CAM & ARP aging timers

Unanswered Question
May 11th, 2010

My network has VLANs span across the access layer. We have seen the return path traffic of unicast flooding to all access layer switches

and end points under each. According the Cisco document this problem can be fixed by tune the ARP aging timer less than the CAM aging timer. Currently, on the Catalyst 6500 the ARP aging timer is 4 hours and CAM aging timer is 300 seconds. It looks like 300 seconds is too short for ARP aging timer. What is the best practice aging timer for ARP & CAM for fixing unicast flooding issue.

I have this problem too.
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Richard Burts Tue, 05/11/2010 - 14:28

Thereare logical and good reasons why the ARP timer (reflecting layer 3 forwarding device operation) is relatively long ( 4 hours) while the CAM (or mac-address table) aging timer is pretty short (300 seconds). The difference in the timers can sometimes lead to issues including unicast flooding of frames. A frequent solution is to make the timer match, or perhaps to make the ARP timer shorter than the CAM timer.

Why would you say that 300 seconds is too short for the ARP timer?

If you are experiencing unicast flooding then I would say that best practice would be to set your ARP timer down to match the CAM timer (at 300 seconds) or perhaps set ARP timer slightly less than 300 seconds.

HTH

Rick

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