The Exterior Gateway Protocol (EGP) exists in order to convey net-reachability information between neighboring gateways, possibly in different autonomous systems. The protocol includes mechanisms to acquire neighbors, monitor neighbor reachability and exchange net-reachability information in the form of Update messages. The protocol is based on periodic polling using Hello/I-Heard-You (I-H-U) message exchanges to monitor neighbor reachability and Poll commands to solicit Update responses.

The format of the header is shown in the following illustration:

EGP version
The version number. This version is version 2.

Type
Identifies the message type.

Code
Identifies the message code.

Status
Contains message-dependent status information.

Checksum
The EGP checksum is the 16-bit one's complement of the one's complement sum of the EGP message starting with the EGP version number field. When computing the checksum the checksum field itself should be zero.

Autonomous System Number
Assigned number identifying the particular autonomous system.

Sequence Number
Send state variable (commands) or receive state variable (responses and indications