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Versions: 00 01 02 03 04                                                
LSR Working Group                                            U. Chunduri
Internet-Draft                                             Futurewei USA
Intended status: Informational                               J. Tantsura
Expires: May 28, 2021                                       Apstra, Inc.
                                                                S. Hegde
                                                        Juniper Networks
                                                       November 24, 2020

             IS-IS Multi Topology Deployment Considerations


   This document analyzes IS-IS Multi Topology (MT) applicability in
   various IS-IS deployments.  This document explores the nuances around
   the terminology and usage of various IS-IS address families,
   topologies with different considerations, for choosing the right
   combination for a specific deployment scenario.

   This document also discusses various ways one can deploy IPv6 only
   IS-IS topology.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [RFC2119],
   RFC8174 [RFC8174] when, and only when they appear in all capitals, as
   shown here.

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 May 28, 2021.

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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Need for MT in IS-IS networks . . . . . . . . . . . . . . . .   3
   3.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Topologies and Address Families . . . . . . . . . . . . . . .   4
     4.1.  Single Topology Mode and Multiple Address Families  . . .   4
     4.2.  Multiple Topology Mode  and Multiple Address Families . .   5
       4.2.1.  Transition Mode . . . . . . . . . . . . . . . . . . .   6
     4.3.  IPv6 Only Topology  . . . . . . . . . . . . . . . . . . .   6
   5.  IS-IS MT and LFA  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   IS-IS originally developed for OSI [ISO.10589.1992] and extensions
   have been made available to support IPv4 [RFC1195].  A method for
   exchanging IPv6 routing information using the IS-IS routing protocol
   is specified in [RFC5308].  How to run a set of independent IP
   topologies with topology specific adjacencies, within a single IS-IS
   domain has been defined in IS-IS MT [RFC5120].

   There are number of networks, including mobile backhaul networks
   seeking to use IPv6 only solutions.  It is possible to conceive,
   various parts of the backhaul networks use IPv4 and appropriate
   migration strategy needed before eventually moving towards IPv6 only

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   network.  While any IGP can be used in these networks, this document
   covers only IS-IS protocol aspects.

   Various layer-3 DC fabric routing options (refs: openfabric, spine-
   leaf, controller-based) by changing or optimizing some aspects w.r.t
   adjacency formation, flooding optimizations, or/and mechanisms to
   automatically compute the location of the node in the fat tree
   topology are proposed recently and this document brings some of the
   multi topology deployment aspects relevant to these networks.  Please
   note, part of the discussion around IS-IS MT is not specific to DC or
   CLOS fabrics and generally applicable to any IS-IS deployment but
   discussed here because of multiple proposals to use various forms of
   IS-IS in this context.

2.  Need for MT in IS-IS networks

   For mobile transport backhaul networks seeking only IPv6 network or
   transitioning from parts of the network with only IPv4, IS-IS MT is
   needed.  For layer-3 DC fabric underlay, which provide reachability,
   only one address family (either IPv4 or IPv6) SHOULD be sufficient.
   However if either only IPv6 address family is needed in the underlay
   or deploying both IPv4 and IPv6 address families are desired
   discussion in Section 4 is relevant.

   It is an unlikely requirement, where DC fabric to be partitioned
   logically to have different topologies in the underlay but this can
   happen in various scenarios as listed in Section 4.1.  If one does
   the same to meet a particular requirement, it introduces a
   manageability complexity of these logical topologies.  IS-IS MT
   [RFC5120] also designed to address the above need and discussion in
   Section 4.2 is relevant.  It is worth noting, majority of the IS-IS
   deployments use MT primarily to have a separate logical topology for
   IPv6 address family.

