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Document Type RFC Internet-Draft (isis WG)
Author Les Ginsberg 
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IS-IS for IP Internets                                          C. Hopps
Internet-Draft                                               L. Ginsberg
Intended status: Standards Track                           Cisco Systems
Expires: July 6, 2011                                    January 2, 2011

                         IS-IS BFD Enabled TLV


   This document describes a type-length-value (TLV) for use in the
   IS-IS routing protocol that allows for the proper use of the
   Bidirectional Forwarding Detection protocol (BFD).  There exist
   certain scenarios in which IS-IS will not react appropriately to a
   BFD detected forwarding plane failure without use of either this TLV
   or some other method.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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

   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 July 6, 2011.

Copyright Notice

   Copyright (c) 2011 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

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   ( 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.  The Problem . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  The Solution  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  State Definitions . . . . . . . . . . . . . . . . . . . . . 4
     3.2.  Adjacency Establishment and Maintenance . . . . . . . . . . 4
     3.3.  Advertisement of Topology Specific IS Neighbors . . . . . . 5
   4.  Transition  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Graceful Restart  . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  The BFD Enabled TLV . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   10. Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The Bidirectional Forwarding Detection (BFD) protocol [RFC5880] is a
   protocol that allows for detection of a forwarding plane failure
   between two routers.  A router can use [RFC5880] to validate that a
   peer router's forwarding ability is functioning.

   One specific application of BFD as described in [RFC5882] is to
   verify the forwarding ability of an IS-IS [RFC1195] router's
   adjacencies; however, the method described in [RFC5882] does not
   allow for certain failure scenarios.  We will define a TLV that will
   allow for proper response to the detection of all forwarding failures
   where the use of BFD is employed with IS-IS.

2.  The Problem

   We observe that to allow for mixed use (i.e., some routers running
   BFD and some not) [RFC5882] does not require a BFD session be
   established prior to the establishment of an IS-IS adjacency.  Thus,
   if a router A has neighbors B and C, and B does not support BFD, A
   would still form adjacencies with B and C, and would only establish a
   BFD session with C.

   The problem with this solution is that it assumes that the
   transmission and receipt of IS-IS Hellos (IIHs) shares fate with
   forwarded data packets.  This is not a fair assumption to make given
   that the primary use of BFD is to protect IPv4 (and IPv6) forwarding
   and IS-IS does not utilize IPv4 or IPv6 for sending or receiving its

   Thus, if we consider our previous example, and if C is currently
   experiencing an IPv4 forwarding failure that allows for IIHs to be
   sent and received, when A first starts (or restarts) A will assume
   that C simply does not support BFD, will form an adjacency with C,
   and may incorrectly forward IPv4 traffic through C.

3.  The Solution

   A simple solution to this problem is for an IS-IS router to advertise
   that it has BFD enabled on a given interface.  It can do this through
   the inclusion of a TLV in its IIHs as described in this document.

   When sending an IIH on a BFD enabled interface, a router which
   supports this extension MUST include the BFD enabled TLV in its IIH.
   The contents of the TLV MUST indicate what topologies/protocols
   [RFC5120] have been enabled for BFD by including the appropriate

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   Multi-Topology Identifier (MTID)/ Network Layer Protocol Identifier
   (NLPID) pairs.

   When sending an IIH on an interface on which BFD is NOT enabled a
   router MUST NOT include the BFD enabled TLV.

3.1.  State Definitions

   The following definitions apply to each IS-IS neighbor:

   For each locally supported MTID/NLPID pair, an
   "ISIS_TOPO_NLPID_BFD_REQUIRED" variable is assigned.  If BFD is
   supported by both the local system and the neighbor for the MTID/
   NLPID this variable is set to "TRUE".  Otherwise the variable is set
   to "FALSE".

   For each locally supported MTID, an "ISIS_TOPO_BFD_REQUIRED" variable
   is set to the logical "OR" of all "ISIS_TOPO_NLPID_BFD_REQUIRED"
   variables associated with that MTID.

   An "ISIS_BFD_REQUIRED" variable is set to the logical "AND" of all
   "ISIS_TOPO_BFD_REQUIRED" variables.

   For each locally supported MTID/NLPID pair, an
   "ISIS_TOPO_NLPID_STATE" variable is assigned.  If
   "ISIS_TOPO_NLPID_BFD_REQUIRED" is "TRUE", this variable follows the
   BFD session state for that MTID/NLPID ("UP == TRUE").  Otherwise the
   variable is set to "TRUE".

   For each locally supported topology (MTID), an "ISIS_TOPO_USEABLE"
   variable is set to the logical "AND" of the set of
   "ISIS_TOPO_NLPID_STATE" variables associated with that MTID.

   An "ISIS_NEIGHBOR_USEABLE" variable is set to the logical "OR" of all
   "ISIS_TOPO_USEABLE" variables.

3.2.  Adjacency Establishment and Maintenance

   Whenever "ISIS_BFD_REQUIRED" is "TRUE" the following extensions to
   the rules for adjacency establishment and maintenance MUST apply:

   o  "ISIS_NEIGHBOR_USEABLE" MUST be "TRUE" before the adjacency can
      transition from "INIT" to "UP" state

   o  When the IS-IS adjacency is "UP" and "ISIS_NEIGHBOR_USEABLE"
      becomes "FALSE" the IS-IS adjacency MUST transition to "DOWN".

