BGP (BGP4 or BGP-4): Border Gateway Protocol and ERP (Exterior Gateway Protocol)

BGP (BGP4 or BGP-4): Border Gateway Protocol The Border Gateway Protocol (BGP), runs over TCP, is an inter-Autonomous System routing protocol. BGP is the only protocol that is designed to deal with a network of the Internet's size, and the only protocol that can deal well with having multiple connections to unrelated routing domains. It is built on experience gained with EGP. The primary function of a BGP system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASs) that reachability information traverses. This information is sufficient to construct a graph of AS connectivity from which routing loops may be pruned and some policy decisions at the AS level may be enforced. BGP4 provides a new set of mechanisms for supporting classless interdomain routing (CIDR). These mechanisms include support for advertising an IP prefix and eliminates the concept of network "class" within BGP. BGP-4 also introduces mechanisms which allow aggregation of routes, including aggregation of AS paths. These changes provide support for the proposed supernetting scheme. Protocol Structure - BGP Border Gateway Protocol Marker (16 byte) Length (2 byte) Type (1 byte) Marker -- Message containing a value predictable by the receiver of the message. Length -- The length of the message including the header. Type --The message type. Possible messages are: Open, Update, Notification, KeepAlive. After a transport protocol connection is established, the first message sent by each side via BGP is an OPEN message. If the OPEN message is acceptable, a KEEPALIVE message confirming the OPEN is sent back. Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION messages may be exchanged. The format of each type of messages could be found in the reference documents BGP (BGP4) is defined by IETF (http://www.ietf.org ) RFC 1771. Reference http://www.javvin.com/protocol/rfc1771.pdf : A Border Gateway Protocol 4 (BGP-4) http://www.javvin.com/protocol/rfc1772.pdf : Application of the Border Gateway Protocol in the Internethttp://www.javvin.com/protocol/rfc1773.pdf : Experience with the BGP-4 protocol http://www.javvin.com/protocol/rfc1774.pdf : BGP-4 Protocol Analysis

EGP: Exterior Gateway Protocol Overview

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 - ERP Exterior Gateway Protocol

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 message

The common portion of the EGP message format: 

8	16	24	32bit
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).