HSRP Versions
There are two versions of HSRP: version 1 and version 2. The difference between them in terms of general functionality is small, but there are several improvements that Cisco made when updating to version 2. Some of these improvements include:
HSRP version 2 now advertises and learns millisecond timer values, improving the stability of HSRP.
The total number of HSRP group numbers increases from 256 to 4096 (not necessarily to utilize this many groups on a single interface, but to match local VLAN numbers—on sub-interfaces—for easy troubleshooting and documentation).
HSRP version 2 includes the addition of an identifier field (which contains the physical MAC address of the source) within the HSRP packet that allows the ability to easily track which device sent an HSRP packet (this is because the active HSRP router will send traffic using the virtual MAC address).
HSRP version 2 changes the multicast address that is used for HSRP communication to avoid conflict with Cisco Group Management Protocol (CGMP).
Some other differences are not really improvements, per se. HSRP version 1 utilizes the MAC address range from 0000.0C07.AC00 through 0000.0C07.ACFF (00 = Group 0, FF = Group 255), while HSRP version 2 utilizes the MAC address range from 0000.0C9F.F000 through 0000.0C9F.FFFF (000 = Group 0, FFF = Group 4095). HSRP version 1 uses the multicast address 224.0.0.2, while HSRP version 2 uses the multicast address 224.0.0.102. HSRP version 1 and version 2 are not compatible and use different packet formats.