CHAPTER
23-1
Catalyst 3550 Multilayer Switch Software Configuration Guide
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23
Configuring HSRP
This chapter describes how to use Hot Standby Router Protocol (HSRP) to p rovide routing redundancy
for routing IP traffic without being dependent on the availability of any sin gle router. To use this feature,
you must have the enhanced multilayer software image installed on your switch. All Catalyst 3550
Gigabit Ethernet switches ship with the enhanced multilayer software image (EMI) installed. Catalyst 3550
Fast Ethernet switches can be shipped with either the standard multilayer software image (SMI) or EMI
pre-installed. You can order the Enhanced Multilayer Software Image Upgrade kit to upgrade Catalyst 3550
Fast Ethernet switches from the SMI to the EMI.
Note You can also use a version of HSRP in Layer 2 mode to configure a redundant com mand sw itch to
take over cluster management if the cluster command switch fails. For more information about
clustering, see Chapter5, “Clustering Switches.”
Note For complete syntax and usage information for the commands used in this chapter, refer to the
Catalyst 3550 Multilayer Switch Command Reference and the Cisco IOS IP and IP Routing
Command Reference for Release 12.1.
This chapter consists of these sections:
•Understanding HSRP, page 23-1
•Configuring HSRP, page 23-3
•Displaying HSRP Configurations, page 23-10

Understanding HSRP

HSRP is Cisco’s standard method of providing high network availability by providing first-hop
redundancy for IP hosts on an IEEE 802 LAN configured wi th a de faul t ga tewa y IP addre ss. HS RP
routes IP traffic without relying on the availability of any single router. It enables a set of router
interfaces to work together to present the appearance of a single virtual router or default gateway to the
hosts on a LAN. When HSRP is configured on a network or segment, it provides a virtual Media Access
Control (MAC) address and an IP address that is shared among a group of configured routers. HSRP
allows two or more HSRP-configured routers to use the MAC address and IP net work address of a virtual
router. The virtual router does not exist; it represents the common target for routers that are co nfig ured
to provide backup to each other. One of the routers is selected to be the active router and another to be
the standby router, which assumes control of the group MAC address and IP add re ss s hould the
designated active router fail.