3com S7906E 설치 설명서

 

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After learning the VPN routing information from the CE, the ingress PE adds RDs and VPN targets for 

these standard IPv4 routes to form VPN-IPv4 routes, and maintains them for the VPN instance created 

for 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 target 

attribute that it maintains for the VPN instance and determines whether to add the routes to the routing 

table of the VPN instance. 

PEs use IGP to ensure the connectivity between them. 

A remote CE can learn VPN routes from the egress PE in a number of ways. The routes can be static 

routes, RIP routes, OSPF routes, IS-IS routes, or EBGP routes. The exchange of routing information 

between the egress PE and the remote CE is the same as that between the local CE and the ingress 

PE. 

In some networking scenarios, multiple sites of a VPN may be connected to multiple ISPs in different 

ASs, or to multiple ASs of an ISP. Such an application is called multi-AS VPN. 

RFC 2547bis presents three inter-provider VPN solutions: 

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VRF-to-VRF: ASBRs manage VPN routes between them through subinterfaces. This solution is 

also called inter-provider VPN option A. 

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EBGP advertisement of labeled VPN-IPv4 routes: ASBRs advertise labeled VPN-IPv4 routes to 

each other through MP-EBGP. This solution is also called inter-provider VPN option B. 

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Multi-hop EBGP advertisement of labeled VPN-IPv4 routes: PEs advertise labeled VPN-IPv4 

routes to each other through MP-EBGP. This solution is also called inter-provider VPN option C. 

The following describes these three solutions. 

In 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 other 

as a CE of its own and advertises IPv4 routes through conventional EBGP. Within an AS, packets are 

forwarded using two-level label forwarding as VPN packets. Between ASBRs, conventional IP 

forwarding is used. 

Ideally, each inter-provider VPN has a pair of subinterfaces to exchange VPN routing information.