Hello David
The “Oldest Path” attribute written out in full is the one that says: “When both paths are external, prefer the path that was received first (the oldest one).”
So this attribute is only valid for routes external to the local AS. Now your question comes down to this: Is it possible for this to be a tie? Well, this attribute is designed in such a way that it’s extremely unlikely to result in a tie. This is because it is based on the timing of when the route was received.
In a real-world scenario, it’s practically impossible for a router to receive two routes from two different peers at the exact same microsecond. Thus, this tiebreaker will almost always be able to determine a preference between routes, unless perhaps in the highly unlikely event of some form of synchronization or a bug.
So to answer your question, it is not impossible, but it is extremely unlikely that this tie breaker will be unable to resolve the issue. So in virtually all cases, for external paths, this will be the last attribute to be checked.
Well, the attributes are used to choose the best “path” not the best “peering”. So it doesn’t really matter what peerings the particular router has, but what is taken into account is if the path is external to the local AS. An iBGP router that has no eBGP peerings can still receive two paths to a particular external prefix, so the attribute is still examined. Does that make sense?
I hope this has been helpful!
Laz