Cisco Systems RJ-45-to-AUX manual Solution, Token Ring Standby Groups 255, 245

The Solution

HSRP is a Cisco proprietary protocol developed for redundancy. It defines a group of routers working as one virtual router. It enables host interfaces to continue communicating outside the local segment even if a host interface’s default route fails or the link is down. Basically, HSRP is a group of routers working together as a unit to provide fault tolerance.

HSRP works by assigning a group of routers a virtual IP address and a virtual MAC address. The routers in this group route packets to a virtual IP address so packets are still routed through the network even when their destination router is pushed off a shelf in the wiring closet. (Routers can fail for many reasons, most of which relate to electrical and component issues.)

HSRP also uses a priority scheme to determine which HSRP−configured router is the default active router. When you assign the standby interface a default priority that is higher than the priority of all other HSRP−configured route processor interfaces, that router is set as the active router. Once this active router is configured, multicast messages and advertising priorities are exchanged among HSRP−configured route processor interfaces. Now, if an active router fails to send a hello message (covered later in this chapter) within the configurable period of time, the standby router with the highest priority will be forced to become an active router.

HSRP is one of the best solutions when host interfaces on a local LAN segment require continuous access to the network resources. As shown in Figure 12.1, when a failure does occur, HSRP automatically lets the elected standby route processor assume the role and function of the offline router.

Figure 12.1: Two routers between various types of servers and clients. The standby route processor assumes the failed default route processor’s position in the network.

HSRP classifies the route processors on the network into standby groups. More than one standby group can be assigned to each route processor. The number of standby groups that can be assigned is limited by the physical topology being used. Table 12.1 shows the number of standby groups that can be configured on each type of physical LAN topology. Each group contains a route processor in each of the following roles:

Table 12.1: The maximum number of standby groups that can be assigned to a routeprocessor based on the physical topology.

Topology

Ethernet

FDDI

Token Ring

Standby Groups

255

255

3

245