42-90
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 42 Configuring IP Unicast Routing
Configuring Protocol-Independent Features
more CPU processing power to be dedicated to packet forwarding. In a switch stack, the hardware uses
distributed CEF (dCEF) in the stack. In dynamic networks, fast switching 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 and dCEF use the Forwarding Information
Base (FIB) lookup table to perform destination-based switching of IP packets.
The two main components in CEF and dCEF 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 or switch stack uses Application Specific Integrated Circuits (ASICs) to achieve
Gigabit-speed line rate IP traffic, CEF or dCEF forwarding applies only to the software-forwarding path,
that is, traffic that is forwarded by the CPU.
CEF or distributed CEF is enabled globally by default. If for some reason it is disabled, you can re-enable
it by using the ip cef or ip cef distributed global configuration command.
The default configuration is CEF or dCEF 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 or dCEF on interfaces except
for debugging purposes.
Beginning in privileged EXEC mode, follow these steps to enable CEF or dCEF 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
or
ip cef distributed
Enable CEF operation on a Catalyst 3560-X switch,
or
enable CEF operation on a Catalyst 3750-X switch.
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.