Networking - Border Gateway Protocol (BGP)

Border Gateway Protocol (BGP) is a routing protocol used to exchange routing information between different networks on the internet. It is mainly used to connect large networks called Autonomous Systems (AS). An Autonomous System is a collection of networks under the control of a single organization, such as an internet service provider, a large company, or a university.

BGP is known as an Exterior Gateway Protocol because it works between different autonomous systems, unlike interior routing protocols such as RIP or OSPF that operate inside a single network.

Each Autonomous System is given a unique number called an Autonomous System Number (ASN). When routers use BGP, they exchange information about which networks they can reach and the path that data must follow to reach those networks. This path information helps routers choose the best route to send data across the internet.

BGP works using a path vector mechanism. Instead of only calculating the shortest path, BGP keeps track of the entire route that data packets will travel through different autonomous systems. This helps prevent routing loops and allows network administrators to control how traffic flows between networks.

BGP routers communicate with each other using TCP on port 179. Because it uses TCP, BGP ensures reliable communication when exchanging routing information. Routers that exchange BGP information are called BGP peers or neighbors.

There are two main types of BGP:

External BGP (eBGP)
External BGP is used to exchange routing information between routers that belong to different autonomous systems. This is commonly used between internet service providers and large networks connected to the internet.

Internal BGP (iBGP)
Internal BGP is used within the same autonomous system to distribute routing information learned from external BGP routers to other routers inside the organization.

BGP is considered the backbone routing protocol of the internet because it manages how data travels between thousands of networks around the world. It allows organizations to apply routing policies, control traffic paths, and maintain stable internet connectivity.

Although BGP is powerful, it can be complex to configure and manage. Proper configuration is important because mistakes in BGP routing can affect large parts of the internet. For this reason, network engineers carefully design and monitor BGP networks.