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
||Autonomous System number
| Sequence number
The version number. This version is version 2.
Identifies the message type.
Identifies the message code.
Contains message-dependent status information.
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
Autonomous System Number
Assigned number identifying the particular
Send state variable (commands) or receive state
variable (responses and indications