What is BGP (Border Gateway Protocol)?

Border Gateway Protocol (BGP) is a standardized exterior gateway protocol used to exchange routing and reachability information between autonomous systems (ASes) on the Internet. An autonomous system is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the Internet. BGP is specifically designed for interdomain routing, allowing different autonomous systems to communicate and make informed decisions about the best paths to reach specific destination networks.

Here is a detailed technical explanation of BGP:

  1. Path Vector Protocol:
    • BGP is classified as a path vector protocol, meaning it uses a vector (list) of autonomous systems through which routing information has passed to reach a destination network.
    • Each BGP router maintains a table of network prefixes and associated path attributes.
  2. TCP-based Communication:
    • BGP uses TCP (Transmission Control Protocol) as its transport protocol. This ensures reliable and connection-oriented communication between BGP peers.
    • BGP operates on port 179.
  3. Neighbor Establishment:
    • BGP routers form neighbor relationships with each other to exchange routing information.
    • Neighbors are configured manually by specifying the IP address of the neighbor router.
  4. UPDATE Messages:
    • BGP routers exchange UPDATE messages to convey routing information.
    • The UPDATE message includes information about reachable network prefixes and the associated path attributes.
  5. Path Attributes:
    • BGP path attributes describe the characteristics of a route and influence the decision-making process for route selection.
    • Common attributes include AS Path, Next Hop, Origin, Local Preference, and others.
  6. AS Path:
    • The AS Path attribute lists the autonomous systems through which the route has traversed.
    • Helps in avoiding loops and making informed decisions about route selection.
  7. Route Decision Process:
    • BGP routers follow a decision process to select the best route for a particular destination.
    • Factors include the AS Path length, origin type, path origin, and various BGP policies configured by the network administrator.
  8. Route Aggregation:
    • BGP supports route aggregation, allowing multiple contiguous IP prefixes to be represented by a single, summarized route.
    • This helps in reducing the size of the BGP routing table.
  9. Route Filtering and Policy Control:
    • BGP provides extensive control mechanisms for filtering and controlling the advertisement and acceptance of routes.
    • Network administrators can implement policies to influence route selection based on various criteria.
  10. Route Flap Damping:
    • BGP routers implement a mechanism called route flap damping to minimize the impact of unstable routes (routes that frequently go up and down).
  11. Security Measures:
    • BGP security is a critical concern, and measures like BGP prefix filtering, prefix validation, and the Resource Public Key Infrastructure (RPKI) are employed to prevent malicious activities such as route hijacking.