1-9Routing information exchange from the ingress PE to the egress PEAfter learning the VPN routing information from the CE, the ingress PE adds RDs and VPN targets forthese standard IPv4 routes to form VPN-IPv4 routes, and maintains them for the VPN instance createdfor the CE.Then, the ingress PE advertises the VPN-IPv4 routes to the egress PE through MP-BGP.Finally, the egress PE compares the export target attribute of the VPN-IPv4 routes with the import targetattribute that it maintains for the VPN instance and determines whether to add the routes to the routingtable of the VPN instance.PEs use IGP to ensure the connectivity between them.Routing information exchange from the egress PE to the remote CEA remote CE can learn VPN routes from the egress PE in a number of ways. The routes can be staticroutes, RIP routes, OSPF routes, IS-IS routes, or EBGP routes. The exchange of routing informationbetween the egress PE and the remote CE is the same as that between the local CE and the ingressPE.Multi-AS VPNIn some networking scenarios, multiple sites of a VPN may be connected to multiple ISPs in differentASs, or to multiple ASs of an ISP. Such an application is called multi-AS VPN.RFC 2547bis presents three inter-provider VPN solutions:z VRF-to-VRF: ASBRs manage VPN routes between them through subinterfaces. This solution isalso called inter-provider VPN option A.z EBGP advertisement of labeled VPN-IPv4 routes: ASBRs advertise labeled VPN-IPv4 routes toeach other through MP-EBGP. This solution is also called inter-provider VPN option B.z Multi-hop EBGP advertisement of labeled VPN-IPv4 routes: PEs advertise labeled VPN-IPv4routes to each other through MP-EBGP. This solution is also called inter-provider VPN option C.The following describes these three solutions.Inter-provider VPN option AIn this kind of solution, PEs of two ASs are directly connected and each PE is also the ASBR of its AS.The PEs acting as ASBRs are connected through multiple subinterfaces. Each of them treats the otheras a CE of its own and advertises IPv4 routes through conventional EBGP. Within an AS, packets areforwarded using two-level label forwarding as VPN packets. Between ASBRs, conventional IPforwarding is used.Ideally, each inter-provider VPN has a pair of subinterfaces to exchange VPN routing information.
