Cisco Cisco IOS Software Release 12.0(2b) Guide De Dépannage

   IP Distributed switching is enabled 

   IP Fast switching turbo vector 

   IP Normal CEF switching turbo vector 

   IP multicast fast switching is enabled 

   IP multicast distributed fast switching is disabled 

   IP route−cache flags are Fast, Distributed, No CEF

In this output, the "No CEF" flag indicates that CEF has been disabled because of the no ip
route−cache cef command on a particular interface. The "CEF" flag indicates that CEF runs.
In a steady state, both flags should not appear. Cisco bug ID CSCdr80269 (registered
customers only) resolves rare conditions that can lead to both flags appearing. For more
information on bug IDs, refer to the Bug Toolkit (registered customers only) .

Confirm that a majority of the packets that flow through the router are CEF−switched.

Use the show interface x/x stat command and determine the number of packets and bytes
that the router forwarded through "Processor" instead of "Route cache." Note that "Route
cache" includes both fast−switched and CEF−switched packets.

router#show interface stats

FastEthernet0/0           

Switching path Pkts In  Chars In Pkts Out Chars Out 

Processor 95084 26211621 33493 3386174 

Route cache 24581 1132797 24542 13297583 

Distributed cache 0 0 0 0 

Total  119665 27344418 58035 16683757

a. 

Use the show ip cache command to determine if there is an IP cache entry, which indicates
that the packet follows the fast−switching path. Fast switching builds on an on−demand route
cache to expedite packet forwarding through a router. The driver code that runs on the
interface hardware transfers control temporarily to the fast−switching code, which searches
the route cache for a frame and other information constructed from a previously transmitted
packet. If the route cache contains an entry, the fast−switching code attempts to send the
packet directly to the destination interface.

router#show ip cache

IP routing cache 0 entries, 0 bytes 

Minimum invalidation interval 2 seconds, maximum interval 5 seconds, 

   quiet interval 3 seconds, threshold 0 requests 

Invalidation rate 0 in last second, 0 in last 3 seconds 

Prefix/Length       Age       Interface       Next Hop

These steps outline a more specific procedure for verifying that packets are being forwarded
using CEF.

b. 

Enable CEF with the ip cef command.

a. 

Issue the clear ip cache command to clear the fast−switched cache entries.

b. 

Start your traffic stream.

c. 

Issue the show ip cache command. Confirm that no entries are displayed in the fast−switched
cache since the packets are CEF−switched.

d. 

Issue the show interface stats command and confirm incremental hits for route cache
inbound.

Note: The route cache counter includes both fast−switched and CEF−switched packets.

e. 

Disable CEF with the no ip route−cache cef command on the inbound interface.

f. 

Issue the show interface stats command and confirm incremental hits for route cache.

g. 

Issue the show ip cache command and confirm that you see entries since Cisco IOS has fallen
back to fast switching.

h. 

Issue the no ip route−cache command on the outbound interface to disable fast switching.
Packets on the matching inbound interface are process switched.

i. 

3.