BGP (Border Gateway Protocol) is an Inter-AS (Autonomous System) routing protocol.
• It obtains the subnet reachability information from neighboring AS.
• It also propagates the reachability information to all routers within an AS.
• The peers determine the routes to each other AS system.
• The BGP uses the attributes for routing the paths between the Autonomous Systems (AS’s). The two most important attributes are AS-PATH and NEXT-HOP.
• The routers use the AS-PATH attribute for detecting and preventing the looping advertisements. When the router finds its own AS number which is present in the list, the advertisement is rejected by the router.
Thus, detection of loops in paths can be detected in BGP protocol with the help of AS-PATH attribute.
• The advertisement passed for the prefix values contains the AS’s in the AS-PATH.
• When the value of prefix is passed into an AS, it adds the ASN (Autonomous System Number) to the AS-PATH attribute.
• The routers also uses the AS-PATH attribute in order to choose a path among the multiple paths with the same prefix.
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