In BGP, which attribute is evaluated first when selecting the best path?

In BGP (Border Gateway Protocol), the weight attribute is evaluated first when selecting the best path. Weight is a Cisco-specific attribute that is used to influence the path selection process significantly. It is a local attribute that can influence the routing decisions on a router, and it is not propagated to other routers.

When multiple paths to a destination are available, the router considers the path with the highest weight value as the preferred path. This means that if one path has a weight of 200 and another has a weight of 100, the path with the weight of 200 will be favored, regardless of other attributes such as AS path length or local preference.

This makes the weight attribute a powerful and effective tool for network administrators who want to control routing decisions on their routers, particularly in scenarios where specific paths should be prioritized based on local policies or preferences. The subsequent evaluation steps in BGP path selection, such as local preference, AS path length, and origin code, occur only after the weight attribute has been taken into consideration.