BGP - oldest path attribute

BGP is a routing protocol that uses a particular list of attributes of specific prefixes to determine the best path. One of those attributes is known as the "oldest path" attribute.

This attribute, when described in full, 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.

An interesting question that arises is the following: 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.

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.

An important note here is that this attribute, 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 attribute is still examined for pure iBGP routers.