Understanding IPv4 VPN Services Across Multiple Autonomous Systems

Inter-AS services, sometimes known as interprovider services, support VPNs that cross AS boundaries. VPNs might need to cross AS boundaries because of a customer deployment that involves geographically separated ASs. The VPN sites can be provided by the same service provider or by different service providers as part of a joint VPN service offering. Inter-AS services are also useful to service providers that use confederations of sub-ASs to reduce the IBGP mesh inside the AS.

You can support these inter-AS services in three different ways, known as inter-AS option A, option B, and option C. Option C is preferred to option B; option B is preferred to option A. For inter-AS options B and C, you must explicitly configure MPLS on all the inter-AS links.

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