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Versatile Routing and Services with BGP: Understanding and Implementing BGP in SR-OS by Colin Bookham, Alcatel-Lucent

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Chapter 8

Graceful Restart and Error Handling

Graceful Restart is a mechanism applied to many protocols and relies on the fact that modern routers separate the control-plane (RIB) from the data-plane (FIB). Given this assumption, it is entirely possible that a router undergoing a control plane restart can maintain its forwarding state as intact during the restart, thereby significantly reducing the impact of the restart. Equally important, however, is that neighbors of the restarting router do not tear down protocol adjacencies and that they retain any routes learned from the restarting router. When the restart has been completed, the restarting router also relies on its neighbors to readvertise all prefixes previously advertised to it so that it can repopulate its RIB-IN. This is accomplished using the Graceful Restart mechanisms, which vary on a per-protocol basis but the capability is generically referred to as Non-Stop Forwarding (NSF).

An alternative method is Non-Stop Routing (NSR), which involves redundant Route Processors (CPMs) within a router being fully synchronized so that if the active processor undergoes a control-plane restart the standby processor is able to assume the active state immediately, without interruption to, and fully transparent to adjacent neighbors. It follows, therefore, that if a router is NSR-capable, it does not rely on its Graceful Restart capable neighbors to readvertise prefixes after a restart, simply because the synchronized Route Processors ...

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