Intermediate System to Intermediate System (IS-IS) Transient Blackhole Avoidance
RFC 3277

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Network Working Group                                       D. McPherson
Request for Comments: 3277                                           TCB
Category: Informational                                       April 2002

           Intermediate System to Intermediate System (IS-IS)
                     Transient Blackhole Avoidance

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document describes a simple, interoperable mechanism that can be
   employed in Intermediate System to Intermediate System (IS-IS)
   networks in order to decrease the data loss associated with
   deterministic blackholing of packets during transient network
   conditions.  The mechanism proposed here requires no IS-IS protocol
   changes and is completely interoperable with the existing IS-IS
   specification.

1. Introduction

   When an IS-IS router that was previously a transit router becomes
   unavailable as a result of some transient condition such as a reboot,
   other routers within the routing domain must select an alternative
   path to reach destinations which have previously transited the failed
   router.  Presumably, the newly selected router(s) comprising the path
   have been available for some time and, as a result, have complete
   forwarding information bases (FIBs) which contain a full set of
   reachability information for both internal and external (e.g., BGP)
   destination networks.

   When the previously failed router becomes available again, it is only
   seconds before the paths that had previously transited the router are
   again selected as the optimal path by the IGP.  As a result,
   forwarding tables are updated and packets are once again forwarded
   along the path.  Unfortunately, external destination reachability
   information (e.g., learned via BGP) is not yet available to the
   router, and as a result, packets bound for destinations not learned
   via the IGP are unnecessarily discarded.

McPherson                    Informational                      [Page 1]
RFC 3277          IS-IS Transient Blackhole Avoidance         April 2002

   A simple interoperable mechanism to alleviate the offshoot associated
   with this deterministic behavior is discussed below.

2. Discussion

   This document describes a simple, interoperable mechanism that can be
   employed in IS-IS [1, 2] networks in order to avoid transition to a
   newly available path until other associated routing protocols such as
   BGP have had sufficient time to converge.

   The benefits of such a mechanism can be realized when considering the
   following scenario depicted in Figure 1.

                                 D.1
                                  |
                              +-------+
                              | RtrD  |
                              +-------+
                              /      \
                             /        \
                        +-------+    +-------+
                        | RtrB  |    | RtrC  |
                        +-------+    +-------+
                             \        /
                              \      /
                              +-------+
                              | RtrA  |
                              +-------+
                                   |
                                  S.1

                 Figure 1: Example Network Topology

   Host S.1 is transmitting data to destination D.1 via a primary path
   of RtrA->RtrB->RtrD.  Routers A, B and C learn of reachability to
   destination D.1 via BGP from RtrD.  RtrA's primary path to D.1 is
   selected because when calculating the path to BGP NEXT_HOP of RtrD,
   the sum of the IS-IS link metrics on the RtrA-RtrB-RtrD path is less
   than the sum of the metrics of the RtrA-RtrC-RtrD path.

   Assume RtrB becomes unavailable and as a result the RtrC path to RtrD
   is used.  Once RtrA's FIB is updated and it begins forwarding packets
   to RtrC, everything should behave properly as RtrC has existing
   forwarding information regarding destination D.1's availability via
   BGP NEXT_HOP RtrD.

McPherson                    Informational                      [Page 2]
RFC 3277          IS-IS Transient Blackhole Avoidance         April 2002

   Assume now that RtrB comes back online.  In only a few seconds, IS-IS
   neighbor state has been established with RtrA and RtrD and database
   synchronization has occurred.  RtrA now realizes that the best path
   to destination D.1 is via RtrB, and therefore updates it FIB
   appropriately.  RtrA begins to forward packets destined to D.1 to
   RtrB.  Though, because RtrB has yet to establish and synchronize its
   BGP neighbor relationship and routing information with RtrD, RtrB has
   no knowledge regarding reachability of destination D.1, and therefore