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New Member

MAC flooding and agingtime

We found that we have MAC flooding as a result of running HSRP and doing equal cost routing with OSPF. This is a known issue with the default ARP timeout of 4 hours and the MAC agingtime of 5 minutes. We have looked into removing asymmetric routing and have implemented ARP and MAC times of 1 hour for both. This has helped the situation, but the MAC is becoming "unlearned" in less than an hour (see attachment). Does a change in MAC agingtime require a reboot on specific or all switch platforms?

Doing unequal cost routing to the active HSRP does not seem like a good solution since it does not do load sharing and would require an OSPF convergence after the primary route went down, opposed to just switching immediatly to the equal route.

I see the MAC is not in the table when the flooding occurs. Could something else besides aging cause the MAC to be unlearned?

1 REPLY
New Member

Re: MAC flooding and agingtime

I found that switches go into "fast aging" (15s) when there is a topology change notification (TCN). Using Portfast for all clients ports is important for reducing TCNs.

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