What is Border Gateway Protocol?
Border Gateway Protocol (BGP) is a routing protocol used to transfer data and information between different host gateways, the Internet, or autonomous systems. BGP is a Path Vector Protocol (PVP) that maintains paths to various hosts, networks and gateway routers and determines the routing decision on this basis. It does not use Interior Gateway Protocol (IGP) metrics for routing decisions, it only decides on the route based on path, network policies and rule sets. Sometimes BGP is described as a reachability protocol rather than a routing protocol.
BGP roles include:
Since it is a PVP, BGP communicates the entire autonomous system / network path topology to other networks
Maintains its routing table with topologies of all externally connected networks
Supports Classless interdomain routing (CIDR), which assigns Internet Protocol (IP) addresses to connected Internet devices
When used to facilitate communication between different autonomous systems, BGP is called External BGP (EBGP). When used in host networks / autonomous systems, BGP is referred to as Internal BGP (IBGP).
BGP was created to extend and replace the Exterior Gateway Protocol (EGP).