What type of ARP message does HSRP send when the standby router switches to the active role?

When the standby router in HSRP (Hot Standby Router Protocol) switches to the active role, it sends a gratuitous ARP message. This type of ARP message is used to inform other devices on the local network about the new MAC address corresponding to the IP address of the HSRP virtual router.

The gratuitous ARP updates the ARP cache of devices on the network with the active router's MAC address without having to wait for any reply from other devices. This is especially important in HSRP scenarios, as it helps ensure that traffic routed to the virtual IP address transitions smoothly to the new active router, thus improving network reliability and minimizing downtime.

In contrast, other types of ARP messages—such as broadcasts, multicasts, and unicasts—serve different purposes. Broadcast ARP requests are sent to discover available hosts, multicast messages are aimed at communicating with a specific group of devices, and unicast messages are directed to a single device. However, during an active role transition in HSRP, the use of gratuitous ARP is crucial for promptly updating the ARP caches of hosts and ensuring they forward packets to the new active router correctly.