Best-Path Selection · Lesson 02 of 02
Hot potato and cold potato
When two exits are otherwise equal, the IGP metric to the next hop breaks the tie — and that choice has a name and a consequence.
Written lesson
Nearest exit wins
Deep in the best-path list sits "lowest IGP metric to the BGP next hop." When two paths survive to that point, BGP picks the exit that is cheapest to reach inside your own AS — it hands the traffic off as soon as possible. This is hot-potato routing: get rid of the packet quickly, let the neighbour AS carry it the long way. It minimises the load on your own backbone.
Written lesson
Cold potato is a policy choice
The opposite, cold-potato routing, keeps traffic on your own network longer to hand it off closer to the destination — usually to honour a peering agreement or deliver a better experience. BGP does not do this by default; you engineer it with LOCAL_PREF or MED so that the further exit wins despite a higher IGP metric. Recognising which behaviour a design wants is a common exam and real-world distinction.
MED is the cold-potato tool between AS's
A neighbour that sets MED on the routes it sends you is asking you to do cold-potato on its behalf — to carry traffic to the exit it prefers. You honour it only if your policy (and the higher steps) let MED decide.