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Versions: (draft-wei-rift-applicability) 00 01

RIFT WG                                                 Yuehua. Wei, Ed.
Internet-Draft                                              Zheng. Zhang
Intended status: Informational                           ZTE Corporation
Expires: 5 October 2020                                Dmitry. Afanasiev
                                                                  Yandex
                                                            Tom. Verhaeg
                                                        Juniper Networks
                                                     Jaroslaw. Kowalczyk
                                                           Orange Polska
                                                              P. Thubert
                                                           Cisco Systems
                                                            3 April 2020


                           RIFT Applicability
                    draft-ietf-rift-applicability-01

Abstract

   This document discusses the properties, applicability and operational
   considerations of RIFT in different network scenarios.  It intends to
   provide a rough guide how RIFT can be deployed to simplify routing
   operations in Clos topologies and their variations.

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
   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 5 October 2020.

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/



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   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statement of Routing in Modern IP Fabric Fat Tree
           Networks  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Applicability of RIFT to Clos IP Fabrics  . . . . . . . . . .   3
     3.1.  Overview of RIFT  . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Applicable Topologies . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Horizontal Links  . . . . . . . . . . . . . . . . . .   6
       3.2.2.  Vertical Shortcuts  . . . . . . . . . . . . . . . . .   6
       3.2.3.  Generalizing to any Directed Acyclic Graph  . . . . .   6
     3.3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.3.1.  DC Fabrics  . . . . . . . . . . . . . . . . . . . . .   8
       3.3.2.  Metro Fabrics . . . . . . . . . . . . . . . . . . . .   8
       3.3.3.  Building Cabling  . . . . . . . . . . . . . . . . . .   8
       3.3.4.  Internal Router Switching Fabrics . . . . . . . . . .   8
       3.3.5.  CloudCO . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Deployment Considerations . . . . . . . . . . . . . . . . . .  11
     4.1.  South Reflection  . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Suboptimal Routing on Link Failures . . . . . . . . . . .  12
     4.3.  Black-Holing on Link Failures . . . . . . . . . . . . . .  14
     4.4.  Zero Touch Provisioning (ZTP) . . . . . . . . . . . . . .  15
     4.5.  Miscabling Examples . . . . . . . . . . . . . . . . . . .  15
     4.6.  Positive vs. Negative Disaggregation  . . . . . . . . . .  18
     4.7.  Mobile Edge and Anycast . . . . . . . . . . . . . . . . .  19
     4.8.  IPv4 over IPv6  . . . . . . . . . . . . . . . . . . . . .  21
     4.9.  In-Band Reachability of Nodes . . . . . . . . . . . . . .  21
     4.10. Dual Homing Servers . . . . . . . . . . . . . . . . . . .  22
     4.11. Fabric With A Controller  . . . . . . . . . . . . . . . .  23
       4.11.1.  Controller Attached to ToFs  . . . . . . . . . . . .  24
       4.11.2.  Controller Attached to Leaf  . . . . . . . . . . . .  24
     4.12. Internet Connectivity With Underlay . . . . . . . . . . .  24
       4.12.1.  Internet Default on the Leaf . . . . . . . . . . . .  25
       4.12.2.  Internet Default on the ToFs . . . . . . . . . . . .  25
     4.13. Subnet Mismatch and Address Families  . . . . . . . . . .  25
     4.14. Anycast Considerations  . . . . . . . . . . . . . . . . .  25
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  27
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28



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1.  Introduction

   This document intends to explain the properties and applicability of
   "Routing in Fat Trees" [RIFT] in different deployment scenarios and
   highlight the operational simplicity of the technology compared to
   traditional routing solutions.  It also documents special
   considerations when RIFT is used with or without overlays,
   controllers and corrects topology miscablings and/or node and link
   failures.

2.  Problem Statement of Routing in Modern IP Fabric Fat Tree Networks

   Clos and Fat-Tree topologies have gained prominence in today's
   networking, primarily as result of the paradigm shift towards a
   centralized data-center based architecture that is poised to deliver
   a majority of computation and storage services in the future.

   Today's current routing protocols were geared towards a network with
   an irregular topology and low degree of connectivity originally.
   When they are applied to Fat-Tree topologies:

   *  they tend to need extensive configuration or provisioning during
      bring up and re-dimensioning.

   *  spine and leaf nodes have the entire network topology and routing
      information, which is in fact, not needed on the leaf nodes during
      normal operation.

   *  significant Link State PDUs (LSPs) flooding duplication between
      spine nodes and leaf nodes occurs during network bring up and
      topology updates.  It consumes both spine and leaf nodes' CPU and
      link bandwidth resources and with that limits protocol
      scalability.

3.  Applicability of RIFT to Clos IP Fabrics

   Further content of this document assumes that the reader is familiar
   with the terms and concepts used in OSPF [RFC2328] and IS-IS
   [ISO10589-Second-Edition] link-state protocols and at least the
   sections of [RIFT] outlining the requirement of routing in IP fabrics
   and RIFT protocol concepts.

3.1.  Overview of RIFT

   RIFT is a dynamic routing protocol for Clos and fat-tree network
   topologies.  It defines a link-state protocol when "pointing north"
   and path-vector protocol when "pointing south".




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   It floods flat link-state information northbound only so that each
   level obtains the full topology of levels south of it.  That
   information is never flooded East-West or back South again.  So a top
   tier node has full set of prefixes from the SPF calculation.

   In the southbound direction the protocol operates like a "fully
   summarizing, unidirectional" path vector protocol or rather a
   distance vector with implicit split horizon whereas the information
   propagates one hop south and is 're-advertised' by nodes at next
   lower level, normally just the default route.

                +-----------+          +-----------+
                |    ToF    |          |    ToF    |         LEVEL 2
      +         +-----+--+--+          +-+--+------+
      |         |     |  |  |          | |  |      |      ^
      +         |     |  |  +-------------------------+   |
      Distance  |  +-------------------+ |  |      |  |   |
      Vector    |  |  |  |               |  |      |  |   +
      South     |  |  |  |      +--------+  |      |  |   Link+State
      +         |  |  |  |      |           |      |  |   Flooding
      |         |  |  +-------------+       |      |  |   North
      v         |  |     |      |   |       |      |  |   +
              +-+--+-+   +------+   +-------+   +--+--+-+ |
              |SPINE |   |SPINE |   | SPINE |   | SPINE | |  LEVEL 1
      +       ++----++   ++---+-+   +--+--+-+   ++----+-+ |
      +        |    |     |   |        |  |      |    |   |     ^ N
      Distance |    +-------+ |        |  +--------+  |   |     |   E
      Vector   |          | | |        |         | |  |   |  +------>
      South    |  +-------+ | |        | +-------+ |  |   |     |
      +        |  |         | |        | |         |  |   |     +
      v       ++--++      +-+-++      ++-+-+     +-+--++  +
              |LEAF|      |LEAF|      |LEAF|     |LEAF |     LEVEL 0
              +----+      +----+      +----+     +-----+

                          Figure 1: Rift overview

   A middle tier node has only information necessary for its level,
   which are all destinations south of the node based on SPF
   calculation, default route and potential disaggregated routes.

