By default, which BGP attribute is only compared when two route advertisements are received from the same neighboring AS?
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A. B. C. D.A
The correct answer is B. AS-Path.
BGP (Border Gateway Protocol) is an inter-AS routing protocol that is used to exchange routing information between different Autonomous Systems (ASes) on the Internet. BGP uses various attributes to make routing decisions, and these attributes are included in the BGP updates that are exchanged between BGP routers.
When a BGP router receives multiple route advertisements for the same destination prefix from different neighboring ASes, it must compare these advertisements to determine the best path to that prefix. By default, BGP compares the attributes in the following order:
AS-Path: The AS-Path attribute is a sequence of AS numbers that the route has traversed on its way to the advertising router. BGP prefers the route with the shortest AS-Path length, as this indicates the fewest number of AS hops to reach the destination.
Origin: The Origin attribute indicates the origin of the route. BGP prefers routes with the lowest origin value, where IGP (Interior Gateway Protocol) is lower than EGP (Exterior Gateway Protocol), which is lower than Incomplete.
MED (Multi-Exit Discriminator): The MED attribute is used to indicate the preferred exit point for traffic leaving the AS. BGP prefers routes with the lowest MED value, as this indicates the most preferred exit point.
Next Hop: The Next Hop attribute is the IP address of the next-hop router that should be used to reach the destination prefix. BGP requires that the next-hop IP address be reachable, and it prefers routes with a next-hop that is directly connected to the advertising router.
Local Preference: The Local Preference attribute is used to indicate the routing policy of the local AS. BGP prefers routes with the highest Local Preference value.
Weight: The Weight attribute is a Cisco-specific attribute that is used to influence the local routing policy of a router. BGP prefers routes with the highest Weight value.
Communities: The Communities attribute is a set of tags that are attached to a route advertisement to indicate additional information about the route. BGP does not use the Communities attribute to make routing decisions, but it can be used to implement routing policies within an AS.
In the given scenario, the only attribute that is compared when two route advertisements are received from the same neighboring AS is the AS-Path attribute. This means that if a BGP router receives two route advertisements for the same prefix from the same neighboring AS, it will prefer the route with the shortest AS-Path length, regardless of any other attribute values.
Therefore, the correct answer is B. AS-Path.