Operational experience proves that the demand-cache mechanisms described previously did not scale well in highly dynamic routing environments such as the Internet. Fast-switching caches must generally be invalidated when there is a change in the routing table. Although route holddown can prevent cyclic churn, rebuilding the cache is computationally expensive because packets that initiate cache entries must be process-switched.
CEF resolves this problem by building and maintaining a forwarding information base (FIB) with entries that include a one-to-one correspondence with entries in the IP routing table. Each entry in the FIB points to an IP next-hop that exists in an adjacency table. The adjacency table contains the information necessary for MAC-layer rewrites (see Figure 5-7).
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