Abstract: An Interior Gateway Protocol (IGP) is used or configured on routers belonging to a single Autonomous System (AS). An Autonomous System (AS) is a collection of routers, whose prefixes and router policies are under common administrative control. IGPs like OSPF, RIPv1 and v2, EIGRP etc… are configured within an AS. But the internet is a collection of interconnected Autonomous Systems (ASs). BGP (Border Gateway Protocol) which is an Exterior Gateway Protocol (EGP) was built to route between AS’s. BGP is considered to be a “Path Vector” routing protocol unlike IGP’s which follow distance vector or Link-State routing protocols. In this paper we connect two or more Autonomous Systems using EBGP between the border routers. Before establishing BGP connection the routers within the AS must be configured with IGP such as OSPF. We also configure IBGP within the Autonomous System and eventually differentiate between EBGP and IBGP. The concept of synchronization is practically implemented and explained. We also stress on establishment of BGP neighborship or BGP Peers (IBGP and EBGP peers). BGP peer messages like OPEN, KEEPALIVE, UPDATE and NOTIFICATION messages are explained in brief.

Keywords: Interior Gateway Protocol (IGP), Exterior Gateway Protocol (EGP), OPEN, KEEPALIVE, UPDATE and NOTIFICATION messages.