   RIFT combines the advantage of both Link-State and Distance Vector:

   *  Fastest Possible Convergence

   *  Automatic Detection of Topology

   *  Minimal Routes/Info on TORs




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   *  High Degree of ECMP

   *  Fast De-commissioning of Nodes

   *  Maximum Propagation Speed with Flexible Prefixes in an Update

   And RIFT eliminates the disadvantages of Link-State or Distance
   Vector:


   *  Reduced and Balanced Flooding

   *  Automatic Neighbor Detection


   So there are two types of link state database which are "north
   representation" N-TIEs and "south representation" S-TIEs.  The N-TIEs
   contain a link state topology description of lower levels and S-TIEs
   carry simply default routes for the lower levels.

   There are a bunch of more advantages unique to RIFT listed below
   which could be understood if you read the details of [RIFT].

   *  True ZTP

   *  Minimal Blast Radius on Failures

   *  Can Utilize All Paths Through Fabric Without Looping

   *  Automatic Disaggregation on Failures

   *  Simple Leaf Implementation that Can Scale Down to Servers

   *  Key-Value Store

   *  Horizontal Links Used for Protection Only

   *  Supports Non-Equal Cost Multipath and Can Replace MC-LAG

   *  Optimal Flooding Reduction and Load-Balancing

3.2.  Applicable Topologies

   Albeit RIFT is specified primarily for "proper" Clos or "fat-tree"
   structures, it already supports PoD concepts which are strictly
   speaking not found in original Clos concepts.





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   Further, the specification explains and supports operations of multi-
   plane Clos variants where the protocol relies on set of rings to
   allow the reconciliation of topology view of different planes as most
   desirable solution making proper disaggregation viable in case of
   failures.  This observations hold not only in case of RIFT but in the
   generic case of dynamic routing on Clos variants with multiple planes
   and failures in bi-sectional bandwidth, especially on the leafs.

3.2.1.  Horizontal Links

   RIFT is not limited to pure Clos divided into PoD and multi-planes
   but supports horizontal links below the top of fabric level.  Those
   links are used however only as routes of last resort northbound when
   a spine loses all northbound links or cannot compute a default route
   through them.

   A possible configuration is a "ring" of horizontal links at a level.
   In presence of such a "ring" in any level (except ToF level) neither
   N-SPF nor S-SPF will provide a "ring-based protection" scheme since
   such a computation would have to deal necessarily with breaking of
   "loops" in Dijkstra sense; an application for which RIFT is not
   intended.

   A full-mesh connectivity between nodes on the same level can be
   employed and that allows N-SPF to provide for any node loosing all
   its northbound adjacencies (as long as any of the other nodes in the
   level are northbound connected) to still participate in northbound
   forwarding.

3.2.2.  Vertical Shortcuts

   Through relaxations of the specified adjacency forming rules RIFT
   implementations can be extended to support vertical "shortcuts" as
   proposed by e.g.  [I-D.white-distoptflood].  The RIFT specification
   itself does not provide the exact details since the resulting
   solution suffers from either much larger blast radius with increased
   flooding volumes or in case of maximum aggregation routing bow-tie
   problems.

3.2.3.  Generalizing to any Directed Acyclic Graph

   RIFT is an anisotropic routing protocol, meaning that it has a sense
   of direction (Northbound, Southbound, East-West) and that it operates
   differently depending on the direction.

   *  Northbound, RIFT operates as a Link State IGP, whereby the control
      packets are reflooded first all the way North and only interpreted
      later.  All the individual fine grained routes are advertised.



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   *  Southbound, RIFT operates as a Distance Vector IGP, whereby the
      control packets are flooded only one hop, interpreted, and the
      consequence of that computation is what gets flooded on more hop
      South.  In the most common use-cases, a ToF node can reach most of
      the prefixes in the fabric.  If that is the case, the ToF node
      advertises the fabric default and disaggregates the prefixes that
      it cannot reach.  On the oethr hand, a ToF Node that can reach
      only a small subset of the prefixes in the fabric will preferably
      advertise those prefixes and refrain from aggregating.

      In the general case, what gets advertised South is in more
      details:

      1.  A fabric default that aggregates all the prefixes that are
          reachable within the fabric, and that could be a default route
          or a prefix that is dedicated to this particular fabric.

      2.  The loopback addresses of the Northbound nodes, e.g., for
          inband management.

      3.  The disaggregated prefixes for the dynamic exceptions to the
          fabric Default, advertised to route around the black hole that
          may form

   *  East-West routing can optionally be used, with specific
      restrictions.  It is useful in particular when a sibling has
      access to the fabric default but this node does not.

   A Directed Acyclic Graph (DAG) provides a sense of North (the
   direction of the DAG) and of South (the reverse), which can be used
   to apply RIFT.  For the purpose of RIFT an edge in the DAG that has
   only incoming vertices is a ToF node.

   There are a number of caveats though:

   *  The DAG structure must exist before RIFT starts, so there is a
      need for a companion protocol to establish the logical DAG
      structure.

   *  A generic DAG does not have a sense of East and West.  The
      operation specified for East-West links and the Southbound
      reflection between nodes are not applicable.

   *  In order to aggregate and disaggregate routes, RIFT requires that
      all the ToF nodes share the full knowledge of the prefixes in the
      fabric.  This can be achieved with a ring as suggested by the RIFT
      main specification, by some preconfiguration, or using a




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      synchronization with a common repository where all the active
      prefixes are registered.

3.3.  Use Cases

3.3.1.  DC Fabrics

   RIFT is largely driven by demands and hence ideally suited for
   application in underlay of data center IP fabrics, vast majority of
   which seem to be currently (and for the foreseeable future) Clos
   architectures.  It significantly simplifies operation and deployment
   of such fabrics as described in Section 4 for environments compared
   to extensive proprietary provisioning and operational solutions.

