Which BGP attribute is used first when determining the best path?
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When BGP (Border Gateway Protocol) receives multiple paths to the same destination, it needs to select the best path among them. BGP uses several attributes to determine the best path. Among these attributes, the first one that BGP uses to determine the best path is the "weight" attribute.
The weight attribute is a Cisco proprietary attribute and is used only locally within the router. When a router has multiple paths to the same destination, it assigns a weight value to each path, and the path with the highest weight value is preferred. The weight attribute is used first because it is the simplest and fastest way to determine the best path, and it does not require any complex calculations.
If two paths have the same weight value, BGP uses the following attributes in the given order to determine the best path:
Local Preference: The local preference is an attribute that is used to influence the inbound traffic from BGP peers. A higher local preference value means a higher preference for the path.
AS Path: The AS Path attribute is a list of all the Autonomous Systems (AS) that the path has traversed to reach the destination. BGP prefers the path with the shortest AS Path length.
Origin: The origin attribute indicates how BGP learned about the route. BGP prefers routes that are learned via internal BGP (iBGP) over those learned via external BGP (eBGP). If two paths are learned via iBGP, BGP prefers the one with the lowest origin value.
MED (Multi-Exit Discriminator): MED is an optional attribute that is used to influence the inbound traffic from different autonomous systems. BGP prefers the path with the lowest MED value.
IGP (Interior Gateway Protocol) Metric: The IGP metric is the metric of the Interior Gateway Protocol used within an autonomous system. BGP prefers the path with the lowest IGP metric value.
Router ID: The Router ID is the unique identifier of the BGP router. BGP prefers the path received from the BGP router with the lowest Router ID.
If all the above attributes are the same for multiple paths, BGP can load-balance traffic across all these paths.
In summary, BGP uses the weight attribute first to determine the best path, and if multiple paths have the same weight, it uses the other attributes in the given order to select the best path.