GRE Flows Through NAT

Because PPTP requires the use of GRE flows, the examples in the previous section also work for any GRE traffic flows that traverse NAT.

GRE flows can terminate at an E Series router if NAT is or is not enabled. When the router receives locally terminating inbound GRE packets, the router transmits the packets to the tunnel server module for GRE processing. If the packets require translating, they are again sent through the tunnel server module.

Note: Only inner IP headers are translated for terminating GRE flows; outer IP headers are never translated.

For outbound GRE packets, the process works in reverse. If the packets require translation, the router transmits the packets to the tunnel server module for translation. If the packets are destined for a GRE tunnel, they are again sent through the tunnel server module where an outer header is prepended to the packet and the packet is then sent to the appropriate GRE tunnel.