MEF 26.1 is a specification document developed by the Technical Committee of the MEF that defines the External Network Network Interface (ENNI)
"The Metro Ethernet Network Architecture Framework specifies a reference point that is the interface between two Metro Ethernet Networks (MENs), where each Operator MEN is under the control of a distinct administrative authority. This reference point is termed the External Network Network Interface or ENNI. The ENNI is intended to support the extension of Ethernet services across multiple Operator MENs. This Technical Specification specifies:
The requirements at the ENNI reference point as well as the interface functionality in sufficient detail to ensure interoperability between two Operator MENs including Link OAM.
The connectivity attributes UNI to UNI, UNI to ENNI, and ENNI to ENNI such that multiple Operator MENs can be interconnected and the Ethernet services and attributes in MEF 6.1  and MEF 10.2  can be realized."