On Page 59, Router Security Strategies: Securing IP Network Traffic Planes:
The adjacency table contains information necessary for encapsulation of the packets that must be sent to given next-hop network devices. CEF considers next-hop devices to be neighbors if they are directly connected via a shared IP subnet.
Each adjacency entry stores pre-computed frame headers used when forwarding a packet using a FIB entry referencing the corresponding adjacency entry. The adjacency table is populated as adjacencies are discovered. Each time an adjacency entry is created, such as through the ARP protocol, a link-layer header for that adjacent node is pre-computed and stored in the adjacency table.
Routes might have more than one path per entry, making it possible to use CEF to switch packets while load balancing across multiple paths.
In addition to next-hop interface adjacencies (in other words host-route adjacencies), certain exception condition adjacencies exist to expedite switching for nonstandard conditions. These include, among others: punt adjacencies for handling features that are not supported in CEF (such as IP options), and ìdropî adjacencies for pre?xes referencing the Null0 interface. Packets forwarded to Null0 are dropped, making an effective, ef?cient form of access ?ltering.
Router Security Strategies: Securing IP Network Traffic Planes By Gregg Schudel – CCIE No. 9591, David J. Smith – CCIE No. 1986 ISBN: 9781587053368 Publisher: Cisco Press