In telecommunications, a network-to-network Interface (NNI) is an interface which specifies signaling and management functions between two networks. NNI circuit can be used for interconnection of either signalling (e.g. SS7

SS7

SS-7 can stand for:* Signaling System #7, a set of telephone signaling protocols.* The R-16 missile, with NATO reporting name SS-7 Saddler.* China Railways SS7, an electric locomotive model in China.* Super Socket 7, a chip socket introduced by AMD...

), IP

Internet Protocol

The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

 (e.g. MPLS

Multiprotocol Label Switching

Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...

) or ATM

Asynchronous Transfer Mode

Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...

 networks.

Basically NNI is used for interconnection of P Router

P Router

In Multi-Protocol Label Switching , a P Router or Provider Router is a Label Switch Router that functions as a transit router of the core network. The P Router typically connected to one or more PE Routers....

s (class 4 or higher provider core) in signaling or GMPLS networks.
NNI can be used for interconnection of two VoIP nodes. In GMPLS case that could be Back-to-Back or EBGP or mixed NNI connection scenarios depends on what type of VRF

VRF

In IP-based computer networks, Virtual Routing and Forwarding is a technology that allows multiple instances of a routing table to co-exist within the same router at the same time...

 exchange is used for interconnection. In case of Back-to-Back VRF it is necessary to create VLANs and subsequently sub-interfaces (VLAN headers and DLCI headers for Ethernet and Frame Relay network packets) on the each interface used for the NNI circuit. In case of eBGP NNI interconnection P routers are taught how to dynamically exchange VRF records without VLAN creation.

In cases of mixed or full-mesh scenarios, other NNI types are possible.

Note that there could be different encapsulation types for NNI interconnection, but Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 (GigE) and frame relay

Frame relay

Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...

are basically used.

Network–network Interface (in telecommunications).