GRE is a tunneling protocol developed by Cisco that can encapsulate a wide variety of protocol packet types inside IP tunnels, creating a virtual point-to-point link to Cisco routers at remote points over an IP internetwork. IP tunneling using GRE enables network expansion across a single-protocol backbone environment. It does this by connecting multiprotocol subnetworks in a single-protocol backbone environment.
GRE has these characteristics:
- GRE is defined as an IETF standard (RFC 2784).
- In the outer IP header, 47 is used in the protocol field to indicate that a GRE header will follow.
- GRE encapsulation uses a protocol type field in the GRE header to support the encapsulation of any OSI Layer 3 protocol. Protocol Types are defined in RFC 1700 as "EtherTypes".
- GRE itself is stateless; by default it does not include any flow-control mechanisms.
- GRE does not include any strong security mechanisms to protect its payload.
- The GRE header, together with the tunneling IP header indicated in the figure, creates at least 24 bytes of additional overhead for tunneled packets.