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BGP protocol: BGP (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.

• 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 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 following are the ways for determining the use of AS-PATH in BGP:

• The routers use the AS-PATH attribute for detecting and preventing the looping advertisements. When the router finds the AS is already present in the list, the advertisement is rejected by the router.

• The routers also uses the AS-PATH attribute in order to choose among the multiple paths with the same prefix.


• The NEXT-HOP is the router interface that initiates the AS-PATH.

• The attribute is necessary in providing the critical link between the Inter-AS routing and Intra-AS routing protocols.

The following are the ways for determining the use of NEXT-HOP in BGP:

• The NEXT-HOP attribute indicates the IP address of the first router along the advertised path that is the advertisement received by the external AS to a given prefix.

• When the forwarding table for the first router is configured, the router uses the NEXT-HOP attribute.

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