HSRP version 2 is designed to address the following restrictions in HSRP version 1:
In HSRP version 1, millisecond timer values are not advertised or learned. HSRP version 2 advertises and learns millisecond timer values. This change ensures stability of the HSRP groups in all cases.
In HSRP version 1, group numbers are restricted to the range from 0 to 255. HSRP version 2 expands the group number range from 0 to 4095.
HSRP version 2 provides improved management and troubleshooting. With HSRP version 1, you cannot use HSRP active hello messages to identify which physical router sent the message because the source MAC address is the HSRP virtual MAC address. The HSRP version 2 packet format includes a 6-byte identifier field that is used to uniquely identify the sender of the message. Typically, this field is populated with the interface MAC address.
The multicast address 126.96.36.199 is used to send HSRP hello messages. This address can conflict with Cisco Group Management Protocol (CGMP) leave processing.
Version 1 is the default version of HSRP.
HSRP version 2 uses the new IP multicast address 188.8.131.52 to send hello packets instead of the multicast address of 184.108.40.206, used by HSRP version 1. This new multicast address allows CGMP leave processing to be enabled at the same time as HSRP.
HSRP version 2 permits an expanded group number range, 0 to 4095, and consequently uses a new MAC address range 0000.0C9F.F000 to 0000.0C9F.FFFF. The increased group number range does not imply that an interface can, or should, support that many HSRP groups. The expanded group number range was changed to allow the group number to match the VLAN number on subinterfaces.
When the HSRP version is changed, each group will reinitialize because it now has a new virtual MAC address.
HSRP version 2 has a different packet format than HSRP version 1. The packet format uses a type-length-value (TLV) format. HSRP version 2 packets received by an HSRP version 1 router will have the type field mapped to the version field by HSRP version 1 and subsequently ignored.
The Gateway Load Balancing Protocol (GLBP) also addresses the same restrictions relative to HSRP version 1 that HSRP version 2 does. See the Configuring GLBP document for more information on GLBP.