BGP Explained – How Does Border Gateway Protocol Work?

BGP ensures that data packets travel fast across the internet using the information found in IP addresses. The protocol also helps create a global routing table.

It also advertises routing and reachability information to neighboring routers, enabling them to select the best route for data packets. This functionality is called external BGP.

Autonomous System (AS)

BGP is a standardized exterior gateway protocol that exchanges routing and reachability information among autonomous systems on the internet. Each independent system is defined by a 16-bit autonomous system number, or ASN, assigned to network routers within the autonomous system. Autonomous systems are often managed by single entities such as educational institutions, governmental organizations, or, most commonly, internet service providers (ISPs).

Each AS carries a unique ASN, which identifies the network’s location on the internet. Its routers collect route information from neighboring ASes through peering sessions, which involve connecting with other routers over TCP/IP connections. Once they receive route information, the routers in each autonomous system “advertise” that route to their neighbors. The other routers then use the information to route data packets to the destination network.

BGP routes data across the internet using a system of weight attributes that assign priority to various network paths. Similarly to a GPS application, the BGP system considers road conditions, the cost of using a toll road, and other factors when deciding which direction to take.

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Network Prefixes

When you open a web browser and want to visit a website that isn’t on your local network, the packets of data travel from your router to the router of your internet service provider, then to the router of your provider’s provider, and so on until it reaches its final destination. So what is BGP in networking, and how does it work? BGP is the routing protocol ensuring your data packets arrive quickly and efficiently. It’s been compared to the postal service for the global internet.

To do its job, BGP advertises route information with its neighbors. This information is a network prefix containing 32 or 128 bits representing one IP address. Each router decides how to forward the packets based on this information, then sends them to other networks that use BGP. These different networks are referred to as peers.

Each peer maintains a table with all the network prefixes it knows about. As each peer receives announcements from others, it adds those network prefixes to its plain and then shares that information with its neighbors.

Because this process is done manually, it’s prone to errors and malicious attacks. That’s why some networks are moving away from BGP for external peering and towards more secure alternatives.


Routers interpret the data packets and find out how to get them where they need to go on the internet. They also update their routing tables with information from other routers in the network. The internet is broken into hundreds of thousands of networks called autonomous systems, or ASes. BGP is the mechanism that makes these large networks work together by sharing routing information.

One analogy is to think of the internet as a city, where an AS is like a post office branch and a network prefix is like a particular street or house. BGP enables data to travel from one AS to another by selecting the best route. This is a complex task. Routers use a variety of factors to determine ways, including business considerations and technical characteristics such as hop count.

For BGP to work, routers must trust the routes other routers advertise.

BGP provides several benefits for the internet, such as facilitating peering between different networks and ASes, conserving network bandwidth, supporting Classless Inter-Domain Routing (CIDR), and allowing network administrators to manage routing information. It also supports the exchange of negative reachability announcements and verifies that peers and the network connection are functioning properly.


BGP enables networks to interact with each other and ensures speedy data routing. Like the mail system, it uses address-like identifiers to route data to its intended destination, whether that destination is in another city, state, country, or continent. To do so, BGP advertises a set of network paths to other routers on the internet.

BGP also helps guarantee network stability by responding to no longer available routes. In addition, it adheres to rules and policies set by administrators that allow them to customize and control routing decisions.

To peer, a network must have a public Autonomous System Number assigned by a Regional Internet Registry and (at least) a block of IP addresses. The most common way to peer is through an Internet Exchange Point. This is where multiple networks connect to exchange traffic for free through a single connection.

A network must have a BGP session with the provider running the IXP and be willing to share its routing information to be present at an Internet Exchange Point. A network can also peer directly with other networks by connecting to a route server.