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Chapter 40 Configuring HSRP
Understanding HSRP
HSRP is useful for hosts that do not support a router discovery protocol and cannot switch to a new router
when their selected router reloads or loses power. When HSRP is configured on a network segment, it
provides a virtual MAC address and an IP address that is shared among router interfaces in a group of
router interfaces running HSRP. The router selected by the protocol to be the active router receives and
routes packets destined for the group’s MAC address. For n routers running HSRP, there are n +1 IP and
MAC addresses assigned.
HSRP detects when the designated active router fails, and a selected standby router assumes control of
the Hot Standby group’s MAC and IP addresses. A new standby router is also selected at that time.
Devices running HSRP send and receive multicast UDP-based hello packets to detect router failure and
to designate active and standby routers. When HSRP is configured on an interface, Internet Control
Message Protocol (ICMP) redirect messages are disabled by default for the interface.
You can configure multiple Hot Standby groups among switches and switch stacks that are operating in
Layer 3 to make more use of the redundant routers. To do so, specify a group number for each Hot
Standby command group you configure for an interface. For example, you might configure an interface
on switch 1 as an active router and one on switch 2 as a standby router and also configure another
interface on switch 2 as an active router with another interface on switch 1 as its standby router.
Figure 40-1
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 blade servers 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 blade server C sends packets to blade server 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. Blade server C continues to use the IP address of the virtual router to address packets destined
for blade server B, which Router B now receives and sends to blade server B. Until Router A resumes
operation, HSRP allows Router B to provide uninterrupted service to users on blade server C’s segment
that need to communicate with users on blade server B’s segment and also continues to perform its
normal function of handling packets between the blade server A segment and blade server B.
