Cisco WS-C3550-12G Software Configuration Manual page 598

Chapter 23 Configuring HSRP
Understanding HSRP
Note Routers in an HSRP group can be any router interface that supports HSRP, including Catalyst 3550
routed ports and switch virtual interfaces (SVIs).
HSRP provides high network availability by providing redundancy for IP traffic from hosts on networks.
In a group of router interfaces, the active router is the router of choice for routing packets; the standby
router is the router that takes over the routing duties when an active router fails or when preset conditions
are met.
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 Catalyst 3550 switches 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 23-1 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.
Catalyst 3550 Multilayer Switch Software Configuration Guide
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