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Network Working Group                                      R. White, Ed.
Internet-Draft                                             S. Zandi, Ed.
Intended status: Informational                                  LinkedIn
Expires: July 6, 2018                                    January 2, 2018


                      IS-IS Support for Openfabric
                       draft-white-openfabric-05

Abstract

   Spine and leaf topologies are widely used in hyperscale and cloud
   scale networks.  In most of these networks, configuration is
   automated, but difficult, and topology information is extracted
   through broad based connections.  Policy is often integrated into the
   control plane, as well, making configuration, management, and
   troubleshooting difficult.  Openfabric is an adaptation of an
   existing, widely deployed link state protocol, Intermediate System to
   Intermediate System (IS-IS) that is designed to:

   o  Provide a full view of the topology from a single point in the
      network to simplify operations

   o  Minimize configuration of each Intermediate System (IS) (also
      called a router or switch) in the network

   o  Optimize the operation of IS-IS within a spine and leaf fabric to
      enable scaling

   This document begins with an overview of openfabric, including a
   description of what may be removed from IS-IS to enable scaling.  The
   document then describes an optimized adjacency formation process; an
   optimized flooding scheme; some thoughts on the operation of
   openfabric, metrics, and aggregation; and finally a description of
   the changes to the IS-IS protocol required for openfabric.

Status of This Memo

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

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

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



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

   This Internet-Draft will expire on July 6, 2018.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Contributors  . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Simplification  . . . . . . . . . . . . . . . . . . . . .   3
     1.4.  Additions and Requirements  . . . . . . . . . . . . . . .   4
     1.5.  Sample Network  . . . . . . . . . . . . . . . . . . . . .   4
   2.  Modified Adjacency Formation  . . . . . . . . . . . . . . . .   6
     2.1.  Level 2 Adjacencies Only  . . . . . . . . . . . . . . . .   6
     2.2.  Point-to-point Adjacencies  . . . . . . . . . . . . . . .   6
     2.3.  Three Way Handshake Support . . . . . . . . . . . . . . .   7
     2.4.  Adjacency Formation Optimization  . . . . . . . . . . . .   7
   3.  Advertisement of Reachability Information . . . . . . . . . .   8
   4.  Determining and Advertising Location on the Fabric  . . . . .   9
     4.1.  Calculating Tier Number with a Fixed T0 . . . . . . . . .   9
     4.2.  Calculating the Tier Number in a Five Stage Spine and
           Leaf  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Flooding Optimization . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Flooding Failures . . . . . . . . . . . . . . . . . . . .  13
   6.  Other Optimizations . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Transit Link Reachability . . . . . . . . . . . . . . . .  13
     6.2.  Transiting T0 Intermediate Systems  . . . . . . . . . . .  14
   7.  Openfabric and Route Aggregation  . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

1.1.  Goals

   Spine and leaf fabrics are often used in large scale data centers; in
   this application, they are commonly called a fabric because of their
   regular structure and predictable forwarding and convergence
   properties.  This document describes modifications to the IS-IS
   protocol to enable it to run efficiently on a large scale spine and
   leaf fabric, openfabric.  The goals of this control plane are:

   o  Provide a full view of the topology from a single point in the
      network to simplify operations

   o  Minimize configuration of each IS in the network

   o  Optimize the operation of IS-IS within a spine and leaf fabric to
      enable scaling

1.2.  Contributors

   The following people have contributed to this draft: Nikos
   Triantafillis (reflected flooding optimization), Ivan Pepelnjak
   (three stage fabric modifications), Hannes Gredler (do not reflood
   optimizations), Les Ginsberg (capabilities encoding, circuit local
   reflooding), Naiming Shen (capabilities encoding, circuit local
   reflooding), Uma Chunduri (failure mode suggestions, flooding), Nick
   Russo, and Rodny Molina.

   See [RFC5449], [RFC5614], and [RFC7182] for similar solutions in the
   Mobile Ad Hoc Networking (MANET) solution space.

