BGP Path Selection Process


1. Next_Hop: The next hop of the BGP route has to be in the routing table else if it is unreachable, the route is ignored.
2. Pre-bestpath Cost: If the pre-bestpath cost attribute is present, choose the route with the lowest cost value, if they are the same, the lowest community.
3. Weight: Cisco proprietary, local significant attribute where the largest is prefered.
4. Loc_Pref: If the weights are the same, choose the path with the highest local preference.
5. Local Originated: Routes that were locally originated with network statement, aggregated or redistributed.
6. AS_PATH: Next compare the as-path length and prefer the route with the shortest AS_PATH length.
7. ORIGIN: Choose the route with the lowest origin type if the AS_PATH lengths are the same. IGP<EGP<INCOMPLETE
8. MED: If the origin types are the same, choose the route with the lowest MED value. This will only be compared for routes from the same AS or if bgp always-compare-med is enabled, for all routes.
9. EBGP/iBGP: Prefer EBGP routes over IBGP routes if the routes have the same MED value.
10. IGP: At this point if there are still multiple routes prefer the route with the shortest route to the NEXT_HOP. The IGP will have already determined the shortest path to the next-hop.
11. Cost: If the cost attribute is present not configured to be ignored, choose the lowest cost.
12. Multipath: If multipath is enabled, multiple paths that match up to this point will be installed.
13. Oldest: If multiple external routes remain, choose the oldest one, thus avoids propagating a flapping route. To overwrite this, this step can be ignored with bestpath compare router-id.
14. Router_ID: If multiple routes still exist, the BGP ROUTER_ID will be a tiebreaker. Choose the route advertised by the BGP peer with the lowest Router_ID. If RR present, the originator ID is used.
15. Cluster Length: Minimum RR cluster length is compared next.
16. Lowest Neighbor: Last, the path from the lowest neighbor address.

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About ccie4all
Hello, and welcome to the first post of my CCIE blog This blog has got one simple goal and that is to improve our skills in Cisco Networking field so we can become best engineers on a job market. Wordpress Blog https://ccie4all.wordpress.com/ information about the changes made to Gns3 BGP , MPLS and R&S CCIE labs. In order to access and download all provided materials and receive important updates from Gns3 BGP , MPLS and R&S CCIE labs under GNS3 tab in the main header please go ahead and subscribe to https://ccie4all.wordpress.com/ ! All other posts have not been affected and can be accessed at any given time. Enjoy ! Tom

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