This is covered in my book. If you don't have portfast enabled on all switchports where the clustered firewalls are attached, use of a VMAC can sometimes cause STP issues where upon failover STP blocks the ports for 10-12 seconds back into Listening & Learning mode. This is due to the same unicast MAC address briefly appearing on two switchports at the same time, which can be perceived by STP as a bridging loop. This causes what I term a "slow" failover where all traffic comes to a screeching halt for about 10 seconds upon failover then suddenly starts working through the newly-active member. On Cisco devices the involved switchports will glow amber during the "screech". Portfast is NOT the same as disabling STP completely which you should NEVER do.
Bottom line: Use the default G-ARP unless you experience slow or incomplete failovers (especially for plucked NAT addresses), then try VMAC but be sure to set portfast.
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