Traditional MPLS VPN networks contain customer or client VPN sites traversing a single MPLS VPN backbone. However, in a geographically dispersed network, client VPN sites might connect to different MPLS VPN backbones. Figure 7-1 shows such a network where client sites belonging to VPN-A and VPN-B are connected to different service provider networks.
Figure 7-1. Dispersed Client Sites Belonging to Different Providers
In such cases, to enable continuity of VPN services across multiple service providers, the VPN information has to be mutually redistributed. The Inter-AS or Inter-Provider VPN feature allows the VPN information to be redistributed between adjacent MPLS VPN entities so that client sites belonging to VPN-A and VPN-B that are dispersed across multiple service provider backbones can communicate with each other.
Figure 7-2 shows the MPLS VPN network in which the edge routers PE2-ASBR1-AS1 and PE2-ASBR2-AS2 serve as Autonomous System Boundary Router (ASBR) routers. The ASBR Router PE2-ASBR1-AS1 is responsible for propagating Site 1 VPN information to Site 2 and PE2-ASBR2-AS2 propagates Site 2 VPN information to Site 1.
Figure 7-2. Inter-Provider VPN Network Using Edge Routers as ASBRs
To maintain the continuity of VPN services across multiple service providers, there are four different options to distribute VPNv4 information across the ASBR routers:
- Option 2a: Using the next-hop-self method
- Option 2b: Using the redistribute connected approach
- Option 2c: Multi-hop MP-eBGP
Figure 7-3 shows the various options to distribute VPNv4 information.
Figure 7-3. Inter-AS VPNv4 Distribution Options
Options 1, 2a, and 3 correspond to the following options mentioned in RFC 2547bis:
Forthcoming sections discuss each of the VPNv4 distribution options.