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| 6.3 |  | HSRP Operations |
| 6.3.1 |  | HSRP operations |

The main components of HSRP are an active router, standby router, and a virtual or phantom router. In Figure , the active router does the forwarding of data packets and transmits hello messages. The standby router monitors the status of the active router and quickly begins forwarding packets if the active router fails. The standby router also transmits hello messages to other routers in the HSRP group. The virtual router does not really exist. It simply represents a consistently available router with an IP address and a MAC address to the hosts on a network.

It is possible that several other routers exist in an HSRP standby group. These other routers will monitor HSRP hello messages but do not respond. They function as normal routers that forward packets sent to them but do not forward packets addressed to the virtual router. These additional HSRP routers remain in the "init" state.

How does the standby router know when the active router fails? The answer is that the standby router stops receiving hello messages from the active router and takes on the role of the active router. Because the hosts are using a virtual IP and MAC address, the hosts see minimal or no service disruption.

If both the active and standby routers fail, all other routers in the group will contend for the active and standby roles. The router with the lowest MAC address becomes the active router unless a HSRP priority is configured.

To facilitate load sharing, a single router may be a member of multiple HSRP standby groups on a single segment. Each standby group emulates a single virtual router. There can be up to 255 standby groups on any LAN. Multiple HSRP groups are also supported on VLAN, port-channel, and Bridge Group Virtual Interface (BVI) interfaces.

In Figure , both Router A and Router B are members of Groups 1 and 2. However, Router A is the active forwarding router for Group 1 and the standby router for Group 2. Router B is the active forwarding router for Group 2 and the standby router for Group 1.

Routers are also able to simultaneously provide redundant backup and perform load sharing across different IP subnets, as shown in Figure .

This is possible because for each standby group, an IP address and a single well-known MAC address with a unique group identifier are allocated to the group.

The IP address of a group is in the range of addresses belonging to the subnet in use on the LAN. However, the IP address of the group must differ from the addresses allocated as interface addresses on all routers and hosts on the LAN, including virtual IP addresses assigned to other HSRP groups.

Initially, there is no active router in the standby group. Within the standby group, the router with the highest standby priority becomes the active router. The default priority for an HSRP router is 100. This priority value is configurable. The active router responds to traffic for the virtual router. If an end station sends a packet to the virtual router MAC address, the active router receives and processes that packet. If an end station sends an ARP request with the virtual router IP address, the active router replies with the virtual router MAC address.

In Figure , Router A has a priority of 200 and Router B has a default priority of 100. Router A assumes the active router role and forwards all frames addressed to the well-known MAC address of 0000.0c07.acxx, where xx is the HSRP group identifier. The well-known MAC address of 0000.0c07.acxx is used for all HSRP configurations unless HSRP is configured to use a different MAC address.