Carrier's carrier

If a customer of the MPLS L3VPN service provider is also a service provider, 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 learn the routes of the customer network connected to a Level 2 carrier. It only learns the routes for delivering packets between different sites of the Level 2 carrier. Routes of the customer networks connected to a Level 2 carrier are exchanged through the BGP session established between the routers of the Level 2 carrier. This can greatly reduce the number of routes maintained by the Level 1 carrier network.

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 (a Level 2 carrier) accesses a PE of the Level 1 carrier:

• If the PE and the CE are in a same AS, you must configure IGP and LDP between them.

• If the PE and the CE are not in the same AS, you must configure MP-EBGP to assign labels to routes exchanged between them.

In either case, you must 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. As shown in Figure 42, PE 3 and PE 4 exchange VPN routes of the Level 2 carrier through an IBGP session.

When the Level 2 carrier is an MPLS L3VPN service provider, its PEs must run IGP and LDP to communicate with CEs. As shown in Figure 43, PE 3 and PE 4 exchange VPN routes of the Level 2 carrier through an MP-IBGP session.

	NOTE: If equal cost routes exist between the Level 1 carrier and the Level 2 carrier, HP recommends that you establish equal cost LSPs between them.