BGP Path Selection Question

[scampbell]

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? 

[Georg Pauwen]

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

[scampbell]

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

[scampbell]

Anyone have any insight on these details? 

[Cisco Freak]

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.