3.3.2.  Metro Fabrics

   The demand for bandwidth is increasing steadily, driven primarily by
   environments close to content producers (server farms connection via
   DC fabrics) but in proximity to content consumers as well.  Consumers
   are often clustered in metro areas with their own network
   architectures that can benefit from simplified, regular Clos
   structures and hence RIFT.

3.3.3.  Building Cabling

   Commercial edifices are often cabled in topologies that are either
   Clos or its isomorphic equivalents.  With many floors the Clos can
   grow rather high and with that present a challenge for traditional
   routing protocols (except BGP and by now largely phased-out PNNI)
   which do not support an arbitrary number of levels which RIFT does
   naturally.  Moreover, due to limited sizes of forwarding tables in
   active elements of building cabling the minimum FIB size RIFT
   maintains under normal conditions can prove particularly cost-
   effective in terms of hardware and operational costs.

3.3.4.  Internal Router Switching Fabrics

   It is common in high-speed communications switching and routing
   devices to use fabrics when a crossbar is not feasible due to cost,
   head-of-line blocking or size trade-offs.  Normally such fabrics are
   not self-healing or rely on 1:/+1 protection schemes but it is
   conceivable to use RIFT to operate Clos fabrics that can deal
   effectively with interconnections or subsystem failures in such
   module.  RIFT is neither IP specific and hence any link addressing
   connecting internal device subnets is conceivable.






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3.3.5.  CloudCO

   The Cloud Central Office (CloudCO) is a new stage of telecom Central
   Office.  It takes the advantage of Software Defined Networking (SDN)
   and Network Function Virtualization (NFV) in conjunction with general
   purpose hardware to optimize current networks.  The following figure
   illustrates this architecture at a high level.  It describes a single
   instance or macro-node of cloud CO.  An Access I/O module faces a
   Cloud CO Access Node, and the CPEs behind it.  A Network I/O module
   is facing the core network.  The two I/O modules are interconnected
   by a leaf and spine fabric.  [TR-384]








































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        +---------------------+           +----------------------+
        |         Spine       |           |     Spine            |
        |         Switch      |           |     Switch           |
        +------+---+------+-+-+           +--+-+-+-+-----+-------+
        |      |   |      | | |              | | | |     |       |
        |      |   |      | | +-------------------------------+  |
        |      |   |      | |                | | | |     |    |  |
        |      |   |      | +-------------------------+  |    |  |
        |      |   |      |                  | | | |  |  |    |  |
        |      |   +----------------------+  | | | |  |  |    |  |
        |      |          |               |  | | | |  |  |    |  |
        |  +---------------------------------+ | | |  |  |    |  |
        |  |   |          |               |    | | |  |  |    |  |
        |  |   |   +-----------------------------+ |  |  |    |  |
        |  |   |   |      |               |    |   |  |  |    |  |
        |  |   |   |      |   +--------------------+  |  |    |  |
        |  |   |   |      |   |           |    |      |  |    |  |
        +--+ +-+---+--+ +-+---+--+     +--+----+--+ +-+--+--+ +--+
        |L | | Leaf   | | Leaf   |     |  Leaf    | | Leaf  | |L |
        |S | | Switch | | Switch |     |  Switch  | | Switch| |S |
        ++-+ +-+-+-+--+ +-+-+-+--+     +--+-+--+--+ ++-+--+-+ +-++
         |     | | |      | | |           | |  |     | |  |     |
         |   +-+-+-+--+ +-+-+-+--+     +--+-+--+--+ ++-+--+-+   |
         |   |Compute | |Compute |     | Compute  | |Compute|   |
         |   |Node    | |Node    |     | Node     | |Node   |   |
         |   +--------+ +--------+     +----------+ +-------+   |
         |   || VAS5 || || vDHCP||     || vRouter|| ||VAS1 ||   |
         |   |--------| |--------|     |----------| |-------|   |
         |   |--------| |--------|     |----------| |-------|   |
         |   || VAS6 || || VAS3 ||     || v802.1x|| ||VAS2 ||   |
         |   |--------| |--------|     |----------| |-------|   |
         |   |--------| |--------|     |----------| |-------|   |
         |   || VAS7 || || VAS4 ||     ||  vIGMP || ||BAA  ||   |
         |   |--------| |--------|     |----------| |-------|   |
         |   +--------+ +--------+     +----------+ +-------+   |
         |                                                      |
        ++-----------+                                +---------++
        |Network I/O |                                |Access I/O|
        +------------+                                +----------+


                Figure 2: An example of CloudCO architecture

   The Spine-Leaf architectures deployed inside CloudCO meets the
   network requirements of adaptable, agile, scalable and dynamic.






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4.  Deployment Considerations

   RIFT presents the opportunity for organizations building and
   operating IP fabrics to simplify their operation and deployments
   while achieving many desirable properties of a dynamic routing on
   such a substrate:

   *  RIFT design follows minimum blast radius and minimum necessary
      epistemological scope philosophy which leads to very good scaling
      properties while delivering maximum reactiveness.

   *  RIFT allows for extensive Zero Touch Provisioning within the
      protocol.  In its most extreme version RIFT does not rely on any
      specific addressing and for IP fabric can operate using IPv6 ND
      [RFC4861] only.

   *  RIFT has provisions to detect common IP fabric mis-cabling
      scenarios.

   *  RIFT negotiates automatically BFD per link allowing this way for
      IP and micro-BFD [RFC7130] to replace LAGs which do hide bandwidth
      imbalances in case of constituent failures.  Further automatic
      link validation techniques similar to [RFC5357] could be supported
      as well.

   *  RIFT inherently solves many difficult problems associated with the
      use of traditional routing topologies with dense meshes and high
      degrees of ECMP by including automatic bandwidth balancing, flood
      reduction and automatic disaggregation on failures while providing
      maximum aggregation of prefixes in default scenarios.

   *  RIFT reduces FIB size towards the bottom of the IP fabric where
      most nodes reside and allows with that for cheaper hardware on the
      edges and introduction of modern IP fabric architectures that
      encompass e.g. server multi-homing.

   *  RIFT provides valley-free routing and with that is loop free.
      This allows the use of any such valley-free path in bi-sectional
      fabric bandwidth between two destination irrespective of their
      metrics which can be used to balance load on the fabric in
      different ways.

