CEF Show And Troubleshooting Commands

Here are a couple of commands for viewing and troubleshooting CEF.

Cisco Express Forwarding Commands

By default, Cisco Express Forwarding and Fast Switching are enabled.

(config-if)#no ip route-cache This command disables both Fast Switching and CEF.

no ip route-cache

(config-if)#ip route-cache This command enables Fast Switching only. Cisco Express Forwarding is still disabled if we don’t activate it.
ip route-cache
(config-if)# ip route-cache cef activates Cisco Express Forwarding under the interface
ip route-cache cef command
# show platform tcam utilization displays TCAM limits and actual utilization
show platform tcam utilisation
# debug ip cef table This command monitors changes on the Cisco Express Forwarding adjacency table.

Let’s run the debug command on a multilayer switch:

debug ip cef table

We list ARP entries:

If we remove one entry from ARP table with clear ip arp {IP address}:

clear ip arp 20.1.1.2 before impacting CEF adjacency table

We can see that there’s a change in the Cisco Express Forwarding adjacency table. Indeed, there’s one ARP entry that’s missing:

ARP table after changes in CEF adjacency table

Now, if we issue a ping from the host whose IP address is 20.1.1.2 to its gateway (SVI on the switch), we ‘ll see another Cisco Express Forwarding debug message appearing on the console interface:

And a new ARP entry is written in the ARP cache:

You’ll find theory details on CEF in this link.

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