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).

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