1.3.  Simplification

   In building any scalable system, it is often best to begin by
   removing what is not needed.  In this spirit, openfabric
   implementations MAY remove the following from IS-IS:

   o  External metrics.  There is no need for external metrics in large
      scale spine and leaf fabrics; it is assumed that metrics will be
      properly configured by the operator to account for the correct
      order of route preference at any route redistribution point.

   o  Tags and traffic engineering processing.  Openfabric is only
      designed to provide topology and reachability information.  It is
      not designed to provide for traffic engineering, route preference



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      through tags, or other policy mechanisms.  It is assumed that all
      routing policy will be provided through an overlay system which
      communicates directly with each IS in the fabric, such as PCEP
      [RFC5440] or I2RS [RFC7921].  Traffic engineering is assumed to be
      provided through Segment Routing (SR)
      [I-D.ietf-spring-segment-routing].

1.4.  Additions and Requirements

   To create a scalable link state fabric, openfabric includes the
   following:

   o  A slightly modified adjacency formation process.

   o  Mechanisms for determining which tier within a spine and leaf
      fabric in which the IS is located.

   o  A mechanism that reduces flooding to the minimum possible, while
      still ensuring complete database synchronization among the
      intermediate systems within the fabric.

   Three general requirements are placed here; more specific
   requirements are considered in the following sections.  Openfabric
   implementations:

   o  MUST support [RFC5301] and enable hostname advertisement by
      default if a hostname is configured on the intermediate system.

   o  SHOULD support [RFC6232], purge originator identification for IS-
      IS.

   o  MUST NOT be mixed with standard IS-IS implementations in
      operational deployments.  Openfabric and standard IS-IS
      implementations SHOULD be treated as two separate protocols.

1.5.  Sample Network

   The following spine and leaf fabric will be used to describe these
   modifications.












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   +----+ +----+ +----+ +----+ +----+ +----+
   | 1A | | 1B | | 1C | | 1D | | 1E | | 1F | (T0)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 2A | | 2B | | 2C | | 2D | | 2E | | 2F | (T1)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 3A | | 3B | | 3C | | 3D | | 3E | | 3F | (T2)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 4A | | 4B | | 4C | | 4D | | 4E | | 4F | (T1)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 5A | | 5B | | 5C | | 5D | | 5E | | 5F | (T0)
   +----+ +----+ +----+ +----+ +----+ +----+

                                 Figure 1

   To reduce confusion (spine and leaf fabrics are difficult to draw in
   plain text art), this diagram does not contain the connections
   between devices.  The reader should assume that each device in a
   given layer is connected to every device in the layer above it.  For
   instance:

   o  5A is connected to 4A, 4B, 4C, 4D, 4E, and 4F

   o  5B is connected to 4A, 4B, 4C, 4D, 4E, and 4F

   o  4A is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and
      5F

   o  4B is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and
      5F

   o  etc.

   The tiers or stages of the fabric are also marked for easier
   reference.  T0 is assumed to be connected to application servers, or
   rather they are Top of Rack (ToR) intermediate systems.  The
   remaining tiers, T1 and T2, are connected only to the fabric itself.
   Note there are no "cross links," or "east west" links in the
   illustrated fabric.  The fabric locality detection mechanism
   described here will not work if there are cross links running east/




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   west through the fabric.  Locality detection may be possible in such
   a fabric; this is an area for further study.

2.  Modified Adjacency Formation

   Because Openfabric operates in a tightly controlled data center
   environment, various modifications can be made to the IS-IS neighbor
   formation process to increase efficencicy and simplify the protocol.
   Specifically, Openfabric implementations SHOULD support [RFC3719],
   section 4, hello padding for IS-IS.  Variable hello padding SHOULD
   NOT be used, as data center fabrics are built using high speed links
   on which padded hellos will have little performance impact.  Further
   modifications to the neighbor formation process are considered in the
   following sections.

