RFC 3277 (rfc3277) - Page 2 of 6
Intermediate System to Intermediate System (IS-IS) Transient Blackhole Avoidance
Alternative Format: Original Text Document
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
