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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter23 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 pre fix is defined, prefixes
requiring exception processing are cached with one of the special adjacencies listed in Table 23- 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 a nd 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
This section contains the following subsections:
•Hardware and Software Switching, page 23-4
•Load Balancing, page 23-6
•Software Interfaces, page 23-6
Catalyst 4500 series switches support an ASIC-based Integrated Switching Engi ne 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 23-1 shows a high-level view of the ASIC-based Layer 2 and Layer 3 switching process on the
Integrated Switching Engine.
Table23-1 Adjacency Types for Exception Processing
This adjacency type... Receives this processing...
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 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 featur es 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.