2.1.  Level 2 Adjacencies Only

   Openfabric is designed to work in a single flooding domain over a
   single data center fabric at the scale of thousands of routers with
   hundreds of thousands of routes (so a moderate scale in router and
   route count terms).  Because of the way Openfabric optimizes
   operation in this environment, it is not necessary nor desirable to
   build multiple flooding domains.  For instance, the flooding
   optimizations described later this document require a full view of
   the topology, as does any proposed overlay to inject policy into the
   forwarding plane.  In light of this, the following changes SHOULD BE
   to IS-IS implemetations to support Openfabric:

   o  IIH PDU 17 (level 2 point-to-point circuit hello) should be the
      only IIH PDU type transmitted (see section 9.7 of ISO 10589)

   o  In IIH PDU 17 (level 2 point-to-point circuit hello), the Circuit
      Type field should be set to 2 (see section 9.7 of ISO 10589)

   o  Support for IIH PDU 15 (level 1 broadcast hello) should be removed
      (see section 9.5 of ISO 10589)

   o  Support for IIH PDU 16 (level 2 broadcast hello) should be removed
      (see section 9.6 of ISO 10589)

2.2.  Point-to-point Adjacencies

   Data center network fabrics only contain point-to-point links;
   because of this, there is no reason to support any broadcast link
   types, nor to support the Designated Intermediate System processing,
   including pseudonode creation.  In light ot his, processing related
   to sections 7.2.3 (broadcast networks), 7.3.8 (generation of level 1
   pseudonode LSPs), 7.3.10 (generation of level 2 pseudonode LSPs), and



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   section 8.4.5 (LAN designated intermediate systems) in [ISO10589]
   SHOULD BE removed.

2.3.  Three Way Handshake Support

   It is important that two way connectivity be established before
   synchronizing the link state database, or routing through a link in a
   data center fabric.  To reject optical failures that cause a one way
   connection between two routers, fabricDC must support the three way
   handshake mechanism described in [RFC5303].

2.4.  Adjacency Formation Optimization

   While adjacency formation is not considered particularly burdensome
   in IS-IS, it may still be useful to reduce the amount of state
   transferred across the network when connecting a new IS to the
   fabric.  In its simplest form, the process is:

   o  An IS connected to the fabric will send hellos on all links.

   o  The IS will only complete the three-way handshake with one newly
      discovered neighbor; this would normally be the first neighbor
      which sends the newly connected intermediate system's ID back in
      the three-way handshake process.

   o  The IS will complete its database exchange with this one newly
      adjacent neighbor.

   o  Once this process is completed, the IS will continue processing
      the remaining neighbors as normal.

   o  If synchronization is not achieved within twice the dead timer on
      the local interface, the newly connected IS will repeat this
      process with the second neighbor with which it forms a three-way
      adjacency.

   This process allows each IS newly added to the fabric to exchange a
   full table once; a very minimal amount of information will be
   transferred with the remaining neighbors to reach full
   synchronization.

   Any such optimization is bound to present a tradeoff between several
   factors; the mechanism described here increases the amount of time
   required to form adjacencies slightly in order to reduce the total
   state carried across the network.  An alternative mechanism could
   provide a better balance of the amount of information carried across
   the network for initial synchronization and the time required to
   synchronize a new IS.  For instance, an IS could choose to



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   synchronize its database with two or three adjacent intermediate
   systems, which could speed the synchronization process up at the cost
   of carrying additional data on the network.  A locally determined
   balance between the speed of synchronization and the amount of data
   carried on the network can be acheived by adjusting the number of
   adjacent intermediate systems the newly attached IS synchronizes
   with.

3.  Advertisement of Reachability Information

   IS-IS describes the topology in two different sets of TLVs; the first
   describes the set of neighbors connected to an IS, the second
   describes the set of reachable destination connected to an IS.  There
   are two different forms of both of these descriptions, one of which
   carries what are widely called narrow metrics, the other of which
   carries what are widely called wide metrics.  In a tightly controlled
   data center fabric implementation, such as the ones Openfabric is
   designed to support, no IS that supports narrow metrics will ever be
   deployed or supported; hence there is no reason to support any metric
   type other than wide metrics.

