| Because many organizations are now
seeking the economic benefits of campus networks for
mission-critical communications, high reliability is becoming
increasingly crucial. Within the campus network, much attention has
been focused on providing a network infrastructure that is available
100 percent of the time. However, one of the greatest challenges -
surprisingly - does not come from the network infrastructure, but
from the workstations and network equipment at the user level.
One way to achieve near-100-percent
network uptime is to use the Hot Standby Router Protocol (HSRP) (RFC
2281),
which provides network redundancy for IP networks, ensuring that
user traffic immediately and transparently recovers from first-hop
failures in network edge devices or access circuits.
By sharing an IP address and a Media
Access Control (MAC) (Layer 2) address, two or more routers can act
as a single "virtual" router. The members of the
virtual-router group continually exchange status messages. This way,
one router can assume the routing responsibility of another, if it
goes out of service for either planned or unplanned reasons.
Hosts continue to forward IP packets to a consistent IP and virtual MAC
address, and the changeover between routes is transparent to the end
workstation.
Upon completion of this chapter, you
will be able to identify the virtual router for a given set of
switch block devices, configure HSRP on the switch block devices to
ensure continual inter-virtual LAN (VLAN) routing, maintain graceful
packet forwarding by changing the active and standby HSRP router
roles, and ensure the role of the active router by assigning a
preempt status.
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