What is BGP (Border Gateway Protocol)?

BGP (Border Gateway Protocol) is a core routing protocol used in the Internet and large-scale enterprise networks. It is the protocol that enables the exchange of routing information and facilitates the interconnection of different autonomous systems (ASes) to create the global Internet.

Here are some key points about BGP:

  • Autonomous Systems (ASes): An Autonomous System (AS) is a collection of IP networks under a single administrative entity or organization. An AS can be a large ISP (Internet Service Provider), a corporate network, or any other network that has its own unique routing policies.
  • Path Vector Protocol: BGP is a path vector routing protocol, which means it makes decisions based on the path information along with various attributes associated with routes. It uses the AS path attribute to avoid routing loops and make intelligent routing decisions.
  • External BGP (eBGP) and Internal BGP (iBGP): BGP can be classified into two types based on the relationship between the ASes. eBGP is used for exchanging routing information between different ASes, and it runs between routers in different autonomous systems. iBGP is used for distributing routing information within an AS, and it runs between routers within the same autonomous system.
  • BGP Peering: BGP peers establish TCP connections with each other to exchange routing information. eBGP peers are usually established between routers of different ASes, and iBGP peers are set up between routers within the same AS.
  • Path Selection: BGP employs a set of rules to select the best path for reaching a destination network, considering factors like the shortest AS path, BGP attributes (e.g., local preference, MED – Multi-Exit Discriminator), and routing policies configured by network administrators.
  • Internet Routing Table: BGP is responsible for maintaining the global Internet routing table, which contains the network prefixes and associated paths to reach those prefixes. The size of the Internet routing table has been continuously growing due to the increasing number of ASes and networks connected to the Internet.
  • BGP Route Filtering and Policy Control: BGP allows network administrators to apply route filtering and implement complex routing policies, enabling them to have control over how traffic is routed through their networks.

BGP plays a crucial role in ensuring the global connectivity and stability of the Internet. It is used by Internet Service Providers (ISPs) to exchange routing information, as well as by large enterprises with multiple locations and complex network topologies. The proper configuration and management of BGP are essential for maintaining a well-functioning and secure Internet infrastructure.