   o  The Level 2 Link State PDU (type 20 in section 9.9 of [ISO10589])
      and the scoped flooding PDU (type 10 in section 3.1 of [RFC7356])
      SHOULD BE the only PDU types used to carry link state information
      in a Openfabric implementation

   o  Processing related to the Level 1 Link State PDU (type 18) MAY BE
      removed from Openfabric implementations (see section 9.8 of
      [ISO10589])

   o  Neighbor reachability MUST BE carried in TLV type 22 (see section
      3 of [RFC5305])

   o  IPv4 reachability SHOULD BE carried in TLV type 135 (see section 4
      of [RFC5305]), or TLV type 235 for multitopology implementations
      (see [RFC5120])

   o  IPv6 reachability SHOULD BE carried in TLV type 236 (see
      [RFC5308]), or TLV type 237 for multitopology implemenations (see
      [RFC5120])

   o  Processing related to the neighbor reachability TLV (type 2, see
      sections 9.8 and 9.9 of [ISO10589]) SHOULD BE removed

   o  Processing related to the narrow metric IP reachability TLV (types
      128 and 130) SHOULD BE removed





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   In order to support segment routing, Openfabric needs to be able to
   support the advertisement of a Prefix-SID tied to a local loopback
   address assigned to the IS.  The configuration of the label to
   advertise MAY BE manually configured for the moment or determined
   through autoconfiguration.  A Prefix-SID SHOULD BE advertised if a
   local label is configured using the Prefix Segment Identifier sub-TLV
   (see section 2.1 of [I-D.ietf-isis-segment-routing-extensions]).

4.  Determining and Advertising Location on the Fabric

   The tier to which a IS is connected is useful to enable
   autoconfiguration of intermediate systems connected to the fabric and
   to reduce flooding.  Once the tier of an intermediate system within
   the fabric has been determined, it MUST be advertised using the 4 bit
   Tier field described in section 3.3 of
   [I-D.shen-isis-spine-leaf-ext].  This section describes two
   mechanisms for determining the tier at which a IS is connected in the
   fabric in several steps.

4.1.  Calculating Tier Number with a Fixed T0

   The first method begins with one of the T0 intermediate systems
   advertising its location in the fabric.  This information can either
   be obtained through:

   o  A single T0 intermediate system is manually configured to
      advertise 0x00 in their IS reachability tier sub-TLV, indicating
      they are at the edge of the fabric (a ToR IS).

   o  The T0 intermediate systems detect they are T0 through the
      presence connected hosts (i.e. through a request for address
      assignment or some other means).  If such detection is used, and
      the IS determines it is located at T0, it should advertise 0x00 in
      its IS reachability tier sub-TLV.

   The second method above SHOULD be used with care, as it may not be
   secure, and it may not work in all data center environments.  For
   instance, if a host is mistakenly (or intentionally, as a form of
   attack) attached to a spine IS, or a request for address assignment
   is transmitted to a spine IS during the bootup phase of the device or
   fabric, it is possible to cause a spine IS to advertise itself as a
   T0.  Unless the autodetection of the T0 devices is secured, the
   manual mechanism SHOULD BE used (configuring at least one T0 device
   manually).

   Given at least one T0 device is advertising its tier number, the
   remaining intermediate systems calculate their tier number as
   follows:



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   o  The local IS calculates an SPT (using SPF) setting the cost of
      every link to 1; this effectively calculates a topology only view
      of the network, without considering any configured link costs

   o  Find the closest IS advertising a tier number of 0 in the Spine
      Leaf extension sub-TLV; call this node A, and set FD to this cost

   o  Calculate an SPT (using SPF) from the perspective of A (above),
      and setting the cost of every link to 1; the maximum cost to any
      node should be 2 for a 3 stage fabric, 4 for a 5 stage fabric,
      etc.

   o  Choose any node that is a maximum metric from A (above); call this
      IS B

   o  Find the cost to B on the locally calculated SPT from the first
      step; call this TD

   o  Calculate the tier number of the local node by subtracting FD from
      TD

   In the example network, assume 5A is manually configured as a T0, and
   is advertising its tier number.  From here:

   o  From 1A the path to 5A is 4 hops; this is FD

   o  Run SPF from the perspective of 5A with all link metrics set to 1

   o  The maximum path length is 4; 1F is one such node; set this node
      to B, and set TD to 4

   o  TD - FD is 0 at 1A, so 1A is T0, or a ToR

   This process will work for any spine and leaf fabric without "cross
   links."

