Multiprovider Considerations

Most enterprise requirements are within the scope of a single MPLS VPN service provider. However, some circumstances require multiple regional MPLS VPNs. For example, a large multinational corporation may require high densities of service in North America, Europe, and the Asia Pacific regions. Based on either cost or density, it may be decided that more than one service provider is required.

In the best case, the service providers support an interconnection with appropriate routing and QoS, and the result is offered to the subscriber as a single seamless solution.

In the more likely case, the subscriber network is required to interconnect to both (or multiple) service providers. Designing the interconnection of subscriber VPNs is greatly simplified by limiting the interconnection between any two VPNs to a single location.

Within a subscriber VPN, all CE routers appear to be only one hop away from all other CE routers. If subscriber VPN A touches subscriber VPN B at more than one location, routing metrics are required to determine which gateway should be used for which locations. Difficulties in advertising routing metrics in the CE-to-PE routing protocols cause additional complexities in this design.

The following example illustrates the situation just described: LargeCo is a multinational corporation with many locations in Europe and North America. For density and economy, LargeCo selects service provider Eurcom to provide MPLS VPN services to sites in Europe and Northcom to provide MPLS VPN services to sites in North America. LargeCo wants two interconnections between the subscriber networks for redundancy. One interconnection will be between Paris and New York, and a second connection will be between London and Washington, D.C. Because all sites in each VPN appear to be only one hop away, all sites are advertised through both gateways at equal cost, causing routing problems.

Resolving this problem requires one of the following:

• A limitation to a single interconnection

• Routing metrics to be adjusted to reflect costs for VPN paths

Subscriber routing metrics are passed through the MPLS VPN without modification. The MPLS label-switched path (LSP) nodes are not reflected in the routing path metrics. Routing metrics may require adjustment to reflect appropriate costs and provide appropriate routing. This approach requires a highly detailed understanding of routing protocols and the network topology.