   *  RIFT includes a key-value distribution mechanism which allows for
      many future applications such as automatic provisioning of basic
      overlay services or automatic key roll-overs over whole fabrics.

   *  RIFT is designed for minimum delay in case of prefix mobility on
      the fabric.



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   *  Many further operational and design points collected over many
      years of routing protocol deployments have been incorporated in
      RIFT such as fast flooding rates, protection of information
      lifetimes and operationally easily recognizable remote ends of
      links and node names.

4.1.  South Reflection

   South reflection is a mechanism that South Node TIEs are "reflected"
   back up north to allow nodes in same level without E-W links to "see"
   each other.

   For example, Spine111\Spine112\Spine121\Spine122 reflects Node S-TIEs
   from ToF21 to ToF22 separately.  Respectively,
   Spine111\Spine112\Spine121\Spine122 reflects Node S-TIEs from ToF22
   to ToF21 separately.  So ToF22 and ToF21 see each other's node
   information as level 2 nodes.

   In an equivalent fashion, as the result of the south reflection
   between Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122,
   Spine121 and Spine 122 knows each other at level 1.

4.2.  Suboptimal Routing on Link Failures




























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                  +--------+          +--------+
                  | ToF21  |          |  ToF22 |                LEVEL 2
                  ++--+-+-++          ++-+--+-++
                   |  | | |            | |  | +
                   |  | | |            | |  | linkTS8
     +-------------+  | +-+linkTS3+-+  | |  | +--------------+
     |                |   |         |  | |  +                |
     |    +----------------------------+ |  linkTS7          |
     |    |           |   |         +    +  +                |
     |    |           |   +-------+linkTS4+------------+     |
     |    |           |             +    +  |          |     |
     |    |           |     +------------+--+          |     |
     |    |           |     |       |  linkTS6         |     |
   +-+----++         ++-----++     ++------+          ++-----++
   |Spin111|         |Spin112|     |Spin121|          |Spin122| LEVEL 1
   +-+---+-+         ++----+-+     +-+---+-+          ++---+--+
     |   |            |    |         |   |             |   |
     |   +--------------+  |         +   ++XX+linkSL6+---+ +
     |                | |  |      linkSL5              | | linkSL8
     |   +------------+ |  |         +   +---+linkSL7+-+ | +
     |   |              |  |         |   |               | |
   +-+---+-+         +--+--+-+     +-+---+-+          +--+-+--+
   |Leaf111|         |Leaf112|     |Leaf121|          |Leaf122| LEVEL 0
   +-+-----+         ++------+     +-----+-+          +-+-----+
     +                +                  +              +
   Prefix111         Prefix112     Prefix121          Prefix122

          Figure 3: Suboptimal routing upon link failure use case

   As shown in Figure 3, as the result of the south reflection between
   Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122, Spine121 and
   Spine 122 knows each other at level 1.

   Without disaggregation mechanism, when linkSL6 fails, the packet from
   leaf121 to prefix122 will probably go up through linkSL5 to linkTS3
   then go down through linkTS4 to linkSL8 to Leaf122 or go up through
   linkSL5 to linkTS6 then go down through linkTS4 and linkSL8 to
   Leaf122 based on pure default route.  It's the case of suboptimal
   routing or bow-tieing.

   With disaggregation mechanism, when linkSL6 fails, Spine122 will
   detect the failure according to the reflected node S-TIE from
   Spine121.  Based on the disaggregation algorithm provided by RIFT,
   Spine122 will explicitly advertise prefix122 in Disaggregated Prefix
   S-TIE PrefixesElement(prefix122, cost 1).  The packet from leaf121 to
   prefix122 will only be sent to linkSL7 following a longest-prefix
   match to prefix 122 directly then go down through linkSL8 to Leaf122
   .



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4.3.  Black-Holing on Link Failures

                   +--------+          +--------+
                   | ToF 21 |          | ToF 22 |                LEVEL 2
                   ++-+--+-++          ++-+--+-++
                    | |  | |            | |  | |
                    | |  | |            | |  | linkTS8
     +--------------+ |  +--linkTS3-X+  | |  | +--------------+
     linkTS1          |    |         |  | |  |                |
     |    +-----------------------------+ |  linkTS7          |
     |    |           |    |         |    |  |                |
     |    |      linkTS2   +--------linkTS4-X-----------+     |
     |    |           |              |    |  |          |     |
     |   linkTS5      +-+    +---------------+          |     |
     |    |             |    |       |  linkTS6         |     |
   +-+----++          +-+-----+     ++----+-+          ++-----++
   |Spin111|          |Spin112|     |Spin121|          |Spin122| LEVEL 1
   +-+---+-+          ++----+-+     +-+---+-+          ++---+--+
     |   |             |    |         |   |             |   |
     |   +---------------+  |         |   +----linkSL6----+ |
     linkSL1           | |  |      linkSL5              | | linkSL8
     |   +---linkSL3---+ |  |         |   +----linkSL7--+ | |
     |   |               |  |         |   |               | |
   +-+---+-+          +--+--+-+     +-+---+-+          +--+-+--+
   |Leaf111|          |Leaf112|     |Leaf121|          |Leaf122| LEVEL 0
   +-+-----+          ++------+     +-----+-+          +-+-----+
     +                 +                  +              +
   Prefix111          Prefix112     Prefix121          Prefix122

             Figure 4: Black-holing upon link failure use case

   This scenario illustrates a case when double link failure occurs and
   with that black-holing can happen.

   Without disaggregation mechanism, when linkTS3 and linkTS4 both fail,
   the packet from leaf111 to prefix122 would suffer 50% black-holing
   based on pure default route.  The packet supposed to go up through
   linkSL1 to linkTS1 then go down through linkTS3 or linkTS4 will be
   dropped.  The packet supposed to go up through linkSL3 to linkTS2
   then go down through linkTS3 or linkTS4 will be dropped as well.
   It's the case of black-holing.

   With disaggregation mechanism, when linkTS3 and linkTS4 both fail,
   ToF22 will detect the failure according to the reflected node S-TIE
   of ToF21 from Spine111\Spine112.  Based on the disaggregation
   algorithm provided by RITF, ToF22 will explicitly originate an S-TIE
   with prefix 121 and prefix 122, that is flooded to spines 111, 112,
   121 and 122.



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   The packet from leaf111 to prefix122 will not be routed to linkTS1 or
   linkTS2.  The packet from leaf111 to prefix122 will only be routed to
   linkTS5 or linkTS7 following a longest-prefix match to prefix122.

