Exterior Gateway Protocol (EGP) is for exchanging routing information between two neighbor gateway hosts in a network of autonomous systems. EGP is commonly used between hosts on the Internet to exchange routing table information. EGP 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 routing table contains a list of known routers, the addresses they can reach, and a cost metric associated with the path to each router so that the best available route is chosen. Each router polls its neighbor at intervals between 120 to 480 seconds and the neighbor responds by sending its complete routing table. EGP-2 is the latest version of EGP.
A more recent exterior gateway protocol, the Border Gateway Protocol (BGP), provides additional capabilities than EGP.
Protocol Structure
Here are the EGP message types:
| Name | Function |
| Request | Request acquisition of neighbor and/or initialize polling variables |
| Confirm | Confirm acquisition of neighbor and/or initialize polling variables |
| Refuse | Refuse acquisition of neighbor |
| Cease | Request de-acquisition of neighbor |
| Cease-ack | Confirm de-acquisition of neighbor |
| Hello | Request neigbor reachability |
| I-H-U | Confirm neigbor reachability |
| Poll | Request net-reachability update |
| Update | Net-reachability update |
| Error | Error messa |
The common portion of the EGP message format:
| 8 | 8 | 8 | 8 |
| Version | Type | Code | Status |
| Checksum | Autonomous System number | ||
| Sequence number | (The rest of the format is msg type specific.) | ||
- Version -- The version number of EGP. The current 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).
Related Terms: IP, IPv4, TCP, BGP, IGP
Sponsor Source: EGP is defined by IETF (http://www.ietf.org) RFC 904.
Reference: http://www.javvin.com/protocol/rfc904.pdf : Exterior Gateway Protocol formal specification