11-23-2016 11:50 AM - edited 03-05-2019 07:32 AM
I have included a screenshot with a logical BGP diagram and a physical diagram for reference. I have also included relevant BGP commands.
The question I have in this scenario is "why does the HQ router choose .11 as the best path for the default route?" As far as I see, all other path selection methods are equal but the router with the higher router-ID is being chosen over the lower router-ID. What am I missing?
11-23-2016 12:11 PM
Hello,
according to the BGP best path selection algorithm:
When both paths are external, prefer the path that was received first (the oldest one).
This seems to apply in your case.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html
11-23-2016 03:21 PM
I had the same thought about the route age. That said, I want to be fully clear on how this works. Since there are additional path selection metrics, I am curious how these would ever be used. If you have my situation with 2 BGP peers sending the same exact route with all attributes equal, when would you ever use the 11-13 attributes?
Additionally, and this may sound silly, but I want to be sure it's the local router's perspective of the prefix age and not an advertised value from the upstream device? If it is the local router's perspective, I imagine you would never get in a situation where the age is EXACTLY the same from all neighors (do we know how granular this metric is?).
Thanks!
-Steven
11-28-2016 06:46 AM
Anyone have any insight on these details?
11-28-2016 09:23 AM
I guess in this case the route that was received first was chosen as the best patch.
The selection rules says it will use 11-13 when any of these criteria are set:
Skip this step(step 10) if any of these items is true:
You have enabled the bgp best path compare-routerid command.
The router ID is the same for multiple paths because the routes were received from the same router.
There is no current best path.
The current best path can be lost when, for example, the neighbor that offers the path goes down.
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