BGP path selection criteria

Border Gateway Protocol (BGP) uses a set of criteria to determine the best path for routing traffic between different networks on the internet. Here’s a simplified breakdown of the BGP path selection criteria:

1. Highest Weight: This is a Cisco-specific attribute. The path with the highest weight is preferred. Weight is a local attribute and not propagated to other routers.

2. Highest Local Preference: Local preference is used to prefer an exit point from the AS (Autonomous System). The path with the highest local preference value is preferred.

3. Shortest AS Path: The path with the fewest number of AS hops is preferred. This metric counts the number of ASes the route has traversed.

4. Lowest Origin Type: BGP routes can have different origin types: IGP (Interior Gateway Protocol), EGP (Exterior Gateway Protocol), and INCOMPLETE. Routes with an origin type of IGP are preferred over EGP, and EGP is preferred over INCOMPLETE.

5. Lowest Multi-Exit Discriminator (MED): MED is used to convey to external neighbors the preferred path into an AS. A lower MED value is preferred.

6. eBGP over iBGP: Routes learned via eBGP (External BGP) are preferred over routes learned via iBGP (Internal BGP) due to the external nature of the routes.

7. Lowest IGP Metric to BGP Next Hop: This criterion evaluates the shortest path to the BGP next hop within the local AS. The route with the lowest IGP metric to the next hop is preferred.

8. Oldest Route (when other criteria are equal): If all other criteria are equal, BGP prefers the route that has been in the BGP table the longest, or the route with the oldest age.

9. Router ID: As a last resort, if all other criteria are equal, BGP might choose the path with the lowest router ID.