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IS-IS Routing for Spine-Leaf Topology

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Naiming Shen , Les Ginsberg , Sanjay Thyamagundalu
Last updated 2019-03-08
Replaces draft-shen-isis-spine-leaf-ext
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Networking Working Group                                         N. Shen
Internet-Draft                                               L. Ginsberg
Intended status: Standards Track                           Cisco Systems
Expires: September 9, 2019                              S. Thyamagundalu
                                                           March 8, 2019

                 IS-IS Routing for Spine-Leaf Topology


   This document describes a mechanism for routers and switches in a
   Spine-Leaf type topology to have non-reciprocal Intermediate System
   to Intermediate System (IS-IS) routing relationships between the
   leafs and spines.  The leaf nodes do not need to have the topology
   information of other nodes and exact prefixes in the network.  This
   extension also has application in the Internet of Things (IoT).

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
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   This Internet-Draft will expire on September 9, 2019.

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   Copyright (c) 2019 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
   ( 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Motivations . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Spine-Leaf (SL) Extension . . . . . . . . . . . . . . . . . .   4
     3.1.  Topology Examples . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Applicability Statement . . . . . . . . . . . . . . . . .   5
     3.3.  Spine-Leaf TLVs . . . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Spine-Leaf TLV  . . . . . . . . . . . . . . . . . . .   6
       3.3.2.  Leaf-Set TLV  . . . . . . . . . . . . . . . . . . . .   7  Leaf-Set Sub-TLVs . . . . . . . . . . . . . . . .   7
       3.3.3.  Advertising IPv4/IPv6 Reachability  . . . . . . . . .   8
       3.3.4.  Advertising Connection to RF-Leaf Node  . . . . . . .   8
     3.4.  Mechanism . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.4.1.  Pure CLOS Topology  . . . . . . . . . . . . . . . . .  10
     3.5.  Implementation and Operation  . . . . . . . . . . . . . .  11
       3.5.1.  CSNP PDU  . . . . . . . . . . . . . . . . . . . . . .  11
       3.5.2.  Leaf to Leaf connection . . . . . . . . . . . . . . .  12  Local traffic only  . . . . . . . . . . . . . . .  12  Transit traffic allowed . . . . . . . . . . . . .  12
       3.5.3.  Spine Node Hostname . . . . . . . . . . . . . . . . .  13
       3.5.4.  IS-IS Reverse Metric  . . . . . . . . . . . . . . . .  13
       3.5.5.  Spine-Leaf Traffic Engineering  . . . . . . . . . . .  13
       3.5.6.  Other End-to-End Services . . . . . . . . . . . . . .  13
       3.5.7.  Address Family and Topology . . . . . . . . . . . . .  14
       3.5.8.  Migration . . . . . . . . . . . . . . . . . . . . . .  14
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The IS-IS routing protocol defined by [ISO10589] has been widely
   deployed in provider networks, data centers and enterprise campus
   environments.  In the data center and enterprise switching networks,
   a Spine-Leaf topology is commonly used.  This document describes a
   mechanism where IS-IS routing can be optimized for a Spine-Leaf

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   In a Spine-Leaf topology, normally a leaf node connects to a number
   of spine nodes.  Data traffic going from one leaf node to another
   leaf node needs to pass through one of the spine nodes.  Also, the
   decision to choose one of the spine nodes is usually part of equal
   cost multi-path (ECMP) load sharing.  The spine nodes can be
   considered as gateway devices to reach destinations on other leaf
   nodes.  In this type of topology, the spine nodes have to know the
   topology and routing information of the entire network, but the leaf
   nodes only need to know how to reach the gateway devices to which are
   the spine nodes they are uplinked.

   This document describes the IS-IS Spine-Leaf extension that allows
   the spine nodes to have all the topology and routing information,
   while keeping the leaf nodes free of topology information other than
   the default gateway routing information.  The leaf nodes do not even
   need to run a Shortest Path First (SPF) calculation since they have
   no topology information.

