Hierarchical and Inter-AS VPNs
In the previous two chapters we have seen the basic operation of BGP/MPLS L3VPNs, where a service provider offers a VPN service to an enterprise customer as replacement for the mesh of circuits geographically dispersed locations. The problem of connecting geographically dispersed locations is not unique to enterprise customers. Carriers may have similar problems, especially following an acquisition of a new network from a different carrier. In this case, is needed to the new network and could be (and sometimes is) accomplished by buying L2 circuits. However, just like the enterprise case, it is possible to also connect the remote locations via an L3VPN service.
In the following sections we will see scenarios where the VPN customers are themselves Internet service providers (ISPs) or VPN providers and they obtain backbone service from a VPN provider who acts as a ‘carrier of carriers’ [RFC4364]. An important thing to note in the context of a ‘carriers’ carrier’ scenario is the fact that all sites of a customer who is a carrier belong to the same AS. We will discuss inter-AS solutions in a separate section afterwards, because the concepts introduced in the carriers’ carrier discussion will facilitate the understanding of the inter-AS case.
9.2 CARRIERS’ CARRIER – SERVICE PROVIDERS AS VPN CUSTOMERS
The biggest challenge in providing VPN service to customers who are themselves providers is the sheer number of routes that may be advertised ...