3.  Acronyms

          IIH : IS-IS Hello Protocol Data Unit

          LSP : Link State PDU

          MT : Multi Topology

          SPF : Shortest Path First

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4.  Topologies and Address Families

   Terminology around IS-IS topologies and address families is somewhat
   confusing at best.  Just to give an example, MT ID #2 defined in
   [RFC5120] says, it is "Reserved for IPv6 routing topology".  While
   multiple MT ID's can be deployed in a network with IPv6 topologies,
   MT ID #2, perhaps referring to a first such topology with IPv6 only
   address family.  This section details various topology and address
   family options possible with currently available IS-IS specifications
   with respective defined TLVs.

4.1.  Single Topology Mode and Multiple Address Families

   IS-IS with IPv4 address family and with wide-metrics [RFC5305] is
   widely deployed, with TLV 22 defined for IS Reachability and TLV 135
   for IP (IPv4) reachability information . This is essentially a single
   topology for the entire IS-IS area/domain with a single address
   family (IPv4 unicast).

   IS-IS can also be enabled with IPv6 unicast address family in a
   single topology mode along with IPv4 unicast address family.  Here
   IPv6 uses the same underlying topology that is used for IPv4 and this
   can be done as specified in IS-IS IPv6 [RFC5308] which introduces TLV
   236, an IPv6 reachability TLV.  It is important to note same IS-IS
   adjacency is used for both address families and with a single SPF
   (decision process) both IPv4 and IPv6 reachability would be computed.

   However, for the above to work effectively, both IPv4 and IPv6
   address families MUST share a common network topology.  That is to
   use IS-IS for IPv4 and IPv6 routing, any interface configured for
   IPv4 IS-IS MUST also be configured for IPv6 IS-IS, and vice versa.
   All routers within an IS-IS area (Level 1 routing) or domain (Level 2
   routing) MUST also support the same set of address families: IPv4
   only, IPv6 only, or both IPv4 and IPv6.  Any discrepancy in the
   configuration w.r.t above can cause routing black holes and one such
   scenario is discussed below.

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                         | /         \|
                      ...Rx          Ry...
                         | \          /
                         |   \      / |
                         |     \  /   |
                         |      /\    |
                         |    /    \  |
                         |  /        \|
                     ... R1          R2...
                         | \        / |

              Figure 1: IS-IS with multiple address families

   As shown, in the above diagram all routers in the network enabled
   with both IPv4 and IPv6 unicast address families at the IS level and
   single topology would be built.  However, at a link level all but
   except one link, say if IPv6 is not configured on the link between
   the routers Rx and R2; due to a single IS-IS topology, the shortest
   path between Rx and R2 is the direct link and since IPv6 is not
   enabled on that link, Rx and R2 cannot exchange IPv6 data traffic
   even though there's an alternate path between them in the topology
   through Rx, R1, Ry and R2.

   Hence to summarize the restrictions: all routers in the topology MUST
   support only IPv4, only IPv6 or both IPv4 and IPv6 address families
   on all links and node.  In other words, network MUST be congruent.
   While this model is to simpler to operate, might not be flexible
   enough for some IS-IS deployments.  Some examples where congruency is
   not possible as follows:

   a.  When IPv6 is getting introduced in the network legacy nodes that
       are IPv6 incapable.

   b.  Implementation issues causing IPv6 to be disabled on some nodes.

   c.  Hardware scale limitations causing IPv6 to be disabled on some
       low-end nodes.

4.2.  Multiple Topology Mode and Multiple Address Families

   Multi-topology IS-IS uses multiple SPFs to compute routes and removes
   the restriction that all interfaces MUST support all configured
   address families and that all routers in an IS-IS area or domain MUST
   support the same set of address families.  This introduces the
   concept of topology specific adjacency with MT IS Reachability TLV
   222 and MT capable IPv4 Reachability with TLV 235 and MT capable IPv6
   Reachability with TLV 237.

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   When MT IS-IS is enabled with IPv4 and IPv6 address families, the
   routers build two topologies, one for each address family (IPv4 and
   IPv6) and can find the optimum path for each address family even when
   some links in the network support only one of them.  IS-IS MT
   [RFC5120] defines MT ID #0 for backward compatibility, as the
   "standard" topology and this essentially operate as IS-IS single
   topology mode as specified in Section 4.1 and supports both IPv4 and
   IPv6 address families.  MT ID #2 [RFC5120] is defined for IPv6
   address family in MT mode.

