All the routers in an HSRP group have specific roles and interact
in specific manners.
The Virtual HSRP Router
The virtual
router is simply an IP and MAC address pair to which end devices have
configured as their default gateway. The active router will process all packets
and frames sent to the virtual router address. The virtual router processes no
physical frames.

The Active HSRP Router
Within an HSRP standby group, one router
is elected to be the active router. The active router physically forwards
packets sent to the MAC address of the virtual router.
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 this example, 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.
ARP Resolution with HSRP
ARP establishes correspondences between
network addresses, such as an IP address and a hardware Ethernet address. All
devices sending packets over IP maintain a table of resolved addresses,
including routers.
The IP address and corresponding MAC address of the
virtual router is maintained in the ARP table of each router in an HSRP standby
group. As shown in Figure
, the
command show ip arp displays the ARP cache on a router.

In the
example, the output displays an ARP entry for a router that is a member of HSRP
standby group 47 in VLAN10. The virtual router for VLAN10 is identified as
172.16.10.110. The well-known MAC address that corresponds to this IP address
is 0000.0c07.ac2f, where 2f is the HSRP group identifier for standby group 47.
The HSRP group number is the standby group number (47) converted to hexadecimal
(2f).
The Standby and other HSRP Routers in the Group
The
function of the HSRP standby router is to monitor the operational status of the
HSRP group and quickly assume packet-forwarding responsibility if the active
router becomes inoperable. Both the active and standby router transmits hello
messages to inform all other routers in the group of their role and status.
An HSRP standby group may contain other routers that are group members but
are not in an active or standby state. These routers monitor the hello messages
sent by the active and standby routers to ensure that an active and standby
router exist for the HSRP group of which they are a member. These routers do
forward any packets addressed to their own specific IP addresses, but they
do not forward packets addressed to the virtual router. These routers
issue speak messages at every hello interval time.

HSRP Active and Standby Router Interaction
When the active router
fails, the other HSRP routers stop seeing hello messages from the active
router. The standby router will then assume the role of the active router. If
there are other routers participating in the group, those routers then contend
to be the new standby router.
In the event that both the active and
standby routers fail, all routers in the group contend for the active and
standby router roles.
Because the new active router assumes both the IP
and MAC addresses of the virtual router, the end stations see no disruption in
service. The end-user stations continue to send packets to the virtual router
MAC address, and the new active router delivers the packets to the destination.