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.  Motivations

   o  The leaf nodes in a Spine-Leaf topology do not require complete
      topology and routing information of the entire domain since their
      forwarding decision is to use ECMP with spine nodes as default

   o  The spine nodes in a Spine-Leaf topology are richly connected to
      leaf nodes, which introduces significant flooding duplication if
      they flood all Link State PDUs (LSPs) to all the leaf nodes.  It
      saves both spine and leaf nodes' CPU and link bandwidth resources
      if flooding is blocked to leaf nodes.  For small Top of the Rack
      (ToR) leaf switches in data centers, it is meaningful to prevent
      full topology routing information and massive database flooding
      through those devices.

   o  When a spine node advertises a topology change, every leaf node
      connected to it will flood the update to all the other spine
      nodes, and those spine nodes will further flood them to all the
      leaf nodes, causing a O(n^2) flooding storm which is largely

   o  Similar to some of the overlay technologies which are popular in
      data centers, the edge devices (leaf nodes) may not need to

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      contain all the routing and forwarding information on the device's
      control and forwarding planes.  "Conversational Learning" can be
      utilized to get the specific routing and forwarding information in
      the case of pure CLOS topology and in the events of link and node

   o  Small devices and appliances of Internet of Things (IoT) can be
      considered as leafs in the routing topology sense.  They have CPU
      and memory constrains in design, and those IoT devices do not have
      to know the exact network topology and prefixes as long as there
      are ways to reach the cloud servers or other devices.

3.  Spine-Leaf (SL) Extension

3.1.  Topology Examples

             +--------+    +--------+             +--------+
             |        |    |        |             |        |
             | Spine1 +----+ Spine2 +- ......... -+ SpineN |
             |        |    |        |             |        |
             +-+-+-+-++    ++-+-+-+-+             +-+-+-+-++
        +------+ | | |      | | | |                 | | | |
        |  +-----|-|-|------+ | | |                 | | | |
        |  |  +--|-|-|--------+-|-|-----------------+ | | |
        |  |  |  | | |    +---+ | |                   | | |
        |  |  |  | | |    |  +--|-|-------------------+ | |
        |  |  |  | | |    |  |  | |              +------+ +----+
        |  |  |  | | |    |  |  | +--------------|----------+  |
        |  |  |  | | |    |  |  +-------------+  |          |  |
        |  |  |  | | +----|--|----------------|--|--------+ |  |
        |  |  |  | +------|--|--------------+ |  |        | |  |
        |  |  |  +------+ |  |              | |  |        | |  |
       ++--+--++      +-+-+--++            ++-+--+-+     ++-+--+-+
       | Leaf1 |~~~~~~| Leaf2 |  ........  | LeafX |     | LeafY |
       +-------+      +-------+            +-------+     +-------+

                      Figure 1: A Spine-Leaf Topology

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                 +---------+             +--------+
                 | Spine1  |             | Spine2 |
                 +-+-+-+-+-+             +-+-+-+-++
                   | | | |                 | | | |
                   | | | +-----------------|-|-|-|-+
                   | | +------------+      | | | | |
          +--------+ +-+            |      | | | | |
          |   +----------------------------+ | | | |
          |   |        |  +------------------+ | +----+
          |   |        |  |         |  +-------+   |  |
          |   |        |  |         |  |           |  |
        +-+---+-+   +--+--+-+     +-+--+--+     +--+--+-+
        | Leaf1 |   | Leaf2 |     | Leaf3 |     | Leaf4 |
        +-------+   +-------+     +-------+     +-------+

                         Figure 2: A CLOS Topology

3.2.  Applicability Statement

   This extension assumes the network is a Spine-Leaf topology, and it
   should not be applied in an arbitrary network setup.  The spine nodes
   can be viewed as the aggregation layer of the network, and the leaf
   nodes as the access layer of the network.  The leaf nodes use a load
   sharing algorithm with spine nodes as nexthops in routing and

   This extension works when the spine nodes are inter-connected, and it
   works with a pure CLOS or Fat Tree topology based network where the
   spines are NOT horizontally interconnected.

