HSRP - Configure the Hot Standby Routing Protocol

Redundancy Protocol, Cisco Proprietary.
Configuration is quite straight-forward, but there are many ways to tune it, in accordance with your needs:
interface FastEthernet0/0
 ip address 172.25.25.2 255.255.255.0
 standby 1 ip 172.25.25.22 <- Group 1 VIRTUAL IP Address
 standby 1 timers 5 15 <- Can also be done in miliseconds using "standby 1 timers msec 250 800"
 standby 1 priority 150 <- Default it 100, Default
 standby 1 preempt
 standby 1 authentication Cisco
 standby 1 name R2-Act
 standby 2 ip 172.25.25.55
 standby 2 timers 5 15
 standby 2 authentication Cisco
 standby 2 name R5-Act

"07-ac" is the SIGNARURE part of Virtual MAC Address of the HSRP:
#sh standby | i 07
  Active virtual MAC address is 0000.0c07.ac01
    Local virtual MAC address is 0000.0c07.ac01 (v1 default)


To check the current configuration, including the HSRP Status and whether
preempt is configured:
#sh standby brief
                     P indicates configured to preempt.
                     |
Interface   Grp Prio P State    Active          Standby         Virtual IP
Fa0/0       1   100    Standby  172.25.25.2      local           172.25.25.22
Fa0/0       2   200  P Active   local           172.25.25.2      172.25.25.55

If you need to TRACK an interface, just add the interface line, and define for how much you want to decrease the
HSRP priority in order to fail over to the HSRP Peer:
(config-if)#standby 1 track serial 0/1/0.21 60