Hot Standby Router Protocol (HSRP) is a Cisco proprietary protocol that takes two different devices and makes them into one. Configurations make the interfaces appear as if they are all part of one big "virtual router" with the same default gateway. The configuration provides for redundancy in the case of a failed link, failed port or total switch failure.
1). Decide how many groups that you need. The first group is the primary Internet protocol (IP) on the interface. Any additional IPs are secondary entries and each IP has its own group. This group includes the range of IPs that the hosts will also fall under. The first IP of the range is for the standby IP of the "virtual router" and is passed back and forth between the two switches. The next two IPs are for the interface on each router that will participating in the group. All other IPs in the range belongs to the hosts that are connected to the switch.
2). Identify what IPs you will be using for each group. Dividing (subnetting) IPs to ensure that there is not an IP conflict takes time and math skills. Waiting until the last minute to assign IPs for the maintenance can put you in a crunch for time. Plan on one subnet per group to keep the hosts on their own local area network (LAN). Hosts need one IP address each so be sure to include the three IPs to cover your two interfaces and the virtual router ID. Allocation for the interface is needed to ensure you are not short on host addresses.
3). Configure the new group under the existing interfaces.
Group numbers 0-255 can be used and each interface will support up to 15 different groups. The Group numbers that are used do not have to be in any certain order. You can jump around and skip numbers in between or follow contiguous number schemes but no there is no benefit either way. The rule of only 16 groups applies on a per interface basis and you must reuse the same group numbers on the next interface.
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