Difference between the Platform Independent (PI) or Internal VLANs and Encapsulation VLANs in ACI

Here is a comprehensive explanation of the difference between the Platform Independent or Internal VLANs and Encapsulation VLANs.

ACI leaves use internal VLANs for some functions like SVIs (Interface VLAN) and pervasive gateways, these VLANs are known as platform-independent VLANs or PI VLANs or Internal VLANs and they are locally significant to each leaf.

ACI uses external or encapsulation VLANs to classify traffic as it comes into the ACI fabric and these are logically significant between the port and the endpoint this means that the same encapsulation VLAN-id could be used for different EPGs and even different Tenants on different ports therefore ACI uses the PI/Internal VLAN as an internal method of communication for some functions in order to normalize layer 2 communications at the leaf level.

To deal with them for troubleshooting purposes, you want to make sure that you look at encapsulation VLAN column most of the time. Before in legacy switches we used to type the (show vlan) command to see the VLANs on a switch, in ACI I recommend you don’t use this command since it will show you the PI/internal VLANs list which may not tell you much if you want to see the encapsulation VLANs at the Leaf level use either the (show endpoint) command or the (show vlan extended) command which will show you both the PI/Internal and encapsulation VLAN association list.