nwDecIVProtoIfSnooping is a read-write switch that, when enabled, puts the DECnet Phase IV distance-vector-based routing protocol into passive mode on this router port, meaning it still receives route information and builds a route table but does not advertise that table or send updates out this specific port. An admin enables this on a port where they want the router to learn routes without actively participating in the routing exchange, such as a monitoring tap or a segment where advertising would be unwanted. For example, enabling nwDecIVProtoIfSnooping on a port connected to a passive monitoring segment lets the router build a DECnet route table from what it overhears there without ever sending its own updates out that port.