Things that you might miss when studying HSRP

1. HSRP group can contain more than two members; One is active member, one is standby router and the others will be in listening HSRP state.
2. HSRP sends hello messages to 224.0.0.2 (All Routers Multicast Address) using UDP port 1985.
3. HSRP group numbers can be [0,255]
4. HSRP groups are locally significant on the interface. This is very important point to understand. For example, HSRP group 1 on the VLAN 10 interface is not same as the HSRP group 20 on VLAN 11 interface.
5. HSRP router election:

            Default priority is 100
            Active Router is the one with the highest priority

            If there is a priority tie then the router with the highest HSRP Ip address wins.

    6.   HSRP States: Disabled> Init> Listen> Speak> Standby> Active.

    7.   Only standby member monitors the hello messages. Listening members do not monitor them.

    8.   Default hello message every =3s; Hold time=10s
    9.  The actual interface address and the standby address must be configured to be in the same IP subnet.