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Versions: (draft-shen-isis-spine-leaf-ext) 00 01

Networking Working Group                                         N. Shen
Internet-Draft                                               L. Ginsberg
Intended status: Standards Track                           Cisco Systems
Expires: June 21, 2019                                  S. Thyamagundalu
                                                       December 18, 2018

                 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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 21, 2019.

Copyright Notice

   Copyright (c) 2018 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
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   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 TLV  . . . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Spine-Leaf Sub-TLVs . . . . . . . . . . . . . . . . .   7  Leaf-Set Sub-TLV  . . . . . . . . . . . . . . . .   7  Info-Req Sub-TLV  . . . . . . . . . . . . . . . .   8
       3.3.2.  Advertising IPv4/IPv6 Reachability  . . . . . . . . .   8
       3.3.3.  Advertising Connection to RF-Leaf Node  . . . . . . .   8
     3.4.  Mechanism . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.4.1.  Pure CLOS Topology  . . . . . . . . . . . . . . . . .  10
     3.5.  Implementation and Operation  . . . . . . . . . . . . . .  11
       3.5.1.  CSNP PDU  . . . . . . . . . . . . . . . . . . . . . .  11
       3.5.2.  Overload Bit  . . . . . . . . . . . . . . . . . . . .  11
       3.5.3.  Spine Node Hostname . . . . . . . . . . . . . . . . .  11
       3.5.4.  IS-IS Reverse Metric  . . . . . . . . . . . . . . . .  11
       3.5.5.  Spine-Leaf Traffic Engineering  . . . . . . . . . . .  12
       3.5.6.  Other End-to-End Services . . . . . . . . . . . . . .  12
       3.5.7.  Address Family and Topology . . . . . . . . . . . . .  12
       3.5.8.  Migration . . . . . . . . . . . . . . . . . . . . . .  13
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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

   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

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   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
      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

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      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 TLV

   This extension introduces a new TLV, the Spine-Leaf TLV, which may be
   advertised in IS-IS Hello (IIH) PDUs, LSPs, or in Circuit Scoped Link
   State PDUs (CS-LSP) [RFC7356].  It is used by both spine and leaf
   nodes in this Spine-Leaf mechanism.

       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            |
       |         .. Optional Sub-TLVs

   The fields of this TLV are defined as follows:

      Type:    1 octet Suggested value 150 (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
                  clear, this value MUST be set to zero on transmission,
                  and it MUST be ignored on receipt.

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         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.

      Optional Sub-TLV:  Not defined in this document, for future

               sub-TLVs MAY be included when the TLV is in a CS-LSP.
               sub-TLVs MUST NOT be included when the TLV is in an IIH

3.3.1.  Spine-Leaf 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 send/request specific
   reachability information via the sub-TLVs defined below.

   Two Spine-Leaf sub-TLVs are defined.  The Leaf-Set sub-TLV and the
   Info-Req sub-TLV.  Leaf-Set Sub-TLV

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

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

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

      Leaf-Set:  A list of IS-IS System-ID of the leaf node neighbors of
               this spine node.

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   This sub-TLV is used by leaf nodes to request the advertisement of
   more specific prefix information from a selected spine node.  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-Set sub-TLV (See
   Section  The fields of this sub-TLV are defined as follows:

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

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

      Info-Req:  List of IS-IS System-IDs of leaf nodes for which
               connectivity information is being requested.

3.3.2.  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 either in IIHs or CS-LSPs [RFC7356] sent
   from the spine to the requesting leaf node.  Sending such information
   in IIHs has limited scale - all reachability information MUST fit
   within a single IIH.  It is therefore recommended that CS-LSPs be

3.3.3.  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 express this link is not used
   for LSP flooding.  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.

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

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   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 over all of its
   IS-IS adjacencies, but in the case of RF-Leafs only self-originated
   LSPs will exist in its LSP database.

   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'
   [REVERSE-METRIC] can be used for this purpose.

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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
   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

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   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 PDU (CSNP) does 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 is the leaf node LSP.

   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.

3.5.2.  Overload Bit

   The leaf node SHOULD set the 'overload' bit on its LSP PDU, since if
   the spine nodes were to forward traffic not meant for the local node,
   the leaf node does not have the topology information to prevent a
   routing/forwarding loop.

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 [REVERSE-METRIC] to set this metric to maximum to
   discourage the leaf using it as part of the loadsharing.

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   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.

   In the leaf-leaf link as a backup gateway use case, the 'Reverse
   Metric' SHOULD always be set to very high value.

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
   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.

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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

   A new TLV codepoint is defined in this document and needs to be
   assigned by IANA from the "IS-IS TLV Codepoints" registry.  It is
   referred to as the Spine-Leaf TLV and the suggested value is 150.
   This TLV is only to be optionally inserted either in the IIH PDU or
   in the Circuit Flooding Scoped LSP PDU.  IANA is also requested to
   maintain the SL-flag bit values in this TLV, and 0x01, 0x02 and 0x04
   bits are defined in this document.

      Value  Name                   IIH  LSP  SNP  Purge  CS-LSP
      -----  ---------------------  ---  ---  ---  -----  -------
      150    Spine-Leaf              y    y    n    n        y

   This extension 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

   Two new sub-TLVs are defined in this document and needs to be added
   assigned by IANA from the "IS-IS TLV Codepoints".  They are referred
   to in this document as the Leaf-Set sub-TLV and the Info-Req sub-TLV.
   It is suggested to have the values 1 and 2 respectively.

   This document also requests that IANA allocate from the registry of
   link-attribute bit values for sub-TLV 19 of TLV 22 (Extended IS
   reachability TLV).  This new bit is referred to as the "Connect to
   RF-Leaf Node" bit.

      Value  Name                             Reference
      -----  -----                            ----------
      0x3    Connect to RF-Leaf Node          This document

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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 for his 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.

              Shen, N., Amante, S., and M. Abrahamsson, "IS-IS Routing
              with Reverse Metric", draft-ietf-isis-reverse-metric-07
              (work in progress), 2017.

   [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-

   [RFC5029]  Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
              Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
              September 2007, <https://www.rfc-editor.org/info/rfc5029>.

   [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, <https://www.rfc-

   [RFC5301]  McPherson, D. and N. Shen, "Dynamic Hostname Exchange
              Mechanism for IS-IS", RFC 5301, DOI 10.17487/RFC5301,
              October 2008, <https://www.rfc-editor.org/info/rfc5301>.

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   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [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, <https://www.rfc-

   [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, <https://www.rfc-editor.org/info/rfc5310>.

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

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

   [RFC8202]  Ginsberg, L., Previdi, S., and W. Henderickx, "IS-IS
              Multi-Instance", RFC 8202, DOI 10.17487/RFC8202, June
              2017, <https://www.rfc-editor.org/info/rfc8202>.

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, <https://www.rfc-

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   [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, <https://www.rfc-

Authors' Addresses

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

   Email: naiming@cisco.com

   Les Ginsberg
   Cisco Systems
   821 Alder Drive
   Milpitas, CA  95035

   Email: ginsberg@cisco.com

   Sanjay Thyamagundalu

   Email: tsanjay@gmail.com

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