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Catalyst 3560 Switch Software Configuration Guide
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Chapter 36 Configuring IP Unicast Routing
Configuring Protocol-Independent Features
cache entries are frequently invalidated because of routing changes, which can cause traffic to be process
switched using the routing table, instead of fast switched using the route cache. CEF use the Forwarding
Information Base (FIB) lookup table to perform destination-based switching of IP packets.
The two main components in CEF are the distributed FIB and the distributed adjacency tables.
• 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.
• 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.
Because the switch uses Application Specific Integrated Circuits (ASICs) to achieve Gigabit-speed line
rate IP traffic, CEF forwarding applies only to the software-forwarding path, that is, traffic that is
forwarded by the CPU.
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 is CEF enabled on all Layer 3 interfaces. Entering the no ip route-cache cef
interface configuration command disables CEF for traffic that is being forwarded by software. This
command does not affect the hardware forwarding path. Disabling CEF and using the debug ip packet
detail privileged EXEC command can be useful to debug software-forwarded traffic. To enable CEF on
an interface for the software-forwarding path, use the ip route-cache cef interface configuration
command.
Caution Although the no ip route-cache cef interface configuration command to disable CEF on an interface is
visible in the CLI, we strongly recommend that you do not disable CEF on interfaces except for
debugging purposes.
Beginning in privileged EXEC mode, follow these steps to enable CEF globally and on an interface for
software-forwarded traffic if it has been disabled:
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 ip cef Enable CEF operation.
Step 3 interface interface-id Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 4 ip route-cache cef Enable CEF on the interface for software-forwarded traffic.
Step 5 end Return to privileged EXEC mode.
Step 6 show ip cef Display the CEF status on all interfaces.
Step 7 show cef linecard [detail]Display CEF-related interface information.
Step 8 show cef interface [interface-id]Display detailed CEF information for all interfaces or the
specified interface.
Step 9 show adjacency Display CEF adjacency table information.
Step 10 copy running-config startup-config (Optional) Save your entries in the configuration file.