VPN Routing Requirements

VPNs tunnel traffic as follows from one customer site to another customer site, using a public network as a transit network, when certain requirements are met:

1. Traffic is forwarded by standard IP forwarding from the CE devices to the PE routers.

   The CE devices require only a BGP connection to the PE routers.

2. The PE routers establish an LSP through the provider network.

   The provider network must be running either OSPF or IS-IS as an IGP, as well as IBGP sessions through either a full mesh or route reflector. IBGP is required so that the PE routers can exchange route information for routes that originate or terminate in the VPN.

3. When the inbound PE router receives traffic, it performs a route lookup. The lookup yields an LSP next hop, and the traffic is forwarded along the LSP.

   Either LDP or RSVP must be configured to dynamically set up LSPs through the provider network.

4. When the traffic reaches the outbound PE router, the PE router pops the MPLS label and forwards the traffic with standard IP routing.

   Because the tunnel information is maintained at both PE routers, neither the provider core routers nor the CE devices need to maintain any VPN information in their configuration databases.