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BGP (Border Gateway Protocol) is a protocol that manages how packets are routed through the Internet by exchanging routing and reachability information between edge routers. BGP directs packets between autonomous systems (AS)-a network managed by a single enterprise or service provider. Traffic routed within a single network AS is called internal BGP or iBGP. More commonly, BGP is used to connect an AS to another autonomous system and then call it an external BGP or eBGP.

BGP is used to transfer data and information between different host gateways, Internet or autonomous systems. BGP is a path vector protocol (PVP) that maintains paths to different hosts, networks, and gateway routers and determines routing decisions based on it. It does not use the Interior Gateway Protocol (IGP) metric to make routing decisions, but only based on paths, network policies and rule sets.

Sometimes, BGP is described as a reachability protocol rather than a routing protocol.

BGP roles include:

Because it is PVP, BGP transfers the entire autonomous system / network path topology to other networks

Maintain its routing table using the topologies of all externally connected networks

Classless inter-domain routing (CIDR) is supported, 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 a host network / autonomous system, BGP is called internal BGP (IBGP).

BGP was created to extend and replace the external Gateway Protocol (EGP).

Where is BGP used:

BGP provides network stability to ensure that routers can quickly adapt to sending packets over another reconnection if another Internet path fails. BGP makes routing decisions based on paths, rules, or network policies configured by the network administrator. Each BGP router maintains a standard routing table that guides packets in transit. This table, used in conjunction with a separate routing table, is called a routing information base (RIB), which is a data table stored on a server on a BGP router. RIB contains routing information from directly connected external peers as well as internal peers and constantly updates the routing table as changes occur. BGP is based on TCP/IP and uses a client-server topology to communicate with the client-server and initiate a BGP session by sending a request to the server.

BGP routing fundamentals:

BGP sends updated router table information only when there is a change-even so, it sends only the affected information. BGP does not have an automatic discovery mechanism, which means that the connection between peers must be set manually and peer addresses must be written on both sides.

BGP makes the best path decision based on current accessibility, hop count and other path characteristics. Where multiple paths are available, such as within a primary hosting facility, BGP can be used to communicate an organization's preferences based on the path of traffic in and out of its network. BGP even has a mechanism for defining arbitrary labels, called communities, which can control routing advertising behavior through mutual protocols between peers.

BGP-4 is the current version of BGP that supports IPv6 and classless inter-domain routing (CIDR) and supports the continuous availability of IPv4. Using CIDR is a way to have more addresses in a network than using the current IP address allocation scheme.

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