CEF Adjacency Types That Require Special Handling

Adjacency Types That Require Special Handling

In addition to adjacencies associated with next-hop interfaces (host-route adjacencies), other types of adjacencies are used to expedite switching when certain exception conditions exist. When the prefix is defined, prefixes requiring exception processing are cached with one of the special adjacencies listed in Table 4.

Table 4 Adjacency Types for Exception Processing
This adjacency type…

Receives this processing…

Null adjacency

Packets destined for a Null0 interface are dropped. This can be used as an effective form of access filtering.

Glean adjacency

When a router is connected directly to several hosts, the FIB table on the router maintains a prefix for the subnet rather than for the individual host prefixes. The subnet prefix points to a glean adjacency. When packets need to be forwarded to a specific host, the adjacency database is gleaned for the specific prefix.

Punt adjacency

Features that require special handling or features that are not yet supported in conjunction with CEF switching paths are forwarded to the next switching layer for handling. Features that are not supported are forwarded to the next higher switching level.

Discard adjacency

Packets are discarded.

Drop adjacency

Packets are dropped, but the prefix is checked.

CEF Adjacency Types That Require Special Handling