GRE Fun

General routing Encapsulation notes

What goes in a GRE header?

All data sent over a network is broken up into smaller pieces called packets, and all packets have two parts: the payload and the header. The payload is the packet’s actual contents, the data being sent. The header has information about where the packet comes from and what group of packets it belongs to. Each network protocol attaches a header to each packet.

GRE adds two headers to each packet: the GRE header, which is 4 bytes long, and an IP header, which is 20 bytes long. The GRE header indicates the protocol type used by the encapsulated packet. The IP header encapsulates the original packet's header and payload. This means that a GRE packet usually has two IP headers: one for the original packet, and one added by the GRE protocol. Only the routers at each end of the GRE tunnel will reference the original, non-GRE IP header.