Cisco Catalyst 3560-X Software Configuration Manual page 1196

Chapter 1 Configuring HSRP and VRRP
Understanding HSRP
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 address should the designated
active router fail.
Note Routers in an HSRP group can be any router interface that supports HSRP, including Catalyst 3750-X
or 3560-X routed ports and switch virtual interfaces (SVIs). Switches running the LAN base feature set
support only static routing on 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 automatically enabled 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 1-1 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 3750-X and 3560-X Switch Software Configuration Guide
1-2
OL-25303-03