BGP Path Selection Oldest Path

BGP is a routing protocol that uses a particular list of attributes to determine the best path in its path selection.

One of the tie-breakers can be the oldest path.

This tie-breaker works like this:

When both paths are external, prefer the path that was received first (the oldest one).”

So this tie-breaker is only used for routes **external to the local AS.

An interesting question that arises is the following: Is it possible for this to be a tie? Is it possible that two paths have the exact same age? It's extremely unlikely. 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.

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 thing to check to decide the best path.

An important note here is that this tie-breaker, like all attributes is used to choose the best “path” not the best “peering”. So it doesn’t really matter if the BGP router in question has eBGP or iBGP peerings. The attribute is taken into account if the prefix in question 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 tie-breaker is still examined for pure iBGP routers.

https://networklessons.com/bgp/bgp-attributes-and-path-selection/