Chapter 6. Troubleshooting Multiprotocol Label Switching Layer 3 VPNs
Multiprotocol Label Switching (MPLS) Layer 3 VPNs are described in Internet Draft draft-ietf-l3vpn-rfc2547bis (RFC2547bis). MPLS Layer 3 VPNs allow a service provider to provision IP connectivity for multiple customers over a shared IP backbone, while maintaining complete logical separation of customer traffic and routing information. Each customer VPN consists of a several geographically dispersed sites. IP connectivity between sites is provisioned over the provider backbone.
There are two basic VPN models:
The overlay model, in which there is no exchange of routing information between the customer and the service provider
The peer model, in which routing information is exchanged between customer and service provider
MPLS Layer 3 VPNs conform to the peer model, but unlike other peer VPN architectures, each customer's routing information is maintained in separate routing and forwarding tables.
Figure 6-1 illustrates a service provider backbone with two MPLS VPNs provisioned.
In Figure 6-1 there are two VPNs, mjlnet_VPN and cisco_VPN. Each VPN has three sites, with site 1 in each VPN connected to Chengdu_PE, site 2 connected to HongKong_PE, and site 3 connected to Shanghai_PE.
The MPLS VPN topology is very flexible. The service provider can configure intranet and extranet topologies, such as hub-and-spoke and full-mesh, simply by controlling the distribution of customer routes between service provider (edge) routers.
The service provider can also act as a backbone to carry traffic between different sites of another service provider. This is known as the carrier's carrier topology.
Finally, service providers can combine to offer VPN connectivity to a customer, with some customer sites connected to one provider and other customer sites connected to other providers. This is called an interprovider VPN.