A route to the same destination is received from two different BGP sources.
Which criterion is considered first to determine the active BGP route?
Click on the arrows to vote for the correct answer
A. B. C. D.D
When a router receives multiple BGP routes to the same destination from different sources, it needs to choose the best route to use as the active route. To make this decision, the router compares the attributes of each BGP route and selects the one with the best combination of attributes.
The criteria used to determine the active BGP route are known as BGP route selection algorithm or BGP best path selection algorithm. According to the algorithm, the criteria are evaluated in the following order:
Highest local preference value: The local preference attribute is used to indicate the preferred path for inbound traffic. When a router receives multiple BGP routes to the same destination, it selects the route with the highest local preference value as the best path. This attribute is configured by the local AS and is not propagated to other AS.
Shortest AS-path length: The AS-path attribute is used to show the sequence of AS that the route has passed through before reaching the current router. The BGP route with the shortest AS-path length is preferred, as it indicates a more direct path to the destination.
Origin code: The origin code attribute is used to indicate the origin of the BGP route. Routes that are learned through an internal source (i.e., the same AS) are preferred over routes that are learned through external sources (i.e., a different AS).
Lowest multi-exit discriminator (MED) value: The MED attribute is used to indicate the preferred exit point for outbound traffic. When a router receives multiple BGP routes to the same destination from the same AS, it selects the route with the lowest MED value as the best path.
eBGP over iBGP: When a router receives multiple BGP routes to the same destination, it prefers the route learned through an eBGP session over the route learned through an iBGP session.
Lowest IGP metric to next-hop: When a router receives multiple BGP routes to the same destination, it selects the route with the lowest IGP metric (e.g., OSPF or IS-IS) to the next-hop as the best path.
Prefer the oldest route: When all other criteria are equal, the router prefers the route that has been in the BGP table the longest.
The cluster length, as mentioned in option A, is not a criterion used in the BGP best path selection algorithm. It is a feature used in BGP route reflection to avoid routing loops and reduce the number of BGP updates.
The RID (Router ID), as mentioned in option C, is not a criterion used in the BGP best path selection algorithm. It is a unique identifier assigned to each BGP router in the AS and is used to break ties when other criteria are equal.