   Although the example diagram in Figure 1 shows a fully meshed Spine-
   Leaf topology, this extension also works in the case where they are
   partially meshed.  For instance, leaf1 through leaf10 may be fully
   meshed with spine1 through spine5 while leaf11 through leaf20 is
   fully meshed with spine4 through spine8, and all the spines are
   inter-connected in a redundant fashion.

   This extension can also work in multi-level spine-leaf topology.  The
   lower level spine node can be a 'leaf' node to the upper level spine
   node.  A spine-leaf 'Tier' can be exchanged with IS-IS hello packets
   to allow tier X to be connected with tier X+1 using this extension.
   Normally tier-0 will be the TOR routers and switches if provisioned.

   This extension also works with normal IS-IS routing in a topology
   with more than two layers of spine and leaf.  For instance, in
   example diagrams Figure 1 and Figure 2, there can be another Core
   layer of routers/switches on top of the aggregation layer.  From an
   IS-IS routing point of view, the Core nodes are not affected by this

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   extension and will have the complete topology and routing information
   just like the spine nodes.  To make the network even more scalable,
   the Core layer can operate as a level-2 IS-IS sub-domain while the
   Spine and Leaf layers operate as stays at the level-1 IS-IS domain.

   This extension assumes the link between the spine and leaf nodes are
   point-to-point, or point-to-point over LAN [RFC5309].  The links
   connecting among the spine nodes or the links between the leaf nodes
   can be any type.

3.3.  Spine-Leaf TLVs

   This extension introduces two new TLVs, the Spine-Leaf TLV and the
   Leaf-Set TLV.  The Spine-Leaf TLV may be advertised in IS-IS Hello
   (IIH) PDUs; the Leaf-Set TLV may be advertised in IS-IS Circuit
   Scoped Link State PDUs (CS-LSP) [RFC7356].  They are used by both
   spine and leaf nodes in this Spine-Leaf mechanism.

3.3.1.  Spine-Leaf TLV

       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    |            SL Flag            |

   The fields of this TLV are defined as follows:

      Type:    1 octet Suggested value 151 (to be assigned by IANA)

      Length:  1 octet (2 + length of sub-TLVs).

      SL Flags:  16 bits

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      |  Tier |     Reserved    |T|R|L|

         Tier:    A value from 0 to 15.  It represents the spine-leaf
                  tier level.  The value 15 is reserved to indicate the
                  tier level is unknown.  This value is only valid when
                  the 'T' bit (see below) is set.  If the 'T' bit is

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                  clear, this value MUST be set to zero on transmission,
                  and it MUST be ignored on receipt.

         L bit (0x01):  Only leaf node sets this bit.  If the L bit is
                  set in the SL flag, the node indicates it is in 'Leaf-

         R bit (0x02):  Only Spine node sets this bit.  If the R bit is
                  set, the node indicates to the leaf neighbor that it
                  can be used as the default route gateway.

         T bit (0x04):  If set, the value in the "Tier" field (see
                  above) is valid.

3.3.2.  Leaf-Set TLV

       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    |      .. Optional Sub-TLVs

   The Type is suggested value of 152 (to be assigned by IANA).  This
   TLV and associated Sub-TLVs MAY appear in CS-LSP PDUs.  Multiple TLVs
   MAY be sent.  Leaf-Set Sub-TLVs

   If the data center topology is a pure CLOS or Fat Tree, there are no
   link connections among the spine nodes.  If we also assume there is
   not another Core layer on top of the aggregation layer, then the
   traffic from one leaf node to another may have a problem if there is
   a link outage between a spine node and a leaf node.  For instance, in
   the diagram of Figure 2, if Leaf1 sends data traffic to Leaf3 through
   Spine1 node, and the Spine1-Leaf3 link is down, the data traffic will
   be dropped on the Spine1 node.

