ARP Question

[js358]

Hello All:

Quick question. I have what I believe to be a config problem on either one of my routers or firewalls. Trying to figure out which.

My question is this. If I clear the arp cache on the router handling internal routing, and then do a sh arp a few minutes later. All records will have the same age except for a few...some are 0 and remain 0 forever. Why is that?

Thanks

[Richard Burts]

I am not sure how it would relate to whether a config problem was on router or firewall, but I can explain part of your situation. When you clear arp on the router, the IOS flushes the dynamic entries in the table and immediately sends an ARP request to all the addresses which it has just flushed. Assuming that the device responds the router adds the address back into the table. This explains why the same addresses will show up in the ARP table after flushing and will have the same age (they were flushed and re-learned at the same time). I am not clear about the entries with 0 age - perhaps they are the MAC addresses for the router interfaces, or perhaps are static entries. Perhaps you can post the output of show arp and it might be more clear what the explanation should be.

HTH

Rick