how does a host re connect to a standy router/firewall

Unanswered Question
Jun 26th, 2009

Hi all

Can anyone tell me what happens, say if I have 2 distribution routers, each of them has a firewall connected in failover mode, The IP is swapped when the firewall fails to the other, my question is how is the mac re-learnt and passed on to the other switch, and also the ARP entry, how is this updated and what instigates in, If possible can someone walk me through the actions

Many thanks


I have this problem too.
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Giuseppe Larosa Fri, 06/26/2009 - 03:56

Hello Carl,

the device that takes over sends a gratuitous ARP saying that IP address X is reachable at MAC address Y.

This is useful also if the MAC address doesn't change as a way to update LAN switches CAM tables in the middle.

This happens for example for HSRP.

Hope to help


carl_townshend Fri, 06/26/2009 - 04:46

Hi there

so at this time, i gather the switch removes the old mac entry and also overites the existing arp entry automatically ?

Giuseppe Larosa Fri, 06/26/2009 - 05:37

Hello Carl,

the switch changes the CAM table associating the MAC address with the new port.

All L3 devices listening to the gratuitous ARP (that haves a broadcast destination) updates their ARP tables accordingly.

Hope to help


carl_townshend Fri, 06/26/2009 - 05:50

how does this work then, if it sees the mac address on a new port, does it automatically delete the old entry ?


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