Identifies the local ifIndex of the interface through which this route's alternate next-hop should be reached. Admins (or the routing process provisioning this row) use it to specify exactly which outgoing interface the backup path uses, which matters when a device has multiple interfaces that could reach the same next-hop address. For a route with two possible egress interfaces to the same alternate next-hop IP, setting ipFrrAltIfIndex explicitly ensures fast-reroute uses the intended interface rather than an ambiguous one.