Cisco Systems 78-11741-02 Wireless Office Headset User Manual


 
Configuring IP Services
Configuring the Hot Standby Router Protocol
IPC-100
Cisco IOS IP Configuration Guide
Configuring the Hot Standby Router Protocol
The Hot Standby Router Protocol (HSRP) provides high network availability because it routes IP traffic
from hosts on Ethernet, FDDI, or Token Ring networks without relying on the availability of any single
router. HSRP is used in a group of routers for selecting an active router and a standby router. (An active
router is the router of choice for routing packets; a standby router is a 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 (such as ICMP Router Discovery
Protocol [IRDP]) and cannot switch to a new router when their selected router reloads or loses power.
Because existing TCP sessions can survive the failover, this protocol also provides a more transparent
recovery for hosts that dynamically choose a next hop for routing IP traffic.
When the HSRP is configured on a network segment, it provides a virtual MAC address and an IP
address that is shared among a group of routers running HSRP. The address of this HSRP group is
referred to as the virtual IP address. One of these devices is selected by the protocol to be the active
router. The active router receives and routes packets destined for the MAC address of the group. For n
routers running HSRP, n + 1 IP and MAC addresses are assigned.
HSRP detects when the designated active router fails, at which point a selected standby router assumes
control of the MAC and IP addresses of the Hot Standby group. A new standby router is also selected at
that time.
Devices that are running HSRP send and receive multicast UDP-based hello packets to detect router
failure and to designate active and standby routers.
Previously, when HSRP was configured on an interface, ICMP redirect messages were disabled by
default. With Cisco IOS Release 12.1(3)T, ICMP redirection on interfaces configured with HSRP are
enabled by default. See the “Enabling HSRP Support for ICMP Redirect Messages”section later in this
document for more information.
You can configure multiple Hot Standby groups on an interface, thereby making fuller use of redundant
routers and load sharing. To do so, specify a group number for each Hot Standby command you configure
for the interface.
Note Token Ring interfaces allow up to three Hot Standby groups each, the group numbers being 0, 1, and
2.