topic MAC addresses falling off forwarding table in Switches, Hubs, and Modems

MAC addresses falling off forwarding table

steve dowling — Fri, 29 Oct 2010 04:34:28 GMT

While following up some intermittent problems I've found that some IP addresses keep their MAC addresses (through ARP), but lose their physical port association (bridge forwarding table). A section of "sho arp" looks like this:

172.27.179.37 001558-a43118 dynamic C2
172.27.179.38 001641-ec87f1 dynamic G4
172.27.179.39 001a6b-61a349 dynamic G4
172.27.179.69 0050c2-70b5a2 dynamic H5
172.27.179.70 0050c2-70b594 dynamic
172.27.179.71 0050c2-70b593 dynamic
172.27.179.75 0008c7-996969 dynamic C2
172.27.179.159 000400-f4b5f8 dynamic G4
172.27.179.161 001b78-ec0cba dynamic C1

The addresses with no associated ports are unavailable, and these MAC addresses no longer show up in the "sho mac" command.

The problem is; these devices are really alive and should be talking on the network. (Which is why I'm chasing faults)

Device is HP5304 running E.11.21

How do I convince this beastie to keep its forwarding table?

Re: MAC addresses falling off forwarding table

Mohammed Faiz — Fri, 29 Oct 2010 08:50:21 GMT

As you say, if the devices are communicating correctly their MAC addresses shouldn't age out but you could try increasing the mac-age timer as a test.

switch(config)# show system
switch(config)# mac-age-time SECONDS

Re: MAC addresses falling off forwarding table

steve dowling — Mon, 01 Nov 2010 06:03:02 GMT

Thanks Mohammed,

MAC-age-time is 300. This is the default, I think. I've increased this to 600.

Let's see if this problem continues...