4.4.  Zero Touch Provisioning (ZTP)

   Each RIFT node may operate in zero touch provisioning (ZTP) mode.  It
   has no configuration (unless it is a Top-of-Fabric at the top of the
   topology or it is desired to confine it to leaf role w/o leaf-2-leaf
   procedures).  In such case RIFT will fully configure the node's level
   after it is attached to the topology.

   The most import component for ZTP is the automatic level derivation
   procedure.  All the Top-of-Fabric nodes are explicitly marked with
   TOP_OF_FABRIC flag which are initial 'seeds' needed for other ZTP
   nodes to derive their level in the topology.  The derivation of the
   level of each node happens then based on LIEs received from its
   neighbors whereas each node (with possibly exceptions of configured
   leafs) tries to attach at the highest possible point in the fabric.
   This guarantees that even if the diffusion front reaches a node from
   "below" faster than from "above", it will greedily abandon already
   negotiated level derived from nodes topologically below it and
   properly peer with nodes above.

4.5.  Miscabling Examples

        +----------------+              +-----------------+
        |     ToF21      |       +------+      ToF22      |   LEVEL 2
        +-------+----+---+       |      +----+---+--------+
        |       |    |   |       |      |    |   |        |
        |       |    |   +----------------------------+   |
        |   +---------------------------+    |   |    |   |
        |   |   |    |           |           |   |    |   |
        |   |   |    |   +-----------------------+    |   |
        |   |   +------------------------+   |        |   |
        |   |        |   |       |       |   |        |   |
      +-+---+-+    +-+---+-+     |     +-+---+-+    +-+---+-+
      |Spin111|    |Spin112|     |     |Spin121|    |Spin122| LEVEL 1
      +-+---+-+    ++----+-+     |     +-+---+-+    ++----+-+
        |   |       |    |       |       |   |       |    |
        |   +---------+  |     link-M    |   +---------+  |
        |           | |  |       |       |           | |  |
        |   +-------+ |  |       |       |   +-------+ |  |
        |   |         |  |       |       |   |         |  |
      +-+---+-+    +--+--+-+     |     +-+---+-+    +--+--+-+
      |Leaf111|    |Leaf112+-----+     |Leaf121|    |Leaf122| LEVEL 0
      +-------+    +-------+           +-------+    +-------+




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                Figure 5: A single plane miscabling example

   Figure 5 shows a single plane miscabling example.  It's a perfect
   fat-tree fabric except link-M connecting Leaf112 to ToF22.

   The RIFT control protocol can discover the physical links
   automatically and be able to detect cabling that violates fat-tree
   topology constraints.  It react accordingly to such mis-cabling
   attempts, at a minimum preventing adjacencies between nodes from
   being formed and traffic from being forwarded on those mis-cabled
   links.  Leaf112 will in such scenario use link-M to derive its level
   (unless it is leaf) and can report links to spines 111 and 112 as
   miscabled unless the implementations allows horizontal links.

   Figure 6 shows a multiple plane miscabling example.  Since Leaf112
   and Spine121 belong to two different PoDs, the adjacency between
   Leaf112 and Spine121 can not be formed. link-W would be detected and
   prevented.

      +-------+    +-------+           +-------+    +-------+
      |ToF  A1|    |ToF  A2|           |ToF  B1|    |ToF  B2| LEVEL 2
      +-------+    +-------+           +-------+    +-------+
      |       |    |       |           |       |    |       |
      |       |    |       +-----------------+ |    |       |
      |       +--------------------------+   | |    |       |
      |            |                   | |   | |    |       |
      |     +------+                   | |   | +------+     |
      |     |        +-----------------+ |   |      | |     |
      |     |        |   +--------------------------+ |     |
      |  A  |        | B |               | A |        |  B  |
      +-----+-+    +-+---+-+           +-+---+-+    +-+-----+
      |Spin111|    |Spin112|      +----+Spin121|    |Spin122| LEVEL 1
      +-+---+-+    ++----+-+      |    +-+---+-+    ++----+-+
        |   |       |    |        |      |   |       |    |
        |   +---------+  |        |      |   +---------+  |
        |           | |  |      link-W   |           | |  |
        |   +-------+ |  |        |      |   +-------+ |  |
        |   |         |  |        |      |   |         |  |
      +-+---+-+    +--+--+-+      |    +-+---+-+    +--+--+-+
      |Leaf111|    |Leaf112+------+    |Leaf121|    |Leaf122| LEVEL 0
      +-------+    +-------+           +-------+    +-------+
     +--------PoD#1----------+       +---------PoD#2---------+

               Figure 6: A multiple plane miscabling example

   RIFT provides an optional level determination procedure in its Zero
   Touch Provisioning mode.  Nodes in the fabric without their level
   configured determine it automatically.  This can have possibly



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   counter-intuitive consequences however.  One extreme failure scenario
   is depicted in Figure 7 and it shows that if all northbound links of
   spine11 fail at the same time, spine11 negotiates a lower level than
   Leaf11 and Leaf12.

   To prevent such scenario where leafs are expected to act as switches,
   LEAF_ONLY flag can be set for Leaf111 and Leaf112.  Since level -1 is
   invalid, Spine11 would not derive a valid level from the topology in
   Figure 7.  It will be isolated from the whole fabric and it would be
   up to the leafs to declare the links towards such spine as miscabled.

           +-------+    +-------+        +-------+    +-------+
           |ToF  A1|    |ToF  A2|        |ToF  A1|    |ToF  A2|
           +-------+    +-------+        +-------+    +-------+
           |       |    |       |                |            |
           |    +-------+       |                |            |
           +    +  |            |  ====>         |            |
           X    X  +------+     |                +------+     |
           +    +         |     |                       |     |
           +----+--+    +-+-----+                     +-+-----+
           |Spine11|    |Spine12|                     |Spine12|
           +-+---+-+    ++----+-+                     ++----+-+
             |   |       |    |                        |    |
             |   +---------+  |                        |    |
             |           | |  |                        |    |
             |   +-------+ |  |                +-------+    |
             |   |         |  |                |            |
           +-+---+-+    +--+--+-+        +-----+-+    +-----+-+
           |Leaf111|    |Leaf112|        |Leaf111|    |Leaf112|
           +-------+    +-------+        +-+-----+    +-+-----+
                                           |            |
                                           |   +--------+
                                           |   |
                                         +-+---+-+
                                         |Spine11|
                                         +-------+

                           Figure 7: Fallen spine













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4.6.  Positive vs. Negative Disaggregation

   Disaggregation is the procedure whereby [RIFT] advertises more a
   specific route Southwards as an exception to the aggregated fabric-
   default North.  Disaggregation is useful when a prefix within the
   aggregation is reachable via some of the parents but not the others
   at the same level of the fabric.  It is mandatory when the level is
   the ToF since a ToF node that cannot reach a prefix becomes a black
   hole for that prefix.  The hard problem is to know which prefixes are
   reachable by whom.

