Gratuitous ARP

Gratuitous ARP provides an ARP announcement packet containing valid sender hardware and protocol addresses for the host that sent it. Such a request is not intended to solicit a reply, but merely updates the ARP caches of other hosts that receive the packet. Gratuitous ARP is usually formatted as an ARP request, but it may also be formatted as an ARP reply. ARP announcements are typically sent out during startup. This helps to resolve problems which would otherwise occur if, for example, an IP-address-to-MAC-address mapping changed because a network card was replaced. In this example, gratuitous ARP solves the problem of remote hosts that still have the old mapping in their ARP caches. The S- K- and 7100-Series provides for setting the behavior when an ARP announcement is received for an already existing ARP table entry or for a non-existing ARP table entry, referred to as ARP learning.

IP gratuitous ARP is enabled by default for the modification of pre-existing ARP table entries and is disabled by default for the learning of new table entries.

Use the ip gratuitous-arp command in interface configuration command mode to:

• Configure the device to ignore gratuitous ARP announcements received for existing ARP table entries
• Configure the device to change the ARP table only if the gratuitous ARP announcement is a reply
• Configure the device to change the ARP table only if the gratuitous ARP announcement is a request.

Use the ip gratuitous-arp-learning command, in interface configuration command mode, to allow an interface to learn new ARP bindings using gratuitous ARP. The router will learn new ARP bindings from reply, request, or both types of ARP announcements, based upon the option specified in this command.

Gratuitous ARP configuration does not affect normal ARP operations. Normal ARP packets (non gratuitous) will always be learned and updated regardless of gratuitous ARP configuration.