Overview BGP

Border Gateway Protocol (BGP) is a routing protocol that exchanges Network Layer Reachability Information (NLRI) between Autonomous Systems (ASs). Internal Gateway Protocols (IGPs) such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Intermediate System-to-Intermediate System (IS-IS), focus mainly on finding accurate paths, taking network nodes (such as routers, layer-3 switches, multi-NIC hosts) as the routing units. Different from IGPs, External Gateway Protocols (EGPs) focuses mainly on controlling the routing direction, taking AS networks as the routing units.

BGP is used for interconnection between AS networks. It supports routing information exchange between ASs. It is usually used for large-scale network aggregation and network core. Its application layer determines that BGI has the following features when compared with IGP:

• BGP uses the TCP protocol to transmit packets through service port 179. TCP ensures the reliability of transmission, so BGP need not provide an independent transmission control policy for reliable transmission of information.
• BGP updates routes in incremental mode, that is, it informs its neighbors of route changes only when route properties are changed, or a route is added or deleted. This mode greatly decreases network bandwidth that is occupied by BGP in transmitting routes.
• BGP is an AS-based distance vector protocol. It carries AS path properties in routing packets to solve the routing loop problem.
• BGP routes have abundant properties. You can modify the properties by applying a routing policy. In this way, you can control route filtering and selection freely.

BGP has two neighbor types, Interior Border Gateway Protocol (IBGP) and External Border Gateway Protocol (EBGP). Between different types of neighbors, different route advertisements and routing policies are used.