User's Guide

Appendix D Fibre Channel Topology
Quantum DX-Series Software Interface Guide 299
Arbitrated Loop devices still use the three byte address identifiers, but
also use an Arbitrated Loop Physical Address (AL_PA). AL_PAs are one
byte values dynamically assigned each time the Loop is initialized. Once
the Loop is initialized and (hopefully) each L_Port has selected an
AL_PA, public NL_Ports will attempt Fabric Login. If there is an FL_Port,
the Fabric will assign the upper two bytes of the NL_Port's address
identifier and usually allow the low byte to be the NL_Port's AL_PA. (If
not, the Loop will need to be re-initialized so the NL_Port can select the
Fabric assigned AL_PA). If no Fabric exists or if an NL_Port is a private
NL_Port (does not login with the Fabric), the upper two bytes of the
address identifier will remain '0000', and the lower byte will simply be
the NL_Port's AL_PA.
But there still needs to be a way of uniquely identifying a port - even for
much of the above initialization to take place. This is accomplished using
Name_Identifiers, a fixed 64-bit value. Name_Identifiers are used to
uniquely identify nodes (Node_Name), a Port (Port_Name), and a Fabric
(Fabric_Name). Name Identifiers are not used to route frames, but are
used in mapping to upper layer protocols.