Overview
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.