BGP also said to be known as Border Gateway Protocol is an Inter-AS routing protocol.
• It obtains the subnet reachability information from neighboring AS.
• It also propagates the reachability information to all routers within an AS.
BGP route selection using AS-PATH:
The following is the points for the selection of loop-free route using the shortest AS-PATH length by the BGP router for routing.
• Using the AS-PATH length attribute, the BGP router always selects the loop-free route.
• The router may contain more than one route to any one prefix.
• In such scenario, the BGP router sequentially invokes some elimination rules to obtain one route.
• Among the elimination rules, one rule is based on choosing the route with the shortest AS-PATH. Hence the routes in BGP router are always loop-free with the shortest AS-PATH length.
• The path is formed by the connections between two combined AS. A BGP route is formed by the collection of the information of the path such that it is formed to a particular destination.
• The prefix along with the attributes is said to be known as a BGP route.
• To ensure the loop-free inter-domain routing, the information of the path is used by the BGP router.
The path chosen by a AS-path transfers the information to one neighbor in the traffic instead of another neighbor with the shorter AS distance.
Therefore, the BGP router always selects the loop-free route with the shortest AS-PATH length for routing.
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