CEF Switching and the Adj table

[izackvail]

I have a 3750 stack of 4 switches. There are multiple SVI interfaces on this switch. I noticed that none of the physical interfaces report doing any CEF switching. In fact all packets are reported as process switched.

Example:

SW3750G#sh int g1/0/1 stats

GigabitEthernet1/0/1

Switch path Pkts In Chars In Pkts Out Chars Out

Processor 0 0 3497187 257226176

Route cache 0 0 0 0

Total 0 0 3497187 257226176

I do notice however that my SVI interfaces report a good portion of the packets to be CEF switched.

Example:

SW3750G#sh int vlan 1 stats

Vlan1

Switch path Pkts In Chars In Pkts Out Chars Out

Processor 48191591 3704212079 11627986 1243583236

Route cache 28845 3620783 4826 1718427

Total 48220436 3707832862 11632812 1245301663

It looks like CEF is not being used at all on any of the L2 interfaces. I guess I always thought that CEF used the adjacency table when switching packets at L2. Is CEF really only used when switching routed packets?

[Richard Burts · Accepted solution]

Izack

Yes CEF is for layer 3 switching of packets. Therefore layer 2 interfaces do not use CEF to switch packets.

HTH

Rick

[izackvail]

I guess the fact that switching on L2 interfaces is being reported as "process" switched was the thing that threw me off. I know that L2 interfaces would typically use the mac address table to make switching decisions but just didn't understand why that would be reported in that way. Thanks!

[Richard Burts]

Izack

I was a bit surprised that it was reported this way (not what I would have expected). I also notice that it reports packets out and characters out but no packets in or characters in which seems unusual.

I am glad that my response was able to resolve your question. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that there was a response which did resolve the question.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick