| 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.
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