   To address this issue spine and leaf nodes may use the sub-TLVs
   defined below to obtain more specific reachability information.

   Two Leaf-Set sub-TLVs are defined.  The Leaf-Neighbors sub-TLV and
   the Reachability-Req sub-TLV.  Leaf-Neighbors Sub-TLV

   This sub-TLV is used by spine nodes to advertise the current set of
   Leaf neighbors to Leaf nodes.  The fields of this sub-TLV are defined
   as follows:

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      Type:    1 octet Suggested value 1 (to be assigned by IANA)

      Length:  1 octet MUST be a multiple of 6 octets.

      Leaf-Neighbors  A list of IS-IS System-IDs of the leaf node
               neighbors of this spine node.  Reachability-Req Sub-TLV

   This sub-TLV is used by leaf nodes to request the advertisement of
   more specific prefix information from one or more selected spine
   node(s).  The list of leaf nodes in this sub-TLV reflects the current
   set of leaf-nodes for which not all spine node neighbors have
   indicated the presence of connectivity in the Leaf-Neighbors sub-TLV
   (See Section  The fields of this sub-TLV are defined as

      Type:    1 octet Suggested value 2 (to be assigned by IANA)

      Length:  1 octet.  It MUST be a multiple of 6 octets.

      Leaf Nodes  List of IS-IS System-IDs of leaf nodes for which
               reachability information is being requested.

3.3.3.  Advertising IPv4/IPv6 Reachability

   In cases where connectivity between a leaf node and a spine node is
   down, the leaf node MAY request reachability information from a spine
   node as described in Section  The spine node utilizes TLVs
   135 [RFC5305] and TLVs 236 [RFC5308] to advertise this information.
   These TLVs MAY be included in CS-LSPs [RFC7356] sent from the spine
   to the requesting leaf node.

3.3.4.  Advertising Connection to RF-Leaf Node

   For links between Spine and Leaf Nodes on which the Spine Node has
   set the R-bit and the Leaf node has set the L-bit in their respective
   Spine-Leaf TLVs, spine nodes MAY advertise the link with a bit in the
   "link-attribute" sub-TLV [RFC5029] to indicate that this link is not
   used for LSP flooding.  This bit is named the Connect-to-RF-Leaf Node
   bit.  This information can be used by nodes computing a flooding
   topology e.g., [DYNAMIC-FLOODING], to exclude the RF-Leaf nodes from
   the computed flooding topology.

   For links between Spine and Leaf Nodes on which the Spine Node has
   set the R-bit and the Leaf node has set the L-bit in their respective

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   Spine-Leaf TLVs, leaf nodes MAY advertise the link with a bit in the
   "link-attribute" sub-TLV [RFC5029] to indicate that this link is to a
   Spine Node neighbor.  This bit is named the Connect-to-RF-Spine Node
   bit.  This information can be used by leaf nodes when deciding
   whether a leaf to leaf link can be used as an alternate default path
   when a leaf node has no connectivity to any spines.  See
   Section 3.5.2.

3.4.  Mechanism

   Leaf nodes in a spine-leaf application using this extension are
   provisioned with two attributes:

   1)Tier level of 0.  This indicates the node is a Leaf Node.  The
   value 0 is advertised in the Tier field of Spine-Leaf TLV defined

   2)Flooding reduction enabled/disabled.  If flooding reduction is
   enabled the L-bit is set to one in the Spine-Leaf TLV defined above

   A spine node does not need explicit configuration.  Spine nodes can
   dynamically discover their tier level by computing the number of hops
   to a leaf node.  Until a spine node determines its tier level it MUST
   advertise level 15 (unknown tier level) in the Spine-Leaf TLV defined
   above.  Each tier level can also be statically provisioned on the

   When a spine node receives an IIH which includes the Spine-Leaf TLV
   with Tier level 0 and 'L' bit set, it labels the point-to-point
   interface and adjacency to be a 'Reduced Flooding Leaf-Peer (RF-
   Leaf)'.  IIHs sent by a spine node on a link to an RF-Leaf include
   the Spine-Leaf TLV with the 'R' bit set in the flags field.  The 'R'
   bit indicates to the RF-Leaf neighbor that the spine node can be used
   as a default routing nexthop.

