The primary router with the highest configured priority will act as a virtual router with a pre-defined gateway IP address and will respond to the ARP or ND request from machines connected to the LAN with a virtual MAC address.
If the primary router should fail, the router with the next-highest priority would take over the gateway IP address and answer ARP requests with the same MAC address, thus achieving transparent default gateway failover.
In the virtual MAC address, Xs represent the group ID in hex.
This can be useful for dual branch routers each with a single link back to the gateway.
Version 2 of the protocol introduces stability, scalability and diagnostic improvements.