Which protocol utilizes a virtual MAC address for responding to ARP requests?

The protocol that utilizes a virtual MAC address for responding to ARP requests is HSRP (Hot Standby Router Protocol). HSRP is a Cisco proprietary protocol designed to provide high availability by allowing multiple routers to appear as a single virtual router to the end devices on the local network.

In HSRP, a single virtual MAC address is assigned to the virtual router, which is shared among the participating routers (the active and standby routers). When an ARP request is made for the default gateway, the router that is currently active responds with this virtual MAC address. This allows devices on the network to send traffic to the virtual IP address of the HSRP group without needing to know which specific physical router is currently in the forwarding state, thus enhancing network redundancy and failover capabilities.

While VRRP (Virtual Router Redundancy Protocol) and GLBP (Gateway Load Balancing Protocol) also provide similar functionalities with their own mechanisms for virtual gateway management, HSRP specifically employs the virtual MAC address concept for responding to ARP requests, making it the correct answer in this context.