32-3
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 32 Configuring HSRP
Understanding HSRP
You can verify the VLAN ID assigned to a routed port by using the show vlan internal usage privileged
EXEC command.
An interface can belong to multiple HSRP groups, and the same HSRP group can be applied to different
interfaces.
Note Identically-numbered HSRP groups use the same virtual MAC address and might cause errors if you
configure bridge groups.
Figure 32-1 shows a segment of a network configured for HSRP. Each router is configured with the MAC
address and IP network address of the virtual router. Instead of configuring hosts on the network with
the IP address of Router A, you configure them with the IP address of the virtual router as their default
router. When Host C sends packets to Host B, it sends them to the MAC address of the virtual router. If
for any reason, Router A stops transferring packets, Router B responds to the virtual IP address and
virtual MAC address and becomes the active router, assuming the active router duties. Host C continues
to use the IP address of the virtual router to address packets destined for Host B, which Router B now
receives and sends to Host B. Until Router A resumes operation, HSRP allows Router B to provide
uninterrupted service to users on Host C’s segment that need to communicate with users on Host B’s
segment and also continues to perform its normal function of handling packets between the Host A
segment and Host B.
Figure 32-1 Typical HSRP Configuration
Si Si Si
Host B
172.20.130.5
172.20.128.32
Host A
172.20.128.55
172.20.128.1 172.20.128.3 172.20.128.2
Virtual
router
Active
router
Standby
router
Router A Router B
Stacked
Catalyst 3550 or
2900XL/3500XL
switches
Stacked
Catalyst 3550 or
2900XL/3500XL
switches
46650
Host C
Catalyst 3550 switches
with enhanced multilayer
software images
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