A tail circuit is the access segment that connects a customer site to a provider’s network, typically running from the building to the provider’s nearest point of presence (POP) or aggregation point. When the provider has its own facilities at the address, it may supply the tail itself. When it does not, it usually leases the tail from a local carrier and connects it to its network, often at a network-to-network interface (NNI). The term is common in private line, Carrier Ethernet and dedicated internet quotes, though carriers use it a little differently, so it is worth confirming what a given quote means by it.
At a glance
- The tail is the access portion of a circuit, between your site and the provider’s network, not the provider’s backbone.
- It may be supplied by your provider or leased from another carrier, depending on whether the provider reaches your building.
- Tail pricing, availability and construction often drive the cost and install date of the whole service.
- When the tail is leased, a second carrier sits in the delivery and repair path even though you have one contract.
- Usage varies: some carriers use “tail” for any local access segment, others mainly for leased access.
What problem it solves
No provider has its own network into every building. Tail circuits let a provider sell service at addresses outside its footprint by buying access from a carrier that does reach them, then carrying the traffic on its own network from there. For a buyer with many sites, that means one provider can serve most or all locations under one contract, even if the access underneath comes from several carriers.
For the provider, it separates the problem of reaching your building from the service it sells. The tail gets your traffic to the provider; the provider’s network and services do the rest.
How it works
Sourcing. When you request a quote, the provider checks whether your address is on-net or off-net. If it is off-net, the provider requests access pricing from one or more local carriers and builds your quote around the best option. A circuit delivered this way is often called a Type II circuit.
Physical path. The tail usually runs from the carrier’s equipment at your demarc to a carrier facility, then to a hand-off with your provider, often at a point of presence (POP) or data center. Fiber is most common for business services, but tails are also delivered over copper, coax or fixed wireless depending on the service.
Hand-off. Leased tails are typically handed to the provider at an NNI, a shared interface that carries many customers’ circuits between the two carriers, each kept separate. For Carrier Ethernet services this is commonly done with VLAN tags.
Service terms. The access carrier’s commitments sit underneath your provider’s SLA. Whether your provider passes through, improves on or excludes those commitments is set in your contract.
For buyers with many sites to connect, our network aggregation page explains how providers source and combine access across carriers.
When it matters for buyers
- Comparing quotes. Price and install time often depend on who supplies the tail. Ask each bidder whether it is on-net and, if not, which carrier it will use.
- Designing redundancy. Two providers may both lease tails from the same local carrier, so your “diverse” circuits could share a cable. Ask about the access carrier and the physical route.
- Planning moves and new sites. Leased tails can carry construction charges and longer lead times; order early.
- Handling outages. A leased tail adds a party to every repair, so escalation paths and SLA coverage matter.
Questions to ask vendors
- Is this address on your own network, or will you lease the tail from another carrier? Which one?
- Where is the tail handed to your network, and how far is that from our site?
- Does your SLA cover the tail end to end, including repair time on the leased segment?
- Who manages faults on the tail, and how do you escalate with the access carrier?
- Are there construction or special-access charges, and what is the realistic install date?
- If we order a second circuit for diversity, will it use a different access carrier and physical path?
How it differs from the last mile
The last mile describes the final physical stretch of any network into a building, regardless of who owns it or how it is sold. A tail circuit is a provider’s term for the whole access segment it needs to reach your site, which can extend well beyond the building entrance to a distant POP and is often bought from another carrier. Backhaul is different again: it carries traffic onward from an access or aggregation point toward a core network or central site.