4.2.1.  Transition Mode

   Most of the vendors supported MT transition feature (though some
   vendors disabled to avoid confusion around this) in the IS-IS
   networks to facilitate MT deployments without disrupting the single
   topology mode.  The MT transition mode allows a network operating in
   single topology IS-IS IPv6 [RFC5308] to continue to work while
   upgrading routers to include MT IS-IS IPv6 support i.e., MT ID #2
   with [RFC5120] . While in transition mode, both types of TLVs
   (single-topology with TLVs 22/236 and MT with TLVs 222/237) are sent
   in LSPs for all configured IPv6 addresses, nodes can continue to
   process these and operate in single topology mode though being in MT
   mode ("standard" IS-IS topology with MT ID #0).  After all routers in
   the area or domain have been upgraded to support MT IPv6 transition
   mode can be removed from the configuration.  Once all routers in the
   area or domain are operating in MT IPv6 mode, the topological
   restrictions of single-topology mode can be made no longer in effect.

   When transition mode is enabled, the router advertises both MT TLVs
   and the old style IS-IS IPv6 TLVs but the topological restrictions of
   the single topology mode discussed above are in effect.  However,
   there were instances while this mode is enabled and expectations for
   different result in the actual deployments.

4.3.  IPv6 Only Topology

   Though it is theoretically possible to build IPv6 only underlay (with
   TLV 236 for IPv6 reachability prefixes) in single topology mode as
   discussed in Section 4.1, lot of legacy implementations require IPv4
   address families too be configured in single topology mode (ingrained
   code structures for IPv4 address family).  IPv6 only DC underlay
   network can be built with multi topology adjacencies (TLV 222) and
   reachability prefixes (TLV 237) with MT ID #2 as discussed above in
   Section 4.2.  With this, any other address family can be introduced
   including "standard" topology MT ID #0 (Single topology mode with
   both address families) and there are no restrictions on which address
   family has to enable on which link as specified in Section 4.1.

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5.  IS-IS MT and LFA

   IP Fast Reroute (FRR) or Loop Free Alternative (LFA) computation in
   MT mode are described in detail in Section 5.2 of [RFC5120].

6.  Acknowledgements

   Thanks to Acee Lindem, Chris Hopps, Michael Abramson and Les Ginsberg
   for various inputs on this work.

7.  IANA Considerations

   This document has no actions for IANA.

8.  Security Considerations

   Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310].
   Further security analysis for IS-IS protocol is done in [RFC7645].

   This document does not introduce any change in any of the IS-IS
   protocol or IS-IS protocol extensions.  This document also does not
   introduce any new security issues other than as noted in the
   referenced IS-IS protocol extensions.

9.  References

9.1.  Normative References

              International Organization for Standardization,
              "Intermediate system to intermediate system intra-domain-
              routing routine information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)",
              ISO Standard 10589, 1992.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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9.2.  Informative References

   [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,

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

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

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

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC7645]  Chunduri, U., Tian, A., and W. Lu, "The Keying and
              Authentication for Routing Protocol (KARP) IS-IS Security
              Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015,

   [RFC8518]  Sarkar, P., Ed., Chunduri, U., Ed., Hegde, S., Tantsura,
              J., and H. Gredler, "Selection of Loop-Free Alternates for
              Multi-Homed Prefixes", RFC 8518, DOI 10.17487/RFC8518,
              March 2019, <https://www.rfc-editor.org/info/rfc8518>.

Authors' Addresses

   Uma Chunduri
   Futurewei USA
   2330 Central Expressway
   Santa Clara, CA  95050

   Email: umac.ietf@gmail.com

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   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com

   Shraddha Hegde
   Juniper Networks
   Elnath-Exora Business Park Survey
   Bangalore, Karnataka  560103

   Email: shraddha@juniper.net

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