4.2.  Calculating the Tier Number in a Five Stage Spine and Leaf

   In some fabrics, it is possible to calculate which intermediate
   systems are at T0 using a modified Shortest Path First (SPF)
   calculation.  Specifically, if the fabric is configured in five
   stages, as shown in the example network, and is not some form of
   butterfly, Benes, or a three stage fabric, it is possible to
   calcualte if an IS is at T0 using the following process:

   o  Calculate a Shortest Path Tree (SPT) for the entire network with
      all link metrics set to 1; this has the effect of calculating a
      tree based only on hop count



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   o  Find one node that is the farthest from the local node in the
      resulting tree; call this node F, and the distance to this node FD

   o  Calculate an SPT for the entire network with all link metrics set
      to 1 from the perspective of F; call this TD

   If FD == TD, and TD >= 4, this is a greater than three stage fabric;
   the local device SHOULD advertise 0x00 in its IS reachability tier
   sub-TLV.  For instance, in the diagram above, 1A would:

   o  Calculate an SPT with all link metrics set to 1; on this SPT, 5A
      through 5F would all have a distance of 4

   o  Select one of these nodes as F; assume 5F is chosen as F

   o  Set FD to 4, the distance to 5F

   o  Run SPF from the perspective of 5F with all link metrics set to 1

   o  Set TD to 4, the cost from 5F to 1A

   o  TD - FD == 0, so 1A is at T0, and is a ToR

   For the remaining intermediate systems to determine which tier they
   are situated on, they perform the following calculation:

   o  Calculate a Shortest Path Tree (SPT) for the entire network with
      all link metrics set to 1; this has the effect of calculating a
      tree based only on hop count

   o  Find one node that is the farthest from the local node in the
      resulting tree; call this node F, and the distance to this node FD

   o  Calculate an SPT for the entire network with all link metrics set
      to 1 from the perspective of F; call this TD

   The IS SHOULD advertise (TD - FD) in its IS reachability tier sub-
   TLV.

   For example, in the above five stage fabric, 3B would:

   o  Calculate an SPT with all link metrics set to 1; on this SPT, 5A
      through 5F and 1A through 1F would all have a cost of 2

   o  Select one of these nodes as F; assume 5F is chosen as F

   o  Set FD to 2, the distance to 5F




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   o  Run SPF from the perspective of 5F with all link metrics set to 1

   o  Set TD to 4, the cost from 5F to 1A

   o  TD - FD == 2, so 1A is at T2, and is a spine switch

5.  Flooding Optimization

   Flooding is perhaps the most challenging scaling issue for a link
   state protocol running on a dense, large scale fabric.  To reduce the
   flooding of link state information in the form of Link State Protocol
   Data Units (LSPs), Openfabric takes advantage of information already
   available in the link state protocol, the list of the local
   intermediate system's neighbor's neighbors, and the fabric locality
   computed above.  The following tables are required to compute a set
   of reflooders:

   o  Neighbor List (NL) list: The set of neighbors

   o  Neighbor's Neighbors (NN) list: The set of neighbor's neighbors;
      this can be calculated by running SPF truncated to two hops

   o  Do Not Reflood (DNR) list: The set of neighbors who should have
      LSPs (or fragments) who should not reflood LSPs

   o  Reflood (RF) list: The set of neighbors who should flood LSPs (or
      fragments) to their adjacent neighbors to ensure synchronization

