Border Gateway Protocol (BGP) is a protocol used for routing traffic between autonomous systems (AS) on the internet. BGP is crucial for the internet to function as it ensures that data packets take the most efficient path through various networks to their destination. This article will provide a comprehensive guide to BGP basics.
AS and BGP Speakers
Autonomous systems are networks that are under a single administrative control and use a common routing policy. They typically range from small networks to large networks operated by internet service providers (ISPs). AS is identified with an autonomous system number (ASN). BGP speakers are routers that connect to other ASs, exchanging routing information so that they can communicate with each other.
BGP Peering
BGP peering is the exchange of routing information between two BGP speakers that belong to different ASs. It is the core of the BGP protocol. BGP peers establish a TCP connection between themselves, and once established, BGP peers exchange BGP routing information. Peering can either be internal or external.
Internal BGP peering happens within the same AS and enables routing between routers within the same AS. External BGP peering is between routers in different ASs and enables routing between ASs.
BGP Route Selection
BGP uses a sophisticated algorithm to determine the best route for data to travel between ASs. When a BGP speaker receives routing updates, it compares the information received with its current routing table and selects the best route based on a set of rules. Here are some of the rules that BGP uses to select the best path:
1. Rule of Local Preference: BGP considers routing information affecting the same AS, and BGP speaker chooses the route with the highest local preference.
2. Rule of Shortest AS Path: BGP chooses the route with the shortest AS path.
3. Rule of Origin Type: BGP prefers routes with the lowest origin type, which includes IGP (Interior Gateway Protocol) and EGP (Exterior Gateway Protocol).
4. Rule of Multi-Exit Discriminator: BGP uses MED values assigned to the path to determine the best one to use.
5. Rule of Highest IP address: BGP selects the route with the highest IP address.
BGP Configuration
To configure BGP, there are several steps to follow. First, you need to create an autonomous system by obtaining an autonomous system number, which is a 16-bit number that identifies your network on the internet. Then, configure BGP on routers in the AS to establish peering and exchange routing information.
BGP uses router ID, which is a 32-bit number assigned to a router interface, to identify routers in an AS. It is important to ensure that the router ID is unique across the network.
To configure BGP, you also need to decide on the policies that determine how packets are routed. This includes setting up filters and route maps to control the flow of packets based on source, destination, or other attributes.
Conclusion
BGP is an essential protocol for internet routing because it enables efficient routing of packets between different autonomous systems. Understanding BGP basics is crucial for anyone who manages a network or wants to understand how routing works on the internet. This article should provide a comprehensive guide to BGP basics and its role in network routing.