   In the general case, [RIFT] solves that problem by interconnecting
   the ToF nodes so they can exchange the full list of prefixes that
   exist in the fabric and figure when a ToF node lacks reachability and
   to existing prefix.  This requires additional ports at the ToF,
   typically 2 ports per ToF node to form a ToF-spanning ring.  xref
   target='I-D.ietf-rift-rift'/> also defines the Southbound Reflection
   procedure that enables a parent to explore the direct connectivity of
   its peers, meaning their own parents and children; based on the
   advertisements received from the shared parents and children, it may
   enable the parent to infer the prefixes its peers can reach.

   When a parent lacks reachability to a prefix, it may disaggregate the
   prefix negatively, i.e., advertise that this parent can be used to
   reach any prefix in the aggregation except that one.  The Negative
   Disaggregation signaling is simple and functions transitively from
   ToF to ToP and then from Top to Leaf.  But it is hard for a parent to
   figure which prefix it needs to disaggregate, because it does not
   know what it does not know; it results thet the use of a spanning
   ring at the ToF is required to operate the Negative Disaggregation.
   Also, though it is only an implementation problem, the programmation
   of the FIB is complex compared to normal routes, and may incur
   recursions.

   The more classical alternative is, for the parents that can reach a
   prefix that peers at the same level cannot, to advertise a more
   specific route to that prefix.  This leverages the normal longest
   prefix match in the FIB, and does not require a special
   implementation.  But as opposed to the Negative Disaggregation, the
   Positive Disaggregation is difficult and inefficient to operate
   transitively.

   Transitivity is not needed to a grandchild if all its parents
   received the Positive Disaggregation, meaning that they shall all
   avoid the black hole; when that is the case, they collectively build
   a ceiling that protects the grandchild.  But until then, a parent
   that received a Positive Disaggregation may believe that some peers




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   are lacking the reachability and readvertise too early, or defer and
   maintain a black hole situation longer than necessary.

   In a non-partitioned fabric, all the ToF nodes see one another
   through the reflection and can figure if one is missing a child.  In
   that case it is possible to compute the prefixes that the peer cannot
   reach and disaggregate positively without a ToF-spanning ring.  The
   ToF nodes can also acertain that the ToP nodes are connected each to
   at least a ToF node that can still reach the prefix, meaning that the
   transitive operation is not required.

   The bottom line is that in a fabric that is partitioned (e.g., using
   multiple planes) and/or where the ToP nodes are not guaranteed to
   always form a ceiling for their children, it is mandatory to use the
   Negative Disaggregation.  On the other hand, in a highly symmetrical
   and fully connected fabric, (e.g., a canonical Clos Network), the
   Positive Disaggregation methods allows to save the complexity and
   cost associated to the ToF-spanning ring.

   Note that in the case of Positive Disaggregation, the first ToF
   node(s) that announces a more-specific route attracts all the traffic
   for that route and may suffer from a transient incast.  A ToP node
   that defers injecting the longer prefix in the FIB, in order to
   receive more advertisements and spread the packets better, also keeps
   on sending a portion of the traffic to the black hole in the
   meantime.  In the case of Negative Disaggregation, the last ToF
   node(s) that injects the route may also incur an incast issue; this
   problem would occur if a prefix that becomes totally unreachable is
   disaggregated, but doing so is mostly useless and is not recommended.

4.7.  Mobile Edge and Anycast

   When a physical or a virtual node changes its point of attachement in
   the fabric from a previous-leaf to a next-leaf, new routes must be
   installed that supercede the old ones.  Since the flooding flows
   Northwards, the nodes (if any) between the previous-leaf and the
   common parent are not immediately aware that the path via previous-
   leaf is obsolete, and a stale route may exist for a while.  The
   common parent needs to select the freshest route advertisement in
   order to install the correct route via the next-leaf.  This requires
   that the fabric determines the sequence of the movements of the
   mobile node.

   On the one hand, a classical sequence counter provides a total order
   for a while but it will eventually wrap.  On the other hand, a
   timestamp provides a permanent order but it may miss a movement that
   happens too quickly vs.  the granularity of the timing information.
   It is not envisioned in the short term that the average fabric



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   supports a Precision Time Protocol, and the precision that may be
   available with the Network Time Protocol [RFC5905], in the order of
   100 to 200ms, may not be necessarily enough to cover, e.g., the fast
   mobility of a Virtual Machine.

   Section 4.3.3.  "Mobility" of [RIFT] specifies an hybrid method that
   combines a sequence counter from the mobile node and a timestamp from
   the network taken at the leaf when the route is injected.  If the
   timestamps of the concurrent advertisements are comparable (i.e.,
   more distant than the precision of the timing protocol), then the
   timestamp alone is used to determine the relative freshness of the
   routes.  Otherwise, the sequence counter from the mobile node, if
   available, is used.  One caveat is that the sequence counter must not
   wrap within the precision of the timing protocol.  Another is that
   the mobile node may not even provide a sequence counter, in which
   case the mobility itself must be slower than the precision of the
   timing.

   Mobility must not be confused with Anycast.  In both cases, a same
   address is injected in RIFT at different leaves.  In the case of
   mobility, only the freshest route must be conserved, since mobile
   node changed its point of attachement for a leaf ot the next.  In the
   case of anycast, the node may be either multihomed (attached to
   multiple leaves in parallel) or reachable beyond the fabric via
   multiple routes that are redistributed to different leaves; either
   way, in the case of anycast, the multiple routes are equally valid
   and should be conserved.  Without further information from the
   redistributed routing protocol, it is impossible to sort out a
   movement from a redistribution that happens asynchronously on
   different leaves.  [RIFT] expects that anycast addresses are
   advertised within the timing precision, which is typically the case
   with a low-precision timing and a multihomed node.  Beyond that time
   interval, RIFT interprets the lag as a mobility and only the freshest
   route is retained.

