8.1 HSRP Operation
8.1.2 How HSRP addresses these issues
HSRP provides a way for non-IRDP end nodes to keep communicating even if their local routers become unavailable. HSRP works by creating a "virtual" router that has its own IP and MAC addresses, as shown in the Figure.

HSRP routers on a LAN segment or VLAN communicate among themselves to designate three possible router states: one router will go into the "active" state, one router will go into the "standby" state, the others will stay in an "init" state. The active router does the work for the HSRP virtual. If an end node sends a packet to the virtual MAC address of the virtual, the active router receives that packet and processes it. If an end node sends an ARP request for the IP address of the virtual, the active router replies with the MAC address of the virtual. The routers participating in HSRP can be configured to respond with their burn-in MAC address instead of a virtual MAC address.

The active and standby routers send periodic hello messages while the other HSRP routers listen for such messages. If the active router fails and the standby router stops receiving hello messages, the standby router takes over and becomes the active router. Because the new active router assumes both the IP and MAC addresses of the virtual, end nodes see no change at all. They continue to send packets to the MAC address of the virtual, and the new active router delivers those packets.

In our example, Anderson would be configured to use the virtual as its default router. Only the virtual would appear in the Anderson configuration; Broadway would not be listed. Upon booting, the routers might elect Broadway as active router so that Broadway would deliver packets from Anderson. If Broadway or its LAN interface went offline, Central Park would take over as active router, continuing with the delivery of Anderson packets. The changes taking place in the network would remain transparent to Anderson. If a third HSRP router was added to the LAN segment, this router would begin to act as the new standby router but remain in the "init" state.

HSRP also works for proxy ARP. When an active HSRP router receives an ARP request for a node that is not on the local LAN, the router replies with the virtual MAC address instead of its own. If the router that originally sent the ARP reply later loses its connection, the new active router can still deliver the traffic. Although HSRP works with proxy ARP configurations, it is not a recommended configuration due to administrative and security reasons.