so far, the solution illustrated in Figure 3 relies only
   on currently standardized ISIS functionality.  Without new
   functionality, however, the data traffic will traverse only R21.
   This will unnecessarily create a bottleneck at R21 since there is
   still available capacity in the paths crossing the L1-only routers
   R20 and R22.

   Hence, some new functionality is necessary to allow the L1/L2 edge
   nodes (R10-12 and R30-32 in Figure 3) to recognize that the L2
   adjacency to R21 should not be used for forwarding.  The L1/L2 edge
   nodes should forward traffic that would normally be forwarded over
   the L2 adjacency to R21 over L1 links instead.  This would allow the
   forwarding within the L1 area to use the L1-only nodes and links
   shown in Figure 1 as well.  It allows networks to be built that use
   the entire forwarding capacity of the L1 areas, while at the same
   time introducing control plane scaling benefits provided by L2 flood
   reflectors.

   This document defines all extensions necessary to support flood
   reflector deployment:

   o  A 'flood reflector adjacency' for all the adjacencies built for
      the purpose of reflecting flooding information.  This allows these
      'flood reflectors' to participate in the IS-IS control plane
      without being used in the forwarding plane.  This is a purely
      local operation on the L1/L2 ingress; it does not require
      replacing or modifying any routers not involved in the reflection
      process.  Deployment-wise, it is far less tricky to just upgrade
      the routers involved in flood reflection rather than have a flag
      day on the whole ISIS domain.

   o  A full mesh of L1 tunnels between the L1/L2 routers, ideally load-
      balancing across all available L1 links.  This harnesses all
      forwarding paths between the L1/L2 edge nodes without injecting
      unneeded state into the L2 flooding domain or creating 'choke
      points' at the 'flood reflectors' themselves.  A solution without
      tunnels is also possible by judicious scoping of reachability
      information between the levels.

   o  Some way to support reflector redundancy, and potentially some way
      to auto-discover and advertise such adjacencies as flood reflector
      adjacencies.  Such advertisements may allow L2 nodes outside the
      L1 to perform optimizations in the future based on this
      information.




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2.  Further Details

   Several considerations should be noted in relation to such a flood
   reflection mechanism.

   First, this allows multi-area IS-IS deployments to scale without any
   major modifications in the IS-IS implementation on most of the nodes
   deployed in the network.  Unmodified (traditional) L2 routers will
   compute reachability across the transit L1 area using the flood
   reflector adjacencies.

   Second, the flood reflectors are not required to participate in
   forwarding traffic through the L1 transit area.  These flood
   reflectors can be hosted on virtual devices outside the forwarding
   topology.

   Third, astute readers will realize that flooding reflection may cause
   the use of suboptimal paths.  This is similar to the BGP route
   reflection suboptimal routing problem described in
   [ID.draft-ietf-idr-bgp-optimal-route-reflection-19].  The L2
   computation determines the egress L1/L2 and with that can create
   illusions of ECMP where there is none.  And in certain scenarios lead
   to an L1/L2 egress which is not globally optimal.  This represents a
   straightforward instance of the trade-off between the amount of
   control plane state and the optimal use of paths through the network
   often encountered when aggregating routing information.

   One possible solution to this problem is to expose additional
   topology information into the L2 flooding domains.  In the example
   network given, links from router 01 to router 02 can be exposed into
   L2 even when 01 and 02 are participating in flood reflection.  This
   information would allow the L2 nodes to build 'shortcuts' when the L2
   flood reflected part of the topology looks more expensive to cross
   distance wise.

   Another possible variation is for an implementation to approximate
   with the L1 tunnel cost the cost of the underlying topology.

   Redundancy can be achieved by building multiple flood reflectors in
   the L1 area.  Multiple flood reflectors do not need any
   synchronization mechanisms amongst themselves, except standard ISIS
   flooding and database maintenance procedures.

3.  Flood Reflection TLV

   The Flood Reflection TLV is a new top-level TLV that MAY appear in
   IIHs.  The Flood Reflection TLV indicates the flood reflector cluster
   (based on Flood Reflection Cluster ID) that a given router is



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   configured to participate in.  It also indicates whether the router
   is configured to play the role of either flood reflector or flood
   reflector client.  The Flood Reflection Cluster ID and flood
   reflector roles advertised in the IIHs are used to ensure that flood
   reflector adjacencies are only formed between a flood reflector and
   flood reflector client, and that the Flood Reflection Cluster IDs
   match.  The Flood Reflection TLV has the following format:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |C|  RESERVED   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Flood Reflection Cluster ID                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Sub-TLVs ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type:  TBD

   Length:  The length, in octets, of the following fields.