   When using IPv6 [RFC8200], RIFT suggests to leverage "Registration
   Extensions for IPv6 over Low-Power Wireless Personal Area Network
   (6LoWPAN) Neighbor Discovery (ND)" [RFC8505] as the IPv6 ND
   interaction between the mobile node and the leaf.  This provides not
   only a sequence counter but also a lifetime and a security token that
   may be used to protect the ownership of an address.  When using
   [RFC8505], the parallel registration of an anycast address to
   multiple leaves is done with the same sequence counter, whereas the
   sequence counter is incremented when the point of attachement
   changes.  This way, it is possible to differentiate a mobile node
   from a multihomed node, even when the mobility happens within the
   timing precision.  It is also possible for a mobile node to be




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   multihomed as well, e.g., to change only one of its points of
   attachement.

4.8.  IPv4 over IPv6

   RIFT allows advertising IPv4 prefixes over IPv6 RIFT network.  IPv6
   AF configures via the usual ND mechanisms and then V4 can use V6
   nexthops analogous to RFC5549.  It is expected that the whole fabric
   supports the same type of forwarding of address families on all the
   links.  RIFT provides an indication whether a node is v4 forwarding
   capable and implementations are possible where different routing
   tables are computed per address family as long as the computation
   remains loop-free.

                            +-----+        +-----+
                 +---+---+  | ToF |        | ToF |
                     ^      +--+--+        +-----+
                     |      |  |           |     |
                     |      |  +-------------+   |
                     |      |     +--------+ |   |
                     |      |     |          |   |
                    V6      +-----+        +-+---+
                 Forwarding |SPINE|        |SPINE|
                     |      +--+--+        +-----+
                     |      |  |           |     |
                     |      |  +-------------+   |
                     |      |     +--------+ |   |
                     |      |     |          |   |
                     v      +-----+        +-+---+
                 +---+---+  |LEAF |        | LEAF|
                            +--+--+        +--+--+
                               |              |
                  IPv4 prefixes|              |IPv4 prefixes
                               |              |
                           +---+----+     +---+----+
                           |   V4   |     |   V4   |
                           | subnet |     | subnet |
                           +--------+     +--------+

                          Figure 8: IPv4 over IPv6

4.9.  In-Band Reachability of Nodes

   RIFT doesn't precondition that nodes of the fabric have reachable
   addresses.  But the operational purposes to reach the internal nodes
   may exist.  Figure 9 shows an example that the NMS attaches to LEAF1.





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                         +-------+      +-------+
                         | ToF1  |      | ToF2  |
                         ++---- ++      ++-----++
                          |     |        |     |
                          |     +----------+   |
                          |     +--------+ |   |
                          |     |          |   |
                         ++-----++      +--+---++
                         |SPINE1 |      |SPINE2 |
                         ++-----++      ++-----++
                          |     |        |     |
                          |     +----------+   |
                          |     +--------+ |   |
                          |     |          |   |
                         ++-----++      +--+---++
                         | LEAF1 |      | LEAF2 |
                         +---+---+      +-------+
                             |
                             |NMS

                   Figure 9: In-Band reachability of node

   If NMS wants to access LEAF2, it simply works.  Because loopback
   address of LEAF2 is flooded in its Prefix North TIE.

   If NMS wants to access SPINE2, it simply works too.  Because spine
   node always advertises its loopback address in the Prefix North TIE.
   NMS may reach SPINE2 from LEAF1-SPINE2 or LEAF1-SPINE1-ToF1/
   ToF2-SPINE2.

   If NMS wants to access ToF2, ToF2's loopback address needs to be
   injected into its Prefix South TIE.  Otherwise, the traffic from NMS
   may be sent to ToF1.

   And in case of failure between ToF2 and spine nodes, ToF2's loopback
   address must be sent all the way down to the leaves.

4.10.  Dual Homing Servers

   Each RIFT node may operate in zero touch provisioning (ZTP) mode.  It
   has no configuration (unless it is a Top-of-Fabric at the top of the
   topology or the must operate in the topology as leaf and/or support
   leaf-2-leaf procedures) and it will fully configure itself after
   being attached to the topology.







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                     +---+         +---+         +---+
                     |ToF|         |ToF|         |ToF|
                     +---+         +---+         +---+
                     |   |         |   |         |   |
                     |   +----------------+      |   |
                     |             |   |  |      |   |
                     |          +----------------+   |
                     |          |  |   |  |          |
                     +----------+--+   +--+----------+
                     | Spine|ToR1  |   | Spine|ToR2  |
                     +--+------+---+   +--+-------+--+
                 +---+  |      |   |   |  |       |  +---+
                 |      |      |   |   |  |       |      |
                 |   +-----------------+  |       |      |
                 |   |  |   +-------------+       |      |
                 +   |  +   |  |   |-----------------+   |
                 X   |  X   |  +--------x-----+   |  X   |
                 +   |  +   |                 |   |  +   |
                 +---+  +---+                 +---+  +---+
                 |   |  |   |                 |   |  |   |
                 +---+  +---+  ...............+---+  +---+
                 SV(1) SV(2)                 SV(n+1) SV(n)

                       Figure 10: Dual-homing servers

   In the single plane, the worst condition is disaggregation of every
   other servers at the same level.  Suppose the links from ToR1 to all
   the leaves become not available.  All the servers' routes are
   disaggregated and the FIB of the servers will be expanded with n-1
   more spicific routes.

   Sometimes, pleople may prefer to disaggregate from ToR to servers
   from start on, i.e. the servers have couple tens of routes in FIB
   from start on beside default routes to avoid breakages at rack level.
   Full disaggregation of the fabric could be achieved by configuration
   supported by RIFT.

4.11.  Fabric With A Controller

   There are many different ways to deploy the controller.  One
   possibility is attaching a controller to the RIFT domain from ToF and
   another possibility is attaching a controller from the leaf.