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   o  On a Point-to-Point circuit whenever "ISIS_NEIGHBOR_USEABLE" is
      "FALSE", the Three-Way adjacency state MUST be set to "DOWN" in
      the Point-to-Point Three Way Adjacency TLV[RFC5303] in all
      transmitted IIHs.

   o  On a LAN circuit whenever "ISIS_NEIGHBOR_USEABLE" is "FALSE", the
      IS Neighbors TLV advertising the MAC address of the neighbor MUST
      be omitted in all transmitted IIHs.

3.3.  Advertisement of Topology Specific IS Neighbors

   The advertisement of a topology specific IS-neighbor (as well as the
   use of the neighbor in the topology specific decision process) is
   determined by the value of "ISIS_TOPO_USEABLE" for each topology.  If
   "ISIS_TOPO_USEABLE" is "TRUE" then the topology specific neighbor is
   advertised.  If "ISIS_TOPO_USEABLE" is "FALSE" then the topology
   specific neighbor is not advertised.

4.  Transition

   To allow for a non-disruptive transition to the use of BFD some
   amount of time should be allowed before bringing down an "UP"
   adjacency on a BFD enabled interface when the value of
   "ISIS_BFD_REQUIRED" becomes "TRUE" as a result of the introduction of
   the BFD TLV or the modification (by adding a new supported MTID/
   NLPID) of an existing BFD TLV in a neighbor's IIH.  A simple way to
   do this is to not update the adjacency hold-time when receiving such
   an IIH from a neighbor with whom we have an "UP" adjacency until

   If the value of "ISIS_BFD_REQUIRED" becomes "FALSE" as a result of
   the removal the BFD TLV or the modification (by removing a supported
   MTID/NLPID) of an existing BFD TLV in a neighbor's IIH then BFD
   session establishment is no longer required to maintain the adjacency
   or transition the adjacency to the "UP" state.

   If a BFD session is administratively shut down [RFC5880] and the BFD
   session state change impacts the value of "ISIS_NEIGHBOR_USEABLE",
   then IS-IS SHOULD allow time for the corresponding MTID/NLPID to be
   removed from the neighbor's BFD TLV by not updating the adjacency
   hold time until "ISIS_BFD_REQUIRED" becomes "FALSE".  Note that while
   this allows a non-disruptive transition, it still enforces
   consistency between the administrative state of the BFD session and
   the MTID/NLPID(s) advertised in the BFD TLV.  This is necessary to
   provide consistent behavior regardless of whether the BFD AdminDown
   state is introduced before or after an IS-IS adjacency "UP" state has
   been achieved.

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5.  Graceful Restart

   This section describes IS-IS implementation considerations when both
   both IS-IS graceful restart [RFC5306] and BFD are co-deployed.

   In cases where BFD shares fate with the control plane, it can be
   expected that BFD session failure may occur in conjunction with the
   control plane restart.  In such cases premature abort of IS-IS
   graceful restart as a result of BFD session failure is undesirable.
   Therefore, some mechanism to ignore the BFD session failure for a
   limited period of time would be beneficial.  The issue of the
   interaction between graceful restart and BFD is described at length
   in RFC5882.  The implementation of this interaction is outside the
   scope of this document.

6.  The BFD Enabled TLV

   The BFD enabled TLV is formatted as shown below.  The TLV SHALL only
   be included in an IIH and only when BFD is enabled for one or more
   supported MTID/protocols on the interface over which the IIH is being
   sent.  The NLPIDs encoded in the TLV are defined in [ISO9577]

     Type   139 (suggested - to be assigned by IANA)
     Length # of octets in the value field (3 to 255)
     Value  three octets specifying the MTID/NLPID for each
            topology/data protocol for which BFD support is enabled

                                          No. of octets
                |R|R|R|R|   MTID        |     2
                |      NLPID            |     1
                :                       :
                :                       :
                |R|R|R|R|   MTID        |     2
                | NLPID                 |     1

7.  Security Considerations

   The TLV defined within this document describes an addition to the
   IS-IS Hello protocol.  Inappropriate use of this TLV could prevent an
   IS-IS adjacency from forming or lead to failure to detect

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   bidirectional forwarding failures - each of which is a form of denial
   of service.  However, a party who can manipulate the contents of this
   TLV is already in a position to create such a denial of service by
   disrupting IS-IS routing in other ways.

   Note that the introduction of this TLV has no impact on the use/
   non-use of authentication either by IS-IS or by BFD.

8.  IANA Considerations

   The following IS-IS TLV type is defined by this draft.

   Name                                  Value  IIH   LSP  SNP
   ----------------------                -----  ---   ---  ---
   BFD Enabled TLV                         139   y     n    n

   Please update the IS-IS TLV Codepoint Registry accordingly.

9.  Acknowledgements

   The authors wish to thank Jeffrey Haas, Matthew Jones, Dave Katz,
   Jonathan Moon, Stefano Previdi, Mike Shand, Michael Shiplett and
   David Ward, for various input on this document.

10.  Normative References

   [ISO9577]  International Organization for Standardization, "Protocol
              identification in the network layer(ISO/IEC 9577)", ISO/
              IEC 9577:1999, Fourth Edition, Dec 1999.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, December 1990.

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

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

   [RFC5303]  Katz, D., Saluja, R., and D. Eastlake, "Three-Way
              Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
              October 2008.

   [RFC5306]  Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",

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              RFC 5306, October 2008.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC5882]  Katz, D. and D. Ward, "Generic Application of
              Bidirectional Forwarding Detection (BFD)", RFC 5882,
              June 2010.

Authors' Addresses

   Christian E. Hopps
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, California  95134


   Les Ginsberg
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, Ca.  95035


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