RFC 3277 (rfc3277) - Page 3 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 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 discards the packets received from RtrA destined to D.1. If RtrB were to temporarily set its LSP Overload bit while synchronizing BGP tables with its neighbors, RtrA would continue to use the working RtrA->RtrC->RtrD path, and the LSP should only be used to obtain reachability to locally connected networks (rather than for calculating transit paths through the router, as defined in [1]). However, it should be noted that when RtrB goes away, its LSP is still present in the IS-IS databases of all other routers in the routing domain. When RtrB comes back it establishes adjacencies. As soon as its neighbors have an adjacency with RtrB, they will advertise their new adjacency in their new LSP. The result is that all the other routers will receive new LSPs from RtrA and RtrD containing the RtrB adjacency, even though RtrB is still completing its synchronization and therefore has not yet sent its new LSP. At this time SPF is computed and everyone will include RtrB in their tree since they will use the old version of RtrB LSP (the new one has not yet arrived). Once RtrB has finished establishing all its adjacencies, it will then regenerate its LSP and flood it. Then all other routers within the domain will finally compute SPF with the correct information. Only at that time will the Overload bit be taken into account. As such, it is recommended that each time a router establishes an adjacency, it will update its LSP and flood it immediately, even before beginning database synchronization. This will allow for the Overload bit setting to propagate immediately, and remove the potential for an older version of the reloaded routers LSP to be used. After synchronization of BGP tables with neighboring routers (or expiry of some other timer or trigger), RtrB would generate a new LSP, clearing the Overload bit, and RtrA could again begin using the optimal path via RtrB. McPherson Informational
