HSRP - standby group numbers

When applying HSRP as an FHRP mechanism on a Layer 3 switch, it is necessary to assign a standby group number to each SVI that is participating as a redundant gateway.

The standby group numbers are used to calculate the virtual MAC address used for each virtual address.

In most modern switches, it turns out that it is possible to use the same standby number for multiple SVIs without affecting the operation of HSRP at all.

If you have two L3 switches connected to each other, and they are operating as HSRP pairs making their SVIs redundant gateways for their respective subnets, then each redundant pairing operates within its own discrete Layer 2 domain. And since all HSRP communication for setting up the redundancy is confined to that layer 2 domain, there are no issues with duplicate standby numbers.

However, this will cause duplicate virtual MAC addresses since the standby number is used to calculate them. Even so, 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.

Now some older switches choose to maintain a system-wide MAC address table regardless of VLAN, and in such a case, you will experience problems. Also, if two VLANs/subnets are connected at Layer 2 by accident or on purpose, using the same standby group number, and thus the same MAC will cause problems.

Even so, best practice dictates that you should use different standby numbers if only to keep everything tidy and comprehensible. Using the same standby numbers may lead to more human error in configuration than anything else.