Virtual mac and IP are only needed for stretch fabric scenarios.
Stretch fabric in a nutshell would be 2 separate ACI fabrics with an L2 connection between the 2. Each fabric having similar bridge domain configurations and sharing the same gateway on the BD SVI. This common gateway is called Common Pervasive Gateway. More info on this below:
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/1-x/aci-fundamentals/b_ACI-Fundamentals/b_ACI-Fundamentals_chapter_010100.html#concept_C5DF4A259C8044DD82E52B0847A9DD96
Use case example:
ACI fabric-A -> BD-A -> Subnet 192.168.100.254/24
ACI fabric-B -> BD-A -> Subnet 192.168.100.254/24
Lets say you manage both of these fabrics and want to bridge the two together. First, setup the L2 link between the two fabrics. Fabrics are now bridged, but now to have the same gateway IP and MAC existing on both fabrics. To resolve this we implement, common pervasive gateway. Configuration example below.
Fabric-A -> BD-A
192.168.100.254/24 :: Virtual IP = yes
192.168.100.252/24 :: Virtual IP = no
Virtual mac = 0022.bdf8.19ff
Custom mac = 0022.bdf8.19fd
Fabric-B -> BD-A
192.168.100.254/24 :: Virtual IP = yes
192.168.100.253/24 :: Virtual IP = no
Virtual mac = 0022.bdf8.19ff
Custom mac = 0022.bdf8.19fe
Notice that both fabrics share the same VIP and VMAC. Custom mac and physical IP are different.
If this is not the use case, then no need to configure VIP and VMAC. Only configure a subnet without virtual IP enabled and only configure a custom mac.
Jason
