MPLS-Enabled Applications: Emerging Developments and New Technologies

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 from each site: the entire routing table when the customer is an Internet provider and the total number of VPN routes in the customer's network when the customer is a VPN provider. A large number of customer routes from each such customer places a significant burden on the (carriers' carrier) network, both in terms of the memory needed to store the routes at the PEs and in terms of the resources necessary to send and receive advertisements every time any of these routes flap.

In order to be able to scale the solution and support a large number of carrier customers it is necessary to shield the VPN provider (carriers' carrier) from the carrier-customer's routes. The idea is to split the load between the customer and the provider as follows:

1. The carrier-customer handles the route advertisements between the sites, via IBGP (internal BGP) sessions between routers in the different sites. The routes exchanged in these sessions are called 'external routes' because they are not part of the carrier-customer network. They may either be Internet routes or they may be VPN routes belonging to the carrier-customer's own VPN customers. For example, Figure 9.1 shows an ISP as a VPN customer. In this context, prefix 20.1/16 learned over the EBGP peering is considered to be an external route.

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