You can use the following command to turn off sending gratuitous ARP packets after a failover:

Sending gratuitous ARP packets is turned on by default.

In most cases you would want to send gratuitous ARP packets because its a reliable way for the cluster to notify the network to send traffic to the new primary unit. However, in some cases, sending gratuitous ARP packets may be less optimal. For example, if you have a cluster of FortiGate units in Transparent mode, after a failover the new primary unit will send gratuitous ARP packets to all of the addresses in its Forwarding Database (FDB). If the FDB has a large number of addresses it may take extra time to send all the packets and the sudden burst of traffic could disrupt the network.

If you choose to disable sending gratuitous ARP packets you must first enable the link-failed-signal setting. The cluster must have some way of informing attached network devices that a failover has occurred.

For more information about the link-failed-signal setting, see “Updating MAC forwarding tables when a link failover occurs”.