I have a client configs that currently working with HSRP on 2 LAN gateway switches.
Differently interface VLans have been configured on these switches.
Now, what strikes me is that all the interface Vlans on these gateway switches have been configured for the same HSRP groups thpough the ip addresses pairs are in different subnets
What could be the possible drawback on this???
The formulation of an HSRP virtual MAC-address, which corresponds to the virtual IP address of an HSRP group, is a function of the group number.
The template is 0000.0c07.acXX, where "XX" represents the two hexadecimal digits that represent the group number.
So, if under vlan 10's SVI you have "standby 100" configured, the virtual MAC-address will be "0000.0c07.ac64." The hexadecimal equivalent of decimal 100 is 64.
This is what is meant when the Cisco website says that "they share the same HSRP virtual MAC address" when referring to multiple vlan interfaces with the same standby group assigned to it.
Given that, it is true that if you configure vlan 11, 12, 13, 14 and so on with the same group number, the same virtual MAC-address will be used.
This does not cause a problem, even though the same MAC-address is going to be returned in an ARP response for every group that uses the same group number, because the ARP query and response are bound by the L2 broadcast domain of the vlan. So, too, are all L2 communications within the vlan.
Therefore, when a host on a given vlan inserts MAC-address 0000.0c07.ac64 as the destination host address, it will only be forwarded to the standby address of that vlan because MAC-addresses only have significance within the vlan.