Adjacency Resolution - Cisco Catalyst 4500 Series Configuration Manual

Chapter 33 Configuring Cisco Express Forwarding

Adjacency Resolution

A route might have several paths to a destination prefix, such as when a router is configured for
simultaneous load balancing and redundancy. For each resolved path, a pointer is added for the
adjacency corresponding to the next-hop interface for that path. This method is used for load balancing
across several paths.
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 33-1
Adjacency Type
Null adjacency
Glean adjacency
Punt adjacency
Discard adjacency
Drop adjacency
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:
•
•
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 33-1
integrated switching engine.
OL-25340-01
Adjacency Types for Exception Processing
Processing Method
Packets destined for a Null0 interface are dropped. A Null0 interface can
be used as an effective form of access filtering.
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.
Features that require special handling or features that are not yet
supported by CEF switching are sent (punted) to the next higher
switching level.
Packets are discarded.
Packets are dropped.
Ethernet bridging at Layer 2
IP routing at Layer 3
shows a high-level view of the ASIC-based Layer 2 and Layer 3 switching process on the
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
Catalyst 4500 Series Switch Implementation of CEF
Table 33-1.
33-3