   NL is set to contain all neighbors, and sorted deterministically (for
   instance, from the highest IS identifier to the lowest).  All
   intermediate systems within a single fabric SHOULD use the same
   mechanism for sorting the NL list.  NN is set to contain all
   neighbor's neighbors, or all intermediate systems that are two hops
   away, as determined by performing a truncated SPF.  The DNR and RF
   tables are initially empty.  To begin, the following steps are taken
   to reduce the size of NN and NL:

   o  Move any IS in NL with its tier (or fabric location) set to T0 to
      DNR

   o  Remove all intermediate systems from NL and NN that in the
      shortest path to the IS that originated the LSP

   Then, for every IS in NL:

   o  If the current entry in NL is connected to any entries in NN:

      *  Move the IS to RF



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      *  Remove the intermediate systems connected to the IS from NN

   o  Else move the IS to DNR

   When flooding, LSPs transmitted to adjacent neighbors on the RF list
   will be transmitted normally.  Adjacent intermediate systems on this
   list will reflood received LSPs into the next stage of the topology,
   ensuring database synchronization.  LSPs transmitted to adjacent
   neighbors on the DNR list, however, MUST be transmitted using a
   circuit scope PDU as described in [RFC7356].

5.1.  Flooding Failures

   It is possible in some failure modes for flooding to be incomplete
   because of the flooding optimizations outlined.  Specifically, if a
   reflooder fails, or is somehow disconnected from all the links across
   which it should be reflooding, it is possible an LSP is only
   partially flooded through the fabric.  To prevent such situations,
   any IS receiving an LSP transmitted using DNR SHOULD:

   o  Set a short timer; the default should be less than one second

   o  When the timer expires, send a Complete Sequence Number Packet
      (CSNP) to all neighbors

   o  Process any Partial Sequence Number Packets (PSNPs) as required to
      resynchronize

   o  If a resynchronization is required, notify the network operator
      through a network management system

6.  Other Optimizations

6.1.  Transit Link Reachability

   In order to reduce the amount of control plane state carried on large
   scale spine and leaf fabrics, openfabric implementations SHOULD NOT
   advertise reachability for transit links.  These links MAY remain
   unnumbered, as IS-IS does not require layer 3 IP addresses to
   operate.  Each IS SHOULD be configured with a single loopback
   address, which is assigned an IPv6 address, to provide reachability
   to intermediate systems which make up the fabric.

   [RFC3277] SHOULD be supported on devices supporting openfabric with
   unnumbered interface in order to support traceability and network
   management.





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6.2.  Transiting T0 Intermediate Systems

   In data center fabrics, ToR intermediate systems SHOULD NOT be used
   to transit between two T1 (or above) spine intermediate systems.  The
   simplest way to prevent this is to set the overload bit [RFC3277] for
   all the LSPs originated from T0 intermediate systems.  However, this
   solution would have the unfortunate side effect of causing all
   reachability beyond any T0 IS to have the same metric, and many
   implementations treat a set overload bit as a metric of 0xFFFF in
   calculating the Shortest Path Tree (SPT).  This document proposes an
   alternate solution which preserves the leaf node metric, while still
   avoiding transiting T0 intermediate systems.

   Specifically, all T0 intermediate systems SHOULD advertise their
   metric to reach any T1 adjacent neighbor with a cost of 0XFFE.  T1
   intermediate systems, on the other hand, will advertise T0
   intermediate systems with the actual interface cost used to reach the
   T0 IS.  Hence, links connecting T0 and T1 intermediate systems will
   be advertised with an asymmetric cost that discourages transiting T0
   intermediate systems, while leaving reachability to the destinations
   attached to T0 devices the same.

7.  Openfabric and Route Aggregation

   While schemes may be designed so reachability information can be
   aggregated in Openfabric deployments, this is not a recommended
   configuraiton.

8.  Security Considerations

   This document outlines modifications to the IS-IS protocol for
   operation on large scale data center fabrics.  While it does add new
   TLVs, and some local processing changes, it does not add any new
   security vulnerabilities to the operation of IS-IS.  However,
   openfabric implementations SHOULD implement IS-IS cryptographic
   authentication, as described in [RFC5304], and should enable other
   security measures in accordance with best common practices for the
   IS-IS protocol.

