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Layer 3 VPNs

In a Layer 3 VPN, the routing occurs on the service provider’s routers. Therefore, Layer 3 VPNs require more configuration on the part of the service provider, because the service provider’s PE routers must store and process the customer’s routes.

In JUNOS software, Layer 3 VPNs are based on the Internet draft draft-rosen-rfc2547bis, BGP/MPLS VPNs. This Internet draft defines a mechanism by which service providers can use their IP backbones to provide Layer 3 VPN services to their customers. The sites that make up a Layer 3 VPN are connected over a provider’s existing public Internet backbone.

VPNs based on draft-rosen-rfc2547bis are also known as BGP/MPLS VPNs because BGP is used to distribute VPN routing information across the provider’s backbone, and MPLS is used to forward VPN traffic across the backbone to remote VPN sites.

Customer networks, because they are private, can use either public addresses or private addresses, as defined in RFC 1918, Address Allocation for Private Internets. When customer networks that use private addresses connect to the public Internet infrastructure, the private addresses might overlap with the private addresses used by other network users. BGP/MPLS VPNs solve this problem by prefixing a VPN identifier to each address from a particular VPN site, thereby creating an address that is unique both within the VPN and within the public Internet. In addition, each VPN has its own VPN-specific routing table that contains the routing information for that VPN only.

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