A network with High Availability provides alternate means by which all
infrastructure paths and key servers can be accessed at all times. The Hot
Standby Routing Protocol is one of those software features configured to
provide Layer 3 redundancy to network hosts. HSRP optimization provides
immediate or link specific failover as well as a recovery mechanism. Virtual
Router Redundancy Protocol and Gateway Load Balancing Protocol are derivatives
of HSRP providing additional Layer 3 redundancy features such as load
balancing.
When router redundancy is configured, a virtual router is
created. The IP address of the virtual router will be configured as the default
gateway for the workstations on a specific IP segment. When frames are to be
sent from the workstation to the default gateway, the workstation will use ARP
to resolve the MAC address associated with the IP address of the default
gateway. The ARP resolution will return the MAC address of the virtual router.
Frames sent to the MAC address of the virtual router can then be physically
processed by any active or standby router that is part of that virtual router
group.
A protocol is used to identify two or more routers as the
devices responsible for processing frames sent to the Mac or IP address of a
single virtual router. Host devices send traffic to the address of the virtual
router. The physical router that handles forwarding this traffic is transparent
to the end stations. The redundancy protocol provides the mechanism for
determining which router should take the active role in forwarding traffic, and
for determining when that role must be taken over by a standby router. The
transition from one forwarding router to another is transparent to the end
devices.