   If T0 intermediate systems are auto-detected using information
   outside Openfabric, it is possible to attack the calucations used for
   flooding reduction and auto-configuration of intermediate systems.
   For instance, if a request for an address pool is used as an
   indicator of an attached host, and hence receiving such a request
   causes an intermediate system to advertise itself as T0, it is
   possible for an attacker (or a simple mistake) to cause auto-
   configuration to fail.  Any such auto-detection mechanims SHOULD BE




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   secured using appropriate techniques, as described by any protocols
   or mechanisms used.

9.  References

9.1.  Normative References

   [I-D.shen-isis-spine-leaf-ext]
              Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
              Routing for Spine-Leaf Topology", draft-shen-isis-spine-
              leaf-ext-05 (work in progress), January 2018.

   [ISO10589]
              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/
              IEC 10589:2002, Second Edition, Nov 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              DOI 10.17487/RFC2629, June 1999,
              <https://www.rfc-editor.org/info/rfc2629>.

   [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-editor.org/info/rfc5120>.

   [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>.

   [RFC5303]  Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way
              Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
              DOI 10.17487/RFC5303, October 2008,
              <https://www.rfc-editor.org/info/rfc5303>.

   [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>.




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   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

   [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-editor.org/info/rfc5309>.

   [RFC5311]  McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
              Shand, "Simplified Extension of Link State PDU (LSP) Space
              for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
              <https://www.rfc-editor.org/info/rfc5311>.

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
              December 2008, <https://www.rfc-editor.org/info/rfc5316>.

   [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-editor.org/info/rfc7356>.

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

9.2.  Informative References

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
              "IS-IS Extensions for Segment Routing", draft-ietf-isis-
              segment-routing-extensions-15 (work in progress), December
              2017.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-14 (work
              in progress), December 2017.

   [RFC3277]  McPherson, D., "Intermediate System to Intermediate System
              (IS-IS) Transient Blackhole Avoidance", RFC 3277,
              DOI 10.17487/RFC3277, April 2002,
              <https://www.rfc-editor.org/info/rfc3277>.



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   [RFC3719]  Parker, J., Ed., "Recommendations for Interoperable
              Networks using Intermediate System to Intermediate System
              (IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004,
              <https://www.rfc-editor.org/info/rfc3719>.

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

   [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>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC5449]  Baccelli, E., Jacquet, P., Nguyen, D., and T. Clausen,
              "OSPF Multipoint Relay (MPR) Extension for Ad Hoc
              Networks", RFC 5449, DOI 10.17487/RFC5449, February 2009,
              <https://www.rfc-editor.org/info/rfc5449>.

   [RFC5614]  Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
              Extension of OSPF Using Connected Dominating Set (CDS)
              Flooding", RFC 5614, DOI 10.17487/RFC5614, August 2009,
              <https://www.rfc-editor.org/info/rfc5614>.

   [RFC5837]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
              N., and JR. Rivers, "Extending ICMP for Interface and
              Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
              April 2010, <https://www.rfc-editor.org/info/rfc5837>.

   [RFC6232]  Wei, F., Qin, Y., Li, Z., Li, T., and J. Dong, "Purge
              Originator Identification TLV for IS-IS", RFC 6232,
              DOI 10.17487/RFC6232, May 2011,
              <https://www.rfc-editor.org/info/rfc6232>.

   [RFC7182]  Herberg, U., Clausen, T., and C. Dearlove, "Integrity
              Check Value and Timestamp TLV Definitions for Mobile Ad
              Hoc Networks (MANETs)", RFC 7182, DOI 10.17487/RFC7182,
              April 2014, <https://www.rfc-editor.org/info/rfc7182>.

   [RFC7921]  Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
              Nadeau, "An Architecture for the Interface to the Routing
              System", RFC 7921, DOI 10.17487/RFC7921, June 2016,
              <https://www.rfc-editor.org/info/rfc7921>.



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Authors' Addresses

   Russ White (editor)
   LinkedIn

   Email: russ@riw.us


   Shawn Zandi (editor)
   LinkedIn

   Email: szandi@linkedin.com







































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