MPLS-Enabled Applications: Emerging Developments and New Technologies

9.3. MULTI-AS BACKBONES

The previous section showed how a VPN provider can be a VPN customer, and its sites are in the same AS. In this section we will take a look at what happens when the sites are in different ASs. This can be the case when a provider spans several ASs (e.g. following an acquisition) or when two providers cooperate in order to provide a VPN service to a common customer. In the latter case, it is necessary for the providers to agree on the conditions and compensation involved and to determine the management responsibilities. To distinguish the two cases, they are referred to as inter-AS and interprovider respectively.

The problem with multi-AS scenarios is that the routers in the two sites cannot establish an IBGP session to exchange external routes. Instead, an EBGP session must be run. [RFC4364] describes three ways to solve the multi-AS scenario. These methods are often referred to by their respective section number in [RFC4364], as options A, B and C. An important thing to bear in mind when reading this section is that multi-AS scenarios are not targeted in particular at carrier-customers and provide a general solution for VPNs crossing several ASs.

9.3.1. Option A: VRF-to-VRF connections at the ASBR

The simplest method to exchange the VPN routes across an AS boundary is to attach the two ASBRs directly via multiple subinterfaces (e.g. VLANs, or Virtual LANs) and run EBGP between them. Each ASBR associates one of the subinterfaces with the VRF for a VPN ...