This is to prevent protocols like OSPF from routing packets to this address across a serial interface when this system could be functioning as a host . When the status of an interface changes, GateD notifies all the protocols, which take the appropriate action. GateD assumes that interfaces which are not marked UP do not exist. While this might not be the most correct action, it is the way things currently work. GateD ignores any interfaces that have invalid data for the local , remote or broadcast addresses or the subnet mask .
Invalid data includes zeros in any field.