Carrier’s to Carrier

 I. Introduction to carrier's carrier

It is possible that a customer of the MPLS L3VPN service provider is also a service provider. In this case, the MPLS L3VPN service provider is called the provider carrier or the Level 1 carrier, while the customer is called the customer carrier or the Level 2 carrier. This networking model is referred to as carrier’s carrier. In this model, the Level 2 service provider serves as a CE of the Level 1 service provider.

For good scalability, the Level 1 carrier does not inject the external routes of a Level 2 carrier; it only injects routes for switching packets from different sites of the Level 2 carrier. The external routes maintained by a Level 2 carrier are exchanged through BGP sessions established between related routes of the Level 2 carrier. This can greatly reduce the number of routes maintained by the Level 1 carrier network.

II. Implementation of carrier’s carrier

Compared with the common MPLS L3VPN, the carrier’s carrier is different because of the way in which a CE of a Level 1 carrier, that is, a Level 2 carrier, accesses a PE of the Level 1 carrier:

• If the PE and the CE are in a same AS, you need to configure IGP and LDP between them.
•  If the PE and the CE are not in the same AS, you need to configure MP-EBGP to label the routes exchanged between them.

In either case, you need to enable MPLS on the CE of the Level 1 carrier. Moreover, the CE holds the VPN routes of the Level 2 carrier, but it does not advertise the routes to the PE of the Level 1 carrier; it only exchanges the routes with other PEs of the Level 2 carrier.

A Level 2 carrier can be an ordinary ISP or an MPLS L3VPN service provider.

When the Level 2 carrier is an ordinary ISP, its PEs run IGP to communicate with the CEs, rather than MPLS.PE 3 and PE 4 exchange VPN routes of the Level 2 carrier through IBGP sessions.

Scenario where the Level 2 carrier is an ISP

When the Level 2 carrier is an MPLS L3VPN service provider, its PEs need to run IGP and LDP to communicate with CEs.PE 3 and PE 4 exchange VPN routes of the Level 2 carrier through MP-IBGP sessions.

Scenario where the Level 2 carrier is an MPLS L3VPN service provide

Caution:

If there are equal cost routes between the Level 1 carrier and the Level 2 carrier, you are recommended to establish equal cost LSPs between them accordingly.