   There is no change to the IS-IS adjacency bring-up mechanism for
   Spine-Leaf peers.

   A spine node blocks LSP flooding to RF-Leaf adjacencies, except for
   the LSP PDUs in which the IS-IS System-ID matches the System-ID of
   the RF-Leaf neighbor.  This exception is needed since when the leaf
   node reboots, the spine node needs to forward to the leaf node non-
   purged LSPs from the RF-Leaf's previous incarnation.

   Leaf nodes will perform IS-IS LSP flooding as normal to send the LSPs
   over all of its IS-IS adjacencies.  In the case of RF-Leafs only
   self-originated LSPs will exist in its LSP database, and in the case

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   of leaf-leaf connections, there will be neighbor leaf nodes LSPs in
   the LSP database in addition to the self-originated LSPs.

   Spine nodes will receive all the LSP PDUs in the network, including
   all the spine nodes and leaf nodes.  It will perform Shortest Path
   First (SPF) as a normal IS-IS node does.  There is no change to the
   route calculation and forwarding on the spine nodes.

   The LSPs of a node only floods north bound towards the upper layer
   spine nodes.  The default route is generated with loadsharing also
   towards the upper layer spine nodes.

   RF-Leaf nodes do not have any LSP in the network except for its own.
   Therefore there is no need to perform SPF calculation on the RF-Leaf
   node.  It only needs to download the default route with the nexthops
   of those Spine Neighbors which have the 'R' bit set in the Spine-Leaf
   TLV in IIH PDUs.  IS-IS can perform equal cost or unequal cost load
   sharing while using the spine nodes as nexthops.  The aggregated
   metric of the outbound interface and the 'Reverse Metric' [RFC8500]
   can be used for this purpose.

3.4.1.  Pure CLOS Topology

   In a data center where the topology is pure CLOS or Fat Tree, there
   is no interconnection among the spine nodes, and there is not another
   Core layer above the aggregation layer with reachability to the leaf
   nodes.  When flooding reduction to RF-Leafs is in use, if the link
   between a spine and a leaf goes down, there is then a possibility of
   black holing the data traffic in the network.

   As in the diagram Figure 2, if the link Spine1-Leaf3 goes down, there
   needs to be a way for Leaf1, Leaf2 and Leaf4 to avoid the Spine1 if
   the destination of data traffic is to Leaf3 node.

   In the above example, the Spine1 and Spine2 are provisioned to
   advertise the Leaf-Set sub-TLV of the Spine-Leaf TLV.  Originally
   both Spines will advertise Leaf1 through Leaf4 as their Leaf-Set.
   When the Spine1-Leaf3 link is down, Spine1 will only have Leaf1,
   Leaf2 and Leaf4 in its Leaf-Set. This allows the other leaf nodes to
   know that Spine1 has lost connectivity to the leaf node of Leaf3.

   Each RF-Leaf node can select another spine node to request for some
   prefix information associated with the lost leaf node.  In this
   diagram of Figure 2, there are only two spine nodes (Spine-Leaf
   topology can have more than two spine nodes in general).  Each RF-
   Leaf node can independently select a spine node for the leaf
   information.  The RF-Leaf nodes will include the Info-Req sub-TLV in

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   the Spine-Leaf TLV in hellos sent to the selected spine node, Spine2
   in this case.

