BGP - Why is MED non-transitive

MED is a BGP attribute that is non-transitive by design, and there is a reason for this. First of all, MED is often meant to dictate preferences based on factors like link bandwidth or congestion that aren’t necessarily relevant to ASes that may be multiple hops away. So the MED attribute is typically used to implement local policy. For example, one might use it to guide traffic toward a cheaper but slower link for certain kinds of non-critical traffic. Since such policies are typically local to an AS or a set of directly connected ASes, it makes sense for MED to be non-transitive.

For the purposes of comparison, consider AS Path Prepending which is inherently transitive because the AS Path attribute provides critical information for routers to make decisions about the best path to a destination based on the number of ASes that must be passed through.