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                                     +------------+
                                     | Controller |
                                     ++----------++
                                      |          |
                                      |          |
                                 +----++        ++----+
                     -------     | ToF |        | ToF |
                        |        +--+--+        +-----+
                        |        |  |           |     |
                        |        |  +-------------+   |
                        |        |     +--------+ |   |
                        |        |     |          |   |
                                 +-----+        +-+---+
                    RIFT domain  |SPINE|        |SPINE|
                                 +--+--+        +-----+
                        |        |  |           |     |
                        |        |  +-------------+   |
                        |        |     +--------+ |   |
                        |        |     |          |   |
                        |        +-----+        +-+---+
                     -------     |LEAF |        | LEAF|
                                 +-----+        +-----+

                    Figure 11: Fabric with a controller

4.11.1.  Controller Attached to ToFs

   If a controller is attaching to the RIFT domain from ToF, it usually
   uses dual-homing connections.  The loopback prefix of the controller
   should be advertised down by the ToF and spine to leaves.  If the
   controller loses link to ToF, make sure the ToF withdraw the prefix
   of the controller(use different mechanisms).

4.11.2.  Controller Attached to Leaf

   If the controller is attaching from a leaf to the fabric, no special
   provisions are needed.

4.12.  Internet Connectivity With Underlay

   If global addressing is running without overlay, an external default
   route needs to be advertised through rift fabric to achieve internet
   connectivity.  For the purpose of forwarding of the entire rift
   fabric, an internal fabric prefix needs to be advertised in the South
   Prefix TIE by ToF and spine nodes.






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4.12.1.  Internet Default on the Leaf

   In case that an internet access request comes from a leaf and the
   internet gateway is another leaf, the leaf node as the internet
   gateway needs to advertise a default route in its Prefix North TIE.

4.12.2.  Internet Default on the ToFs

   In case that an internet access request comes from a leaf and the
   internet gateway is a ToF, the ToF and spine nodes need to advertise
   a default route in the Prefix South TIE.

4.13.  Subnet Mismatch and Address Families


                 +--------+                     +--------+
                 |        |  LIE          LIE   |        |
                 |   A    | +---->       <----+ |   B    |
                 |        +---------------------+        |
                 +--------+                     +--------+
                    X/24                           Y/24

                         Figure 12: subnet mismatch


   LIEs are exchanged over all links running RIFT to perform Link
   (Neighbor) Discovery.  A node MUST NOT originate LIEs on an address
   family if it does not process received LIEs on that family.  LIEs on
   same link are considered part of the same negotiation independent on
   the address family they arrive on.  An implementation MUST be ready
   to accept TIEs on all addresses it used as source of LIE frames.

   As shown in the above figure, without further checks adjacency of
   node A and B may form, but the forwarding between node A and node B
   may fail because subnet X mismatches with subnet Y.

   To prevent this a RIFT implementation should check for subnet
   mismatch just like e.g.  ISIS does.  This can lead to scenarios where
   an adjacency, despite exchange of LIEs in both address families may
   end up having an adjacency in a single AF only.  This is a
   consideration especially in Section 4.8 scenarios.

4.14.  Anycast Considerations








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                                 + traffic
                                 |
                                 v
                          +------+------+
                          |     ToF     |
                          +---+-----+---+
                          |   |     |   |
             +------------+   |     |   +------------+
             |                |     |                |
         +---+---+    +-------+     +-------+    +---+---+
         |       |    |       |     |       |    |       |
         |Spine11|    |Spine12|     |Spine21|    |Spine22| LEVEL 1
         +-+---+-+    ++----+-+     +-+---+-+    ++----+-+
           |   |       |    |         |   |       |    |
           |   +---------+  |         |   +---------+  |
           |           | |  |         |           | |  |
           |   +-------+ |  |         |   +-------+ |  |
           |   |         |  |         |   |         |  |
         +-+---+-+    +--+--+-+     +-+---+-+    +--+--+-+
         |       |    |       |     |       |    |       |
         |Leaf111|    |Leaf112|     |Leaf121|    |Leaf122| LEVEL 0
         +-+-----+    ++------+     +-----+-+    +-----+-+
           +           +                  +      ^     |
         PrefixA      PrefixB         PrefixA    | PrefixC
                                                 |
                                                 + traffic

                             Figure 13: Anycast

   If the traffic comes from ToF to Leaf111 or Leaf121 which has anycast
   prefix PrefixA.  RIFT can deal with this case well.  But if the
   traffic comes from Leaf122, it arrives Spine21 or Spine22 at level 1.
   But Spine21 or Spine22 doesn't know another PrefixA attaching
   Leaf111.  So it will always get to Leaf121 and never get to Leaf111.
   If the intension is that the traffic should been offloaded to
   Leaf111, then use policy guided prefixes [PGP reference].

5.  Acknowledgements


6.  Contributors

   The following people (listed in alphabetical order) contributed
   significantly to the content of this document and should be
   considered co-authors:

   Tony Przygienda




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   Juniper Networks

   1194 N.  Mathilda Ave

   Sunnyvale, CA 94089

   US

   Email: prz@juniper.net

7.  Normative References

   [ISO10589-Second-Edition]
              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)", November
              2002.

   [TR-384]   Broadband Forum Technical Report, "TR-384 Cloud Central
              Office Reference Architectural Framework", January 2018.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC7130]  Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
              Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
              Forwarding Detection (BFD) on Link Aggregation Group (LAG)
              Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
              2014, <https://www.rfc-editor.org/info/rfc7130>.

   [RIFT]     Przygienda, T., Sharma, A., Thubert, P., Rijsman, B., and
              D. Afanasiev, "RIFT: Routing in Fat Trees", Work in
              Progress, Internet-Draft, draft-ietf-rift-rift-11, 10
              March 2020,
              <https://tools.ietf.org/html/draft-ietf-rift-rift-11>.



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   [I-D.white-distoptflood]
              White, R., Hegde, S., and S. Zandi, "IS-IS Optimal
              Distributed Flooding for Dense Topologies", Work in
              Progress, Internet-Draft, draft-white-distoptflood-01, 30
              September 2019,
              <https://tools.ietf.org/html/draft-white-distoptflood-01>.

8.  Informative References

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

Authors' Addresses

   Yuehua Wei (editor)
   ZTE Corporation
   No.50, Software Avenue
   Nanjing
   210012
   China

   Email: wei.yuehua@zte.com.cn


   Zheng Zhang
   ZTE Corporation
   No.50, Software Avenue
   Nanjing
   210012
   China

   Email: zzhang_ietf@hotmail.com






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   Dmitry Afanasiev
   Yandex

   Email: fl0w@yandex-team.ru


   Tom Verhaeg
   Juniper Networks

   Email: tverhaeg@juniper.net


   Jaroslaw Kowalczyk
   Orange Polska

   Email: jaroslaw.kowalczyk2@orange.com


   Pascal Thubert
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 MOUGINS - Sophia Antipolis
   France

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com
























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