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31-3
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 31 Configuring Cisco Express Forwarding
Catalyst 4500 Series Switch Implementation of CEF
Adjacency Types That Require Special Handling
In addition to adjacencies for 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 31-1.
Unresolved Adjacency
When a link-layer header is prepended to packets, FIB requires the prepend to point to an adjacency
corresponding to the next hop. If an adjacency was created by FIB and was not discovered through a
mechanism such as ARP, the Layer 2 addressing information is not known and the adjacency is
considered incomplete. When the Layer 2 information is known, the packet is forwarded to the route
processor, and the adjacency is determined through ARP.
Catalyst 4500 Series Switch Implementation of CEF
Catalyst 4500 series switches support an ASIC-based integrated switching engine that provides these
features:
• Ethernet bridging at Layer 2
• IP routing at Layer 3
Because the ASIC is specifically designed to forward packets, the integrated switching engine hardware
can run this process much faster than CPU subsystem software.
Figure 31-1 shows a high-level view of the ASIC-based Layer 2 and Layer 3 switching process on the
integrated switching engine.
Table 31-1 Adjacency Types for Exception Processing
Adjacency Type Processing Method
Null adjacency Packets destined for a Null0 interface are dropped. A Null0 interface 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 each individual
host. The subnet prefix points to a glean adjacency. When packets must
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 by CEF switching are sent (punted) to the next higher
switching level.
Discard adjacency Packets are discarded.
Drop adjacency Packets are dropped.