   C (Client):  This bit is set to indicate that the router acts as a
      flood reflector client.  When this bit is NOT set, the router acts
      as a flood reflector.  On a given router, the same value of the
      C-bit MUST be advertised across all interfaces advertising the
      Flood Reflection TLV in IIHs.

   RESERVED:  This field is reserved for future use.  It MUST be set to
      0 when sent and MUST be ignored when received.

   Flood Reflection Cluster ID:  Flood Reflection Cluster Identifier.
      These same 32-bit value MUST be assigned to all of the flood
      reflectors and flood reflector clients in the L1 area.  The value
      MUST be unique across different L1 areas within the IGP domain.
      On a given router, the same value of the Flood Reflection Cluster
      ID MUST be advertised across all interfaces advertising the Flood
      Reflection TLV in IIHs.

   Sub-TLVs:  Optional sub-TLVs.  For future extensibility, the format
      of the Flood Reflection TLV allows for the possibility of
      including optional sub-TLVs.  No sub-TLVs of the Flood Reflection
      TLV are defined in this document.






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   The Flood Reflection TLV MUST NOT appear more than once in an IIH.  A
   router receiving multiple Flood Reflection TLVs in the same IIH
   SHOULD use the values in the first TLV.

4.  Flood Reflection Discovery Sub-TLV

   Flood Reflection Discovery sub-TLV is advertised as a sub-TLV of the
   IS-IS Router Capability TLV-242, defined in [RFC7981].  The Flood
   Reflection Discovery sub-TLV is advertised in L1 LSPs with area
   flooding scope in order to enable the auto-discovery of flood
   reflection capabilities and the automatic creation of L2 tunnels to
   be used as flood reflector adjacencies.  The Flood Reflection
   Discovery sub-TLV has the following format:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |C|  Reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Flood Reflection Cluster ID                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type:  TBD

   Length:  The length, in octets, of the following fields.

   C (Client):  This bit is set to indicate that the router acts as a
      flood reflector client.  When this bit is NOT set, the router acts
      as a flood reflector.

   RESERVED:  This field is reserved for future use.  It MUST be set to
      0 when sent and MUST be ignored when received.

   Flood Reflection Cluster ID:  The Flood Reflection Cluster Identifier
      is the same as that defined in the Flood Reflection TLV.

   The Flood Reflection Discovery sub-TLV MUST NOT appear more than once
   in TLV 242.  A router receiving multiple Flood Reflection Discovery
   sub-TLVs in TLV 242 SHOULD use the values in the first sub-TLV.

5.  Flood Reflection Adjacency Sub-TLV

   The Flood Reflection Adjacency sub-TLV is advertised as a sub-TLV of
   TLVs 22, 23, 25, 141, 222, and 223.  Its presence indicates that a
   given adjacency is a flood reflector adjacency.  It is included in L2




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   area scope flooded LSPs.  Flood Reflection Adjacency sub-TLV has the
   following format:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |C|  Reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Flood Reflection Cluster ID                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type:  TBD

   Length:  The length, in octets, of the following fields.

   C (Client):  This bit is set to indicate that the router advertising
      this adjacency is a flood reflector client.  When this bit is NOT
      set, the router advertising this adjacency is a flood reflector.

   RESERVED:  This field is reserved for future use.  It MUST be set to
      0 when sent and MUST be ignored when received.

   Flood Reflection Cluster ID:  The Flood Reflection Cluster Identifier
      is the same as that defined in the Flood Reflection TLV.

   The Flood Reflection Adjacency sub-TLV MUST NOT appear more than once
   in a given TLV.  A router receiving multiple Flood Reflection
   Adjacency sub-TLVs in a TLV SHOULD use the values in the first sub-
   TLV.

6.  Flood Reflection Discovery

   A router participating in flood reflection MUST be configured as an
   L1/L2 router.  It originates the Flood Reflection Discovery sub-TLV
   with area flooding scope in L1 only.  Normally, all routers on the
   edge of the L1 area (those having traditional L2 adjacencies) will
   advertise themselves as route reflector clients.  Therefore, a flood
   reflector client will have both traditional L2 adjacencies and flood
   reflector L2 adjacencies.

