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The External Network-to-Network Interface (ENNI) is a reference point representing the boundary between two Operator CENs that are operated as separate administrative domains. ENNI is defined in MEF 26.2.
ENNIs are important in the situation that a Subscriber for Ethernet Services has locations that are not all served by a single CEN Operator. In order for the Subscriber to contract for an end-to-end service in this case, multiple CEN Operators are needed to support all of the Subscriber's User Network Interfaces (UNIs).
The following diagram shows a Subscriber with two UNIs that are connected together across three network operators.
From the diagram it can be seen that there are two types of operator interconnections in a service that crosses a multi-CEN network. One type of operator can contain a UNI and an ENNI. The other type of operator has two ENNIs–this is a transit operator. (Note that each operator could have more than one UNI and/or more than one ENNI supporting a multipoint service).
Each side of the ENNI has a reference point called the ENNI-N. (Unlike the UNI which is asymmetric with a UNI-C on one side and UNI-N on the other, the ENNI is symmetric with an ENNI-N on both sides).
The ENNI-N has the following functionality:
MEF 26.2 defines an ENNI Frame as any Ethernet frame exchanged between two CEN Operators across an ENNI interface.