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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 31 Configuring IP Unicast Routing
Configuring Protocol-Independent Features
process-switched by using the routing table, instead of fast-switched by using the route cache. CEF uses
the forwarding information base (FIB) lookup table to perform destination-based switching of IP
packets.
The two main components in CEF are the FIB and adjacency tables.
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The FIB is similar to a routing table or information base and maintains a mirror image of the
forwarding information in the IP routing table. When routing or topology changes occur in the
network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB
maintains next-hop address information based on the information in the IP routing table. Because
the FIB contains all known routes that exist in the routing table, CEF eliminates route cache
maintenance, is more efficient for switching traffic, and is not affected by traffic patterns.
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Nodes in the network are said to be adjacent if they can reach each other with a single hop across a
link layer. CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency
table maintains Layer 2 next-hop addresses for all FIB entries.
CEF is enabled globally by default. If for some reason it is disabled, you can re-enable it by using the ip
cef global configuration command.
The default configuration, which we recommend, is CEF enabled on all Layer 3 interfaces. You can
disable CEF on an interface by using the no ip route-cache cef interface configuration command. You
can enable CEF on an interface by using the ip route-cache cef interface configuration command.
Beginning in privileged EXEC mode, follow these steps to enable CEF on an interface after it has been
disabled:
To disable CEF on an interface, use the no ip route-cache cef interface configuration command.
Configuring the Number of Equal-Cost Routing Paths
When a router has two or more routes to the same network with the same metrics, these routes can be
thought of as having an equal cost. The term parallel path is another way to refer to occurrences of
equal-cost routes in a routing table. If a router has two or more equal-cost paths to a network, it can use
them concurrently. Parallel paths provide redundancy in case of a circuit failure and also enable a router
to load balance packets over the available paths for more efficient use of available bandwidth.
Although the router automatically learns about and configures equal-cost routes, you can control the
maximum number of parallel paths supported by an IP routing protocol in its routing table.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the
Layer 3 interface to configure.
Step 3
ip route-cache cef
Enable CEF on the interface.
Step 4
end
Return to privileged EXEC mode.
Step 5
show ip cef
Display the CEF status on all interfaces.
Step 6
show adjacency
Display CEF adjacency table information.
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.