   A router acting as a flood reflector MUST NOT have any traditional L2
   adjacencies.  It will be an L1/L2 router only by virtue of having
   flood reflector L2 adjacencies.  A router desiring to act as a flood
   reflector will advertise itself as such using the Flood Reflection
   Discovery sub-TLV in L1.




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   A given flood reflector or flood reflector client can only
   participate in a single cluster, as determined by the value of its
   Flood Reflection Cluster ID.

   Upon reception of Flood Reflection Discovery sub-TLVs, a router
   acting as flood reflector client MUST initiate a tunnel towards each
   flood reflector with which it shares an Flood Reflection Cluster ID.
   The L2 adjacencies formed over such tunnels MUST be marked as flood
   reflector adjacencies.  If the client has a direct L2 adjacency with
   the flood reflector it SHOULD use it instead of instantiating a new
   tunnel.

   Upon reception of Flood Reflection Discover TLVs, a router acting as
   a flood reflector client MAY initiate tunnels with L1-only
   adjacencies towards all the other flood reflector clients in its
   cluster.  These tunnels MAY be used for forwarding to improve the
   load-balancing characteristics of the L1 area.

7.  Flood Reflection Adjacency Formation

   In order to simplify both implementations and network deployments, we
   do not allow the formation of complex hierarchies of flood reflectors
   and clients.  All flood reflectors and flood reflector clients in the
   same L1 area MUST share the same Flood Reflector Cluster ID.  A flood
   reflector MUST only form flood reflection adjacencies with flood
   reflector clients.  A flood reflector MUST NOT form any traditional
   L2 adjacencies.  Flood reflector clients MUST only form flood
   reflection adjacencies with flood reflectors.  Flood reflector
   clients may form traditional L2 adjacencies with flood reflector
   clients or nodes not participating in flood reflection.

   The Flood Reflector Cluster ID and flood reflector roles advertised
   in the Flood Reflection TLVs in IIHs are used to ensure that flood
   reflection adjacencies that are established meet the above criteria.

   Once a flood reflection adjacency is established, the flood reflector
   and the flood reflector client MUST advertise the adjacency by
   including the Flood Reflection Adjacency Sub-TLV in the Extended IS
   reachability TLV or MT-ISN TLV.

8.  Redistribution of Prefixes

   In some scenarios, L2 prefixes need to be redistributed into L1 by
   the route reflector clients.  However, if a L1 area edge router
   doesn't have any L2 flood reflector adjacencies, then it cannot be
   the shortest path egress in the L2 topology.  Therefore, flood
   reflector client SHOULD only redistribute L2 prefixes into L1 if it
   has an L2 flood reflector adjacency.  The L2 prefixes advertisements



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   redistributed into L1 SHOULD be normally limited to L2 intra-area
   routes (as defined in [RFC7775]), if the information exists to
   distinguish them from other L2 prefix advertisements.

   On the other hand, in topologies that make use of flood reflection to
   hide the structure of L1 areas while still providing transit
   forwarding across them, we generally do not need to redistribute L1
   prefixes advertisements into L2.

   In deployment scenarios where L1 tunnels are not used, all L1/L2 edge
   nodes MUST be flood reflector clients.

9.  Route Computation

   To ensure loop-free routing, the route reflection client MUST follow
   the normal L2 computation to determine L2 routes.  This is because
   nodes outside the L1 area will generally not be aware that flood
   reflection is being performed.  The flood reflection clients need to
   produce the same result for the L2 route computation as a router not
   participating in flood reflection.  However, a flood reflector client
   will not necessarily use a given L2 route for forwarding.  For an L2
   route that uses a flood reflection adjacency as a next-hop, the flood
   reflection client may use the next-hop from an L1 route instead.

   On the reflection client, after L2 and L1 computation, all flood
   reflector adjacencies used as next-hops for L2 routes MUST be
   examined and replaced with the correct L1 tunnel next-hop to the
   egress.  Alternatively, if the ingress has adequate reachability
   information to ensure forwarding towards destination via L1 routes,
   L2 routes using flood reflector adjacencies as next-hops can be
   omitted entirely.  Due to the rules in Section 7 the computation in
   the resulting topology is relatively simple, the L2 SPF from a flood
   reflector client is guaranteed to reach within a hop the Flood
   Reflector and in the following hop the L2 egress to which it has a L1
   forwarding tunnel.  However, if the topology has L2 paths which are
   not route reflected and look "shorter" than the path through the
   Flood Reflector then the computation will have to track the egress
   out of the L1 domain by a more advanced algorithm.