   The spine node, upon receiving the request from one or more leaf
   nodes, will find the IPv6/IPv4 prefixes advertised by the leaf nodes
   listed in the Info-Req sub-TLV.  The spine node will use the
   mechanism defined in Section 3.3.2 to advertise these prefixes to the
   RF-Leaf node.  For instance, it will include the IPv4 loopback prefix
   of leaf3 based on the policy configured or administrative tag
   attached to the prefixes.  When the leaf nodes receive the more
   specific prefixes, they will install the advertised prefixes towards
   the other spine nodes (Spine2 in this example).

   For instance in the data center overlay scenario, when any IP
   destination or MAC destination uses the leaf3's loopback as the
   tunnel nexthop, the overlay tunnel from leaf nodes will only select
   Spine2 as the gateway to reach leaf3 as long as the Spine1-Leaf3 link
   is still down.

   In cases where multiple links or nodes fail at the same time, the RF-
   leaf node may need to send the Info-Req to multiple upper layer spine
   nodes in order to obtain reachability information for all the
   partially connected nodes.

   This negative routing is more useful between tier 0 and tier 1 spine-
   leaf levels in a multi-level spine-leaf topology when the reduced
   flooding extension is in use.  Nodes in tiers 1 or greater may have
   much richer topology information and alternative paths.

3.5.  Implementation and Operation

3.5.1.  CSNP PDU

   In Spine-Leaf extension, Complete Sequence Number PDUs (CSNP) do not
   need to be transmitted over the Spine-Leaf link to an RF-Leaf.  Some
   IS-IS implementations send periodic CSNPs after the initial adjacency
   bring-up over a point-to-point interface.  There is no need for this
   optimization here since the RF-Leaf does not need to receive any
   other LSPs from the network, and the only LSPs transmitted across the
   Spine-Leaf link are the leaf node LSPs.

   Also in the graceful restart case[RFC5306], for the same reason,
   there is no need to send the CSNPs over the Spine-Leaf interface to
   an RF-Leaf.  Spine nodes only need to set the SRMflag on the LSPs
   belonging to the RF-Leaf that has restarted.

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3.5.2.  Leaf to Leaf connection

   Leaf to leaf node links are useful in host redundancy cases in
   switching networks.  There are no flooding extensions required in
   this case.  Leaf node LSPs will be exchanged over this link using the
   normal operation of the IS-IS Update process.  In the example diagram
   Figure 1, Leaf1 will receive Leaf2's LSPs and Leaf2 will receive
   Leaf1's LSPs.  Each of the Leaf nodes will in turn flood the LSPs
   they receive from their leaf node neighbor to their spine neighbors.
   Prefix reachability advertisements received from the leaf neighbor
   will result in the installation of more specific routes using this
   local Leaf-Leaf link.  SPF will be performed in this case just like
   when the entire network only involves with those two IS-IS nodes.
   This does not affect the normal Spine-Leaf mechanism they perform
   toward the spine nodes.

   Leaf to leaf connections SHOULD be limited to a single leaf neighbor.

   Two modes of operation for the Leaf-Leaf link are possible and are
   described in the following sub-sections.  Local traffic only

   The leaf node sets the 'overload' bit in its LSP PDU so that spine
   nodes will not send traffic destined for the neighboring leaf node
   via its leaf node neighbor.  The Leaf-Leaf link will then be used
   solely for local traffic between the two Leaf Nodes.  Transit traffic allowed

   If a leaf node becomes disconnected from all spine nodes, it is
   possible for spine nodes to route traffic destined for the
   disconnected leaf node via its leaf node neighbor.  However the leaf
   to leaf link SHOULD be the link of last resort.  To support this mode
   the leaf nodes do NOT set the overload bit in their LSPs and they
   advertise a high metric for the leaf to leaf link((2^24 - 2) is
   recommended).  This signals to the Spine Nodes that the leaf to leaf
   link may be used for transit traffic, but also insures that it will
   not be used unless the spine node has no other path to a given leaf

   When the leaf node is disconnected from all spine nodes it MAY
   install a default route towards its leaf-node neighbor in support of
   return traffic to the spine nodes.  When doing so the leaf should
   validate that its leaf neighbor has at least one spine neighbor.
   This can be done by looking for the Connect-to-RF-Spine Node bit in
   the Link Attributes sub-TLVs [RFC5029] advertised in the LSPs of its
   leaf node neighbor.

