GRE Tunnel Concepts
This chapter looks at one type of IP tunnel: generic routing encapsulation (GRE). GRE, defined in RFC 2784, defines an additional header, along with the new IP header, that encapsulates the original packet. Two routers work together, with matching configuration settings, to create a GRE IP tunnel. Then, IPsec configuration can be added to encrypt the traffic.
The discussion of GRE tunnels looks at the concepts from several perspectives. The first section shows how packets can be routed over a GRE tunnel, much like using a serial link inside a secured enterprise network. The rest of this topic then explains how GRE does its work.
Routing over GRE Tunnels
A GRE tunnel exists between two routers, with the tunnel working very ...