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Internal VLAN ACI (PI)


Hello , 

Did Someone have a Logic Explanation , why  we use the PI Vlan (Internal Vlan) on ACI?   and What is the Role of PI Vlan ? 

i Know That the FD (Forwarding Vlan ) have always To be maped to an PI vlan , but the Question is Why ? and Why we need the PI Vlan ? 


Best regards 


2 Replies 2

Robert Burns
Cisco Employee
Cisco Employee

It comes down to separating the HW VLANs assigned/used by the switches ASICs, from the global ones used across the fabric for ACI policy & segmentation.  Keep in mind that ACI separates the logical model from physical, meaning we could assign the SAME EPG across any number of switches using a different Encap VLAN on the front panel ports.  To accomplish there needs to be a switch-local and ACI-global VLAN concept.  There's a bit more detail in these two articles , but that's the jist of it. 



VLAN Encapsulation in ACI

External VLAN: Used for External Communication and Integration

Internal VLAN: It is also called as Platform Independent Vlan whose scope is local to each leaf. 


ACI has no control how Platform VLAN is allocated to traffic going via leaf. APIC allocates PI VLAN per EPG, Per BD and these allocation is local to leaf and is different to each Leaf.

Cisco ACI fabric internally does not use VLANs as traditional switches but it translates externally connected VLANs to Flooding Domain, Bridge Domain and VXLANs. All of this is happening at the ingress to the fabric.


if you run the command "#sho  vlan extended" you can see encap VLAN(traditional VLAN) map to a random PI VLAN.

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