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3.5.3.  Spine Node Hostname

   This extension creates a non-reciprocal relationship between the
   spine node and leaf node.  The spine node will receive leaf's LSP and
   will know the leaf's hostname, but the leaf does not have spine's
   LSP.  This extension allows the Dynamic Hostname TLV [RFC5301] to be
   optionally included in spine's IIH PDU when sending to a 'Leaf-Peer'.
   This is useful in troubleshooting cases.

3.5.4.  IS-IS Reverse Metric

   This metric is part of the aggregated metric for leaf's default route
   installation with load sharing among the spine nodes.  When a spine
   node is in 'overload' condition, it should use the IS-IS Reverse
   Metric TLV in IIH [RFC8500] to set this metric to maximum to
   discourage the leaf using it as part of the loadsharing.

   In some cases, certain spine nodes may have less bandwidth in link
   provisioning or in real-time condition, and it can use this metric to
   signal to the leaf nodes dynamically.

   In other cases, such as when the spine node loses a link to a
   particular leaf node, although it can redirect the traffic to other
   spine nodes to reach that destination leaf node, but it MAY want to
   increase this metric value if the inter-spine connection becomes over
   utilized, or the latency becomes an issue.

3.5.5.  Spine-Leaf Traffic Engineering

   Besides using the IS-IS Reverse Metric by the spine nodes to affect
   the traffic pattern for leaf default gateway towards multiple spine
   nodes, the IPv6/IPv4 Info-Advertise sub-TLVs can be selectively used
   by traffic engineering controllers to move data traffic around the
   data center fabric to alleviate congestion and to reduce the latency
   of a certain class of traffic pairs.  By injecting more specific leaf
   node prefixes, it will allow the spine nodes to attract more traffic
   on some underutilized links.

3.5.6.  Other End-to-End Services

   Losing the topology information will have an impact on some of the
   end-to-end network services, for instance, MPLS TE or end-to-end
   segment routing.  Some other mechanisms such as those described in
   PCE [RFC4655] based solution may be used.  In this Spine-Leaf
   extension, the role of the leaf node is not too much different from
   the multi-level IS-IS routing while the level-1 IS-IS nodes only have
   the default route information towards the node which has the Attach
   Bit (ATT) set, and the level-2 backbone does not have any topology

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   information of the level-1 areas.  The exact mechanism to enable
   certain end-to-end network services in Spine-Leaf network is outside
   the scope of this document.

3.5.7.  Address Family and Topology

   IPv6 Address families[RFC5308], Multi-Topology (MT)[RFC5120] and
   Multi-Instance (MI)[RFC8202] information is carried over the IIH PDU.
   Since the goal is to simplify the operation of IS-IS network, for the
   simplicity of this extension, the Spine-Leaf mechanism is applied the
   same way to all the address families, MTs and MIs.

3.5.8.  Migration

   For this extension to be deployed in existing networks, a simple
   migration scheme is needed.  To support any leaf node in the network,
   all the involved spine nodes have to be upgraded first.  So the first
   step is to migrate all the involved spine nodes to support this
   extension, then the leaf nodes can be enabled with 'Leaf-Mode' one by
   one.  No flag day is needed for the extension migration.

