Library /sys$common/syshlp/ncphelp.hlb LOOP, CIRCUIT, PHYSICAL ADDRESS p-address  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Applies only to broadcast circuits.  Identifies the physical address
 of the destination node to be used for loopback testing of  the
 specified broadcast circuit.  You can use this parameter instead of
 the NODE node-id parameter.