VRRP - using the same ID on different interfaces
Each VRRP group is identified by a unique VRRP group ID number, which is configured on each router participating in the group. However, multiple interfaces on a single router can participate in the same VRRP group, as long as they are configured with the same VRRP group ID number and priority.
An example of where this could be useful is the following:
Let’s say you have a network with two switches connected to a router. Each switch has multiple VLANs, and the router has a separate interface for each VLAN on each switch. To provide redundancy for the network, you configure the router to participate in a VRRP group with the switches.
By using the same VRRP ID on each interface, you can ensure that the router appears as a single virtual router to the switches, even though it has multiple interfaces. This means that if one of the router’s interfaces fails, the switches will automatically switch to using the virtual IP address associated with another interface on the same router, without having to reconfigure their routing tables.
In such a case, you are delivering redundancy between router ports rather than between multiple routers. This still has a single point of failure (the router itself) but it does deliver redundancy for the physical links to the router, and the router interfaces themselves.
Note that the behavior is different when applied to subinterfaces of the same physical interface.