This integer matches the index of the interface a route was learned from, for policies that need to filter routes by ingress interface; it should not be used for protocols that are not interface-specific such as IBGP, and -1 means the attribute is unused. An admin uses it to apply different route treatment depending on which physical or virtual interface a route arrived on. For example, setting csRtPolicyTermFromIntfIndex to the index of a specific frame-relay sub-interface matches only routes learned over that link.