4.  IANA Considerations

   Two new TLV codepoint is defined in this document and needs to be
   assigned by IANA from the "IS-IS TLV Codepoints" registry.  They are
   referred to as the Spine-Leaf TLV and the suggested value is 151, and
   Leaf-Set TLV and suggested value is 152.  The Spine-Leaf TLV is only
   to be optionally inserted in the IIH PDU, and the Leaf-Set TLV is
   only to be optionally inserted in Circuit Flooding Scoped LSP PDU.
   IANA is also requested to maintain the SL-flag bit values in the
   Spine-Leaf TLV, and 0x01, 0x02 and 0x04 bits are defined in this

      Value  Name                   IIH  LSP  SNP  Purge  CS-LSP
      -----  ---------------------  ---  ---  ---  -----  -------
      151    Spine-Leaf              y    n    n    n        n
      152    Leaf-Set                n    n    n    n        y

   This document also proposes to have the Dynamic Hostname TLV, already
   assigned as code 137, to be allowed in IIH PDU.

      Value  Name                   IIH  LSP  SNP  Purge
      -----  ---------------------  ---  ---  ---  -----
      137    Dynamic Name            y    y    n    y

   This documents requests IANA to create a new registry under the IS-IS
   TLV Codepoints registry.  The suggested name of the registry is "Sub-

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   TLVs for TLV 152 (Leaf-Set TLV)".  Initial contents of the new
   registry is defined below:

      Value  Name
      -----  ---------------------
      0      Reserved
      1      Leaf Neighbors
      2      Reachability Req
      3-255  Unassigned

   This document also requests that IANA allocate from the registry of
   link-attribute two new bit values for sub-TLV 19 of TLV 22 (Extended
   IS reachability TLV).

      Value  Name                             Reference
      -----  -----                            ----------
      0x4    Connect to RF-Leaf Node          This document
      0x8    Connect to RF-Spine Node         This document

5.  Security Considerations

   Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
   [RFC5310], and [RFC7602].  This extension does not raise additional
   security issues.

6.  Acknowledgments

   The authors would like to thank Tony Przygienda and Lukas Krattiger
   for their discussion and contributions.  The authors also would like
   to thank Acee Lindem, Russ White, Christian Hopps and Aijun Wang for
   their review and comments of this document.

7.  References

7.1.  Normative References

              ISO "International Organization for Standardization",
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473), ISO/IEC
              10589:2002, Second Edition.", 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,

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   [RFC5029]  Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
              Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
              September 2007, <>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,

   [RFC5301]  McPherson, D. and N. Shen, "Dynamic Hostname Exchange
              Mechanism for IS-IS", RFC 5301, DOI 10.17487/RFC5301,
              October 2008, <>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <>.

   [RFC5306]  Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",
              RFC 5306, DOI 10.17487/RFC5306, October 2008,

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <>.

   [RFC7356]  Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 2014,

   [RFC7602]  Chunduri, U., Lu, W., Tian, A., and N. Shen, "IS-IS
              Extended Sequence Number TLV", RFC 7602,
              DOI 10.17487/RFC7602, July 2015,

   [RFC8202]  Ginsberg, L., Previdi, S., and W. Henderickx, "IS-IS
              Multi-Instance", RFC 8202, DOI 10.17487/RFC8202, June
              2017, <>.

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   [RFC8500]  Shen, N., Amante, S., and M. Abrahamsson, "IS-IS Routing
              with Reverse Metric", RFC 8500, DOI 10.17487/RFC8500,
              February 2019, <>.

7.2.  Informative References

              Li, T., "Dynamic Flooding on Dense Graphs", draft-li-
              dynamic-flooding (work in progress), 2018.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,

   [RFC5309]  Shen, N., Ed. and A. Zinin, Ed., "Point-to-Point Operation
              over LAN in Link State Routing Protocols", RFC 5309,
              DOI 10.17487/RFC5309, October 2008,

Authors' Addresses

   Naiming Shen
   Cisco Systems
   560 McCarthy Blvd.
   Milpitas, CA  95035


   Les Ginsberg
   Cisco Systems
   821 Alder Drive
   Milpitas, CA  95035


   Sanjay Thyamagundalu


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