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Internet Engineering Task Force (IETF) C. Hopps Request for Comments: 6213 L. Ginsberg Category: Standards Track Cisco Systems ISSN: 2070-1721 April 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 (BFD) protocol.  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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

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
 ( 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.

Hopps & Ginsberg Standards Track [Page 1] RFC 6213 IS-IS BFD-Enabled TLV April 2011

Table of Contents

 1. Introduction ....................................................2
    1.1. Requirements Language ......................................2
 2. The Problem .....................................................2
 3. The Solution ....................................................3
    3.1. State Definitions ..........................................3
    3.2. Adjacency Establishment and Maintenance ....................4
    3.3. Advertisement of Topology-Specific IS Neighbors ............4
 4. Transition ......................................................4
 5. Graceful Restart ................................................5
 6. The BFD-Enabled TLV .............................................5
 7. Security Considerations .........................................6
 8. IANA Considerations .............................................6
 9. Acknowledgements ................................................6
 10. Normative References ...........................................7

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.

1.1. Requirements Language

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

2. The Problem

 We observe that, in order 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 it would only
 establish a BFD session with C.

Hopps & Ginsberg Standards Track [Page 2] RFC 6213 IS-IS BFD-Enabled TLV April 2011

 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 that
 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
 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 of 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

Hopps & Ginsberg Standards Track [Page 3] RFC 6213 IS-IS BFD-Enabled TLV April 2011

 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".
 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 Media Access Control (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

Hopps & Ginsberg Standards Track [Page 4] RFC 6213 IS-IS BFD-Enabled TLV April 2011

 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.

5. Graceful Restart

 This section describes IS-IS implementation considerations when 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 RFC 5882.  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].

Hopps & Ginsberg Standards Track [Page 5] RFC 6213 IS-IS BFD-Enabled TLV April 2011

   Type   148
   Length # of octets in the value field (3 to 255)
   Value  3 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
 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 document.
 Name                                  Value  IIH  LSP  SNP  Purge
 ----------------------                -----  ---  ---  ---  -----
 BFD-Enabled TLV                         148   y    n    n     n
 The IS-IS TLV Codepoint registry has been updated 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.

Hopps & Ginsberg Standards Track [Page 6] RFC 6213 IS-IS BFD-Enabled TLV April 2011

10. Normative References

 [ISO9577]  International Organization for Standardization, "Protocol
            identification in the network layer(ISO/IEC 9577)", ISO/
            IEC 9577:1999, Fourth Edition, December 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",
            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, California  95035

Hopps & Ginsberg Standards Track [Page 7]

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