BGP Prefer eBGP over iBGP

Hello Anoop

We must keep in mind the order of operations with which these processes take place.

The BGP router will first consider all of the possible routes that have been learned via BGP. These routes are all listed in the BGP table. From this, using the best path selection algorithm, BGP will choose the best one. Once this is done, that best path will then become a candidate to be installed in the routing table.

This candidate route will then be compared with routes to the same destination that have been learned from other routing protocols (if they exist) using the Administrative Distance. The candidate route with the lowest AD is chosen.

The note that Rene mentions in the lesson, is simply saying that the choice of eBGP over iBGP within the BGP process and the BGP table should not be confused with the process of comparing routes for the routing table using the AD.

Now having said that, we know that eBGP routes have a default AD of 20 and iBGP routes have a default AD of 200. Because the BGP best path algorithm always prefers eBGP over iBGP, you will never have a BGP router choose both an iBGP and eBGP route to the same destination as the best paths. Therefore, you will never need to compare the AD of an iBGP and eBGP route on the same router. BGP itself chooses the best path before you even get to the comparison of ADs. Does that make sense?

I hope this has been helpful!