We have built a couple of high-availability environments with 6500s and Sup720s. We have been concerned with the issue of asymmetric routing and unicast flooding and have observed this behavior on more than one occasion. My question stems from documents located at the following URLs:
The solution given is to bring the ARP timeout value and the mac-address aging value closer to each other with the preference being to increase the mac-address timer. However I am not sure this really solves the problem. If I set my mac addresses to age out after 4 hours then I think the problem will still occur, it will just be 3 hours and 55 minutes later than normal. This is especially true of 24/7 hosts such as servers that are going to be communicated with at least once every 4 hours.
The behavior I have observed is that the packets passing through a router destined for a host reset the ARP age to zero in the router. Therefore a host that is communicated with continuously will never age out its ARP entry but the mac-address in the CAM or mac-address-table will still age out because the switch with the MSFC operating as the standby HSRP gateway never sees a packet with the source mac address of the host. Therefore another ARP request is never sent by the standby MSFC so no ARP reply with the host's source mac address is sent allowing the update of the mac-address table on that switch.
Is there a way to force the ARP table to age its entries regardless of the traffic?
Sr. Network Engineer
Dollar General Corporation