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Configuring Gratuitous ARP

 

Gratuitous Address Resolution Protocol (ARP) requests help detect duplicate IP addresses. A gratuitous ARP is a broadcast request for a router’s own IP address. If a router or switch sends an ARP request for its own IP address and no ARP replies are received, the router- or switch-assigned IP address is not being used by other nodes. However, if a router or switch sends an ARP request for its own IP address and an ARP reply is received, the router- or switch-assigned IP address is already being used by another node.

Gratuitous ARP replies are reply packets sent to the broadcast MAC address with the target IP address set to be the same as the sender’s IP address. When the router or switch receives a gratuitous ARP reply, the router or switch can insert an entry for that reply in the ARP cache. By default, updating the ARP cache on gratuitous ARP replies is disabled on the router or switch.

To enable updating of the ARP cache for gratuitous ARPs:

  1. In configuration mode, go to the [edit interfaces interface-name] hierarchy level.
  2. Include the gratuitous-arp-reply statement.

To restore the default behavior, that is, to disable updating of the ARP cache for gratuitous ARP, delete the gratuitous-arp-reply statement from the configuration:

By default, the router or switch responds to gratuitous ARP requests. However, on Ethernet interfaces, you can disable responses to gratuitous ARP requests.

To disable responses to gratuitous ARP requests:

  1. In configuration mode, go to the [edit interfaces interface-name] hierarchy level.
  2. Include the no-gratuitous-arp-request statement.

To return to the default—that is, to respond to gratuitous ARP requests—delete the no-gratuitous-arp-request statement from the configuration: