BGP (Border Gateway Protocol) is an exterior gateway protocol that connects different autonomous systems (AS) on the internet. It uses a path vector algorithm to determine the best path for a particular destination network in terms of the shortest route. The path vector algorithm considers several criteria to identify the shortest route, such as the number of hops, the AS-path length, and the cost of transit.
BGP maintains a routing table that contains all the possible paths to reach the destination network. Each path is identified by a list of AS numbers that the path traverses. BGP exchanges routing information between different routers in the network to update their routing tables and ensure that all routers have the same view of the network topology.
When BGP receives multiple paths to a destination network, it selects the best path based on the following criteria:
1. Shortest AS-path length: BGP selects the path with the shortest AS-path, as it indicates the least number of hops to reach the destination network.
1. Shortest distance to the next hop: BGP selects the path with the shortest distance to the next hop router. This ensures that the packets take the shortest physical distance to reach the destination.
1. Lowest cost: BGP assigns a cost to each path based on various factors, such as bandwidth, delay, and administrative policies. BGP selects the path with the lowest cost.
1. MED (Multi-Exit Discriminator) value: If multiple paths have the same AS-path length, distance to the next hop, and cost, BGP uses the MED value to select the best path. The MED value is an optional attribute that indicates the preference for a particular path at the exit point of an AS.
By considering the above criteria, BGP ensures that the packets are routed through the shortest possible path to reach the destination network.
