Routing and Forwarding
IP routers are typically capable of multiple routing processes, each of which maintains its own RIB. These are either link-state protocols, such as IS-IS or OSPF; or distance-vector protocols, such as RIP, IGRP, and BGP.
Each routing protocol may have multiple routes to the same destination, and the selection of the best route by each protocol is normally determined on the basis of longest match, followed by other routing protocol metrics. The per-protocol decision algorithm can be quite complex and can depend on many locally configured variables that control routing policy.
Distance-vector routing protocols also may have incoming or outgoing policy filters—that is, they may choose to ignore certain prefixes. Link-state ...
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