The Solution - Cisco RJ-45-to-AUX Brochure

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