Figure 77 illustrates the first method, where you create a VRF for each VPN on each AS boundary router.
Figure 77: Inter-AS Topology with VRFs on Each AS Boundary Router
Within each AS, routes are announced by internal MP-BGP and the data packets are forwarded across an MPLS tunnel. You create a logical connection such as an ATM VC between each pair of VRFs (on separate AS boundary routers); these logical connections can share the same physical connection. The following factors limit the scalability of this method:
MPLS tunnels are unidirectional; Figure 77 shows only the tunnels established to carry traffic from ASBR 2 to PE 1 and from PE 4 to ASBR 3. Note that ASBR 2 and ASBR 3 are both also PE routers. In that sense, ASBR 2 treats ASBR 3 as a CE router, and ASBR 3 treats ASBR 2 as a CE router.