10.  Special Considerations

   In pathological cases setting the overload bit in L1 (but not in L2)
   can partition L1 forwarding, while allowing L2 reachability through
   flood reflector adjacencies to exist.  In such a case a node cannot
   replace a route through a flood reflector adjacency with a L1
   shortcut and the client can use the L2 tunnel to the flood reflector
   for forwarding while it MUST initiate an alarm and declare
   misconfiguration.



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   A flood reflector with directly L2 attached prefixes should advertise
   those in L1 as well since based on preference of L1 routes the
   clients will not try to use the L2 flood reflector adjacency to route
   the packet towards them.  A very, very corner case is when the flood
   reflector is reachable via L2 flood reflector adjacency (due to
   underlying L1 partition) only in which case the client can use the L2
   tunnel to the flood reflector for forwarding towards those prefixes
   while it MUST initiate an alarm and declare misconfiguration.

   Instead of modifying the computation procedures one could imagine a
   flood reflector solution where the Flood Reflector would re-advertise
   the L2 prefixes with a 'third-party' next-hop but that would have
   less desirable convergence properties than the solution proposed and
   force a fork-lift of all L2 routers to make sure they disregard such
   prefixes unless in the same L1 domain as the Flood Reflector.

   Depending on pseudo-node choice in case of a broadcast domain with
   multiple flood reflectors attached this can lead to a partitioned LAN
   and hence a router discovering such a condition MUST initiate an
   alarm and declare misconfiguration.

11.  IANA Considerations

   This document requests allocation for the following IS-IS TLVs and
   Sub-TLVs.

11.1.  New IS-IS TLV Codepoint

   This document requests the following IS-IS TLV:

   Value Name                              IIH LSP SNP Purge
   ----- --------------------------------- --- --- --- -----
   TBD1  Flood Reflection                   y   n   n   n


11.2.  Sub TLVs for TLV 242

   This document request the following registration in the "sub-TLVs for
   TLV 242" registry.

   Type  Description
   ----  -----------
   TBD2  Flood Reflection Discovery








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11.3.  Sub TLVs for TLV 22, 23, 25, 141, 222, and 223

   This document requests the following registration in the "sub-TLVs
   for TLV 22, 23, 25, 141, 222, and 223" registry.

   Type  Description                       22  23  25  141 222 223
   ----  --------------------------------  --- --- --- --- --- ---
   TBD3  Flood Reflector Adjacency          y   y  y(s) y   y   y


12.  Security Considerations

   This document introduces no new security concerns to ISIS or other
   specifications referenced in this document.

13.  Acknowledgements

   The authors thank Shraddha Hegde, Peter Psenak, and Les Ginsberg for
   their thorough review and detailed discussions.

14.  References

14.1.  Informative References

   [ID.draft-ietf-idr-bgp-optimal-route-reflection-19]
              Raszuk et al., R., "BGP Optimal Route Reflection", July
              2019.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.

   [RFC8099]  Chen, H., Li, R., Retana, A., Yang, Y., and Z. Liu, "OSPF
              Topology-Transparent Zone", RFC 8099,
              DOI 10.17487/RFC8099, February 2017,
              <https://www.rfc-editor.org/info/rfc8099>.

14.2.  Normative References







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   [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>.

   [RFC7775]  Ginsberg, L., Litkowski, S., and S. Previdi, "IS-IS Route
              Preference for Extended IP and IPv6 Reachability",
              RFC 7775, DOI 10.17487/RFC7775, February 2016,
              <https://www.rfc-editor.org/info/rfc7775>.

   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,
              <https://www.rfc-editor.org/info/rfc7981>.

Authors' Addresses

   Tony Przygienda
   Juniper
   1137 Innovation Way

   Sunnyvale, CA

   USA


   Email: prz@juniper.net


   Chris Bowers
   Juniper
   1137 Innovation Way

   Sunnyvale, CA

   USA


   Email: cbowers@juniper.net


   Yiu Lee
   Comcast
   1800 Bishops Gate Blvd
   Mount Laurel, NJ  08054
   US

   Email: Yiu_Lee@comcast.com



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   Alankar Sharma
   Comcast
   1800 Bishops Gate Blvd
   Mount Laurel, NJ  08054
   US

   Email: Alankar_Sharma@comcast.com


   Russ White
   Juniper
   1137 Innovation Way

   Sunnyvale, CA

   USA


   Email: russw@juniper.net
































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