| 摘要 |
A service provider's routers (PE 1 , P 1 , P 2 , PE 2 ) provide connections between and share routing information with routers (CE 1 , CE 2 ) of a customer virtual private network (VPN) as well as routers of other customers' VPNs, which may have overlapping address spaces. A service provider's edge router (PE 1 ) informed by the customer's router (CE 1 ) that it will forward packets to a given prefix notifies the other edge router (PE 2 ) that PE 1 can forward packets to that address prefix if the destination is in the VPN to which CE 1 belongs. PE 1 also tells PE 2 to tag any thus-destined packets with a particular tag T 3 . PE 2 stores this information in a forwarding information base that it separately keeps for that VPN so that when PE 2 receives from a router CE 2 in the same VPN a packet whose destination address has that prefix, it tags the packet as requested. But PE 2 also tags it with a tag T 2 that the router P 2 to which PE 2 first sends it has asked PE 2 to apply to packets to be sent to PE 1 . P 2 routes the packet in accordance with T 2 , sending it to P 1 after replacing T 2 with a tag T 1 that P 1 has similarly asked P 2 to use. P 1 removes T 1 from the packet and forwards it in accordance with T 1 to PE 1 , which in turn removes T 3 from the packet and forwards it in accordance with T 3 to CE 1 . In this manner, only the edge routers need to maintain separate routing information for separate VPNs.
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