RFC 3277 (rfc3277) - Page 1 of 6

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

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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
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