A network-to-network interface (NNI) is the interface where two networks connect and hand traffic to each other. In business connectivity, it most often means the hand-off between two carriers, where one carrier delivers circuits it has built to customers’ sites into another carrier’s network. A single NNI link can carry many customers’ circuits at once, each kept separate. NNIs are the interfaces that make carrier interconnection work for data circuits, and they are how a provider can sell you service at a building its own network does not reach.
At a glance
- A UNI is where a customer plugs into a provider; an NNI is where two networks plug into each other.
- In Carrier Ethernet, an external NNI (E-NNI) joins two operators and an internal NNI (I-NNI) joins parts of one operator’s network.
- One NNI port commonly carries many customers’ circuits, separated by VLAN tags or similar mechanisms.
- NNIs are usually located in data centers, carrier hotels or other meet points where both networks have equipment.
- Buyers rarely see them, but they shape how off-net circuits are delivered and repaired.
What problem it solves
No single carrier reaches every building. To serve a site outside its footprint, a provider leases a tail circuit from a carrier that does reach it. That carrier’s network and the provider’s network need a standard, scalable place to meet. Without an NNI, each leased circuit might need its own physical hand-off, which would be costly and slow to provision.
An NNI solves this with a shared, high-capacity link between the two networks. New customer circuits can be added as logical connections on the existing NNI, keeping each customer’s traffic separate while using one physical interface.
How it works
Physical link. The two carriers connect equipment, usually with a cross-connect in a shared facility. NNI ports are typically high-capacity, and many carriers deploy more than one at different locations for capacity and resilience.
Logical separation. For Carrier Ethernet, each customer circuit crossing the NNI is usually identified by an outer service tag, sometimes alongside the customer’s own VLAN tags. Both carriers map the tag to the right circuit on their side, so a circuit such as an EVPL can run end to end across both networks.
Agreements. The carriers agree on technical parameters, such as frame sizes, classes of service and how bandwidth profiles carry across the interface, and on commercial terms such as wholesale access pricing and repair responsibilities. MEF publishes standards that many carriers use to align these details.
Operations. Each carrier monitors its side. When a fault occurs on an off-net circuit, tests from both sides of the NNI help locate the problem, and the agreement between carriers sets how quickly the access carrier must respond.
Our network aggregation page covers how buyers use providers that combine access from many carriers.
When it matters for buyers
- Off-net sites. If a site is served through another carrier, ask where the NNI is and how faults across it are handled. A circuit delivered this way is often called a Type II circuit.
- Diversity planning. Two circuits that cross the same NNI or meet point share a potential failure point, even if their access paths differ.
- Performance. An NNI that is far from your site, or congested, can add latency or loss; ask where it is and how it is sized.
- Large multi-site or wholesale buyers. Organizations that aggregate many circuits may be able to take an NNI-style hand-off directly in a data center.
Questions to ask vendors
- For each off-net site, which access carrier is used and where is the NNI?
- Is there more than one NNI with that carrier, and can our circuit fail over between them?
- How is NNI capacity managed, and is it monitored for congestion?
- Do classes of service and frame sizes carry across the NNI unchanged?
- How are faults on the access carrier’s side tested, escalated and measured in the SLA?
- Could we take circuits from you on an NNI-style port at our data center?
How it differs from carrier interconnection
Carrier interconnection is the overall arrangement by which networks link and exchange traffic: the facilities, contracts and traffic types, including internet peering, transit and voice. An NNI is one specific piece of that: the physical and logical interface at the hand-off point, most often used for data circuits. Put simply, carrier interconnection describes the relationship; the NNI is the port where it happens.