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Chapter 34
Configuring IP Unicast Routing
The two main components in dCEF are the distributed FIB and the distributed adjacency tables.
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Because the Catalyst 3750 switch stack uses Application Specific Integrated Circuits (ASICs) to achieve
Gigabit-speed line rate IP traffic, dCEF forwarding applies only to the software-forwarding path, that is,
traffic that is forwarded by the CPU.
Distributed CEF is enabled globally by default. If for some reason it is disabled, you can re-enable it by
using the ip cef distributed global configuration command.
The default configuration is 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 dCEF on interfaces except for
debugging purposes.
Beginning in privileged EXEC mode, follow these steps to enable dCEF globally and on an interface for
software-forwarded traffic if it has been disabled:
Command
Step 1
configure terminal
Step 2
ip cef distributed
Step 3
interface interface-id
Step 4
ip route-cache cef
Step 5
end
Step 6
show ip cef
Step 7
show cef linecard [slot-number] [detail]
Step 8
show cef interface [interface-id]
Step 9
show adjacency
Step 10
copy running-config startup-config
78-16180-02
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.
Purpose
Enter global configuration mode.
Enable dCEF operation.
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Enable CEF on the interface for software-forwarded traffic.
Return to privileged EXEC mode.
Display the CEF status on all interfaces.
Display CEF-related interface information by stack member for
all switches in the stack or for the specified switch.
(Optional) For slot-number, enter the stack member switch
number.
Display detailed CEF information for all interfaces or the
specified interface.
Display CEF adjacency table information.
(Optional) Save your entries in the configuration file.
Catalyst 3750 Switch Software Configuration Guide
Configuring Protocol-Independent Features
34-61
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