Border Gateway Protocol Fundamentals

Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator.
BGP used for routing within an autonomous system is called Interior Border Gateway Protocol, Internal BGP (iBGP). In contrast, the Internet application of the protocol is called Exterior Border Gateway Protocol, External BGP (eBGP).
The Border Gateway Protocol was first described in 1989 in RFC 1105, and has been in use on the Internet since 1994.[3] IPv6 BGP was first defined in RFC 1654 in 1994, and it was improved to RFC 2283 in 1998.
The current version of BGP is version 4 (BGP4), which was published as RFC 4271 in 2006. RFC 4271 corrected errors, clarified ambiguities and updated the specification with common industry practices. The major enhancement was the support for Classless Inter-Domain Routing (CIDR) and use of route aggregation to decrease the size of routing tables. The new RFC allows BGP4 to carry a wide range of IPv4 and IPv6 “address families”. It is also called the Multiprotocol Extensions which is Multiprotocol BGP (MP-BGP).
BGP neighbors, called peers, are established by manual configuration among routers to create a TCP session on port 179. A BGP speaker sends 19-byte keep-alive messages every 30 seconds (protocol default value, tunable) to maintain the connection[5]. Among routing protocols, BGP is unique in using TCP as its transport protocol.
When BGP runs between two peers in the same autonomous system (AS), it is referred to as Internal BGP (iBGP or Interior Border Gateway Protocol). When it runs between different autonomous systems, it is called External BGP (eBGP or Exterior Border Gateway Protocol). Routers on the boundary of one AS exchanging information with another AS are called border or edge routers or simply eBGP peers and are typically connected directly, while iBGP peers can be interconnected through other intermediate routers. Other deployment topologies are also possible, such as running eBGP peering inside a VPN tunnel, allowing two remote sites to exchange routing information in a secure and isolated manner.
The main difference between iBGP and eBGP peering is in the way routes that were received from one peer are typically propagated by default to other peers:
•New routes learned from an eBGP peer are re-advertised to all iBGP and eBGP peers.
•New routes learned from an iBGP peer are re-advertised to all eBGP peers only.
These route-propagation rules effectively require that all iBGP peers inside an AS are interconnected in a full mesh with iBGP sessions.
How routes are propagated can be controlled in detail via the route-maps mechanism. This mechanism consists of a set of rules. Each rule describes, for routes matching some given criteria, what action should be taken. The action could be to drop the route, or it could be to modify some attributes of the route before inserting it in the routing table.