arp table on routers

carl_townshend · ‎06-19-2008

Hi all, As there is a timer for the arp table on a router, if I was to change the ip address on the inside host but the router still had the old arp entry on there, what would happen? if it could not see the address in the arp table, would it just send out another arp broadcast ?

keeleym · ‎06-19-2008

Hi Carl

A Router will no have MAC addresses for hosts!

A Router knows nothing about end hosts, unless of course you have a end host connected to a Router in a lab scenario. So under normal circumstances, a Routers ARP table will hold the MAC addresses for neighbouring routers.

If you did have a end host connected to a router and you changed the IP address on the end host, I would imagine that the end host would be sending "keep alives" which would update the ARP table on the Router. The router would see these keep alives with teh new MAC address and would update the entry for that interface.

Worst casr scenario is that you would have to wait for the ARP timer to time out (300 seconds by default, I think)

Best Regards,

Michael

michael.leblanc · ‎06-19-2008

Upon receiving a DHCP lease on an IP address, a Windows XP host sends three Gratuitous ARP "Requests".

If you were to change a statically configured address, I suspect the host would send the Gratuitous ARP "Requests" as well, however I have not verified that.

The router should add a new ARP entry in the cache for the "new" IP address.

The ARP entry for the "original" IP address would likely remain in the cache until it timed out.

Imagine a NIC configured with multiple IP addresses. Each address would be mapped to the same MAC address, thus two ARP entries with the same MAC.