What is gratuitous ARP??

[admin_2]

I was reading thru some HSRP doc and the above term was mention.I wonder what is it???

[#2]

It's indeed related to ARP and HSRP.

The router broadcasts an ARP response packet advertising the group's virtual IP address and virtual MAC address.

The packet is sent using the virtual MAC address as the source MAC address in the link layer header, as well as within the ARP packet.

[mpconnelly]

My understanding of gratuitous ARP is that it is sent when the BIA MAC address is used with HSRP and not the (default) virtual MAC address. The problem with using the BIA is that each router in the HSRP group will have a different MAC address. When the standby changes, all local devices must change their ARP tables to the new MAC address. A gratuitous ARP is an un-requested ARP reply to force local systems to change their ARP tables. It does not work with all IP stacks.