Seagate Fibre Channel Interface - Manual (Page 49)

Fibre Channel Interface Manual, Rev. D 35
In the parallel SCSI world, the higher the bus address, the higher the priority the device has. The opposite is
true with FC-AL. See Table 14.
During loop initialization, each NL_Port interested in participating on the loop is assigned one of the 126 AL_PA
values. If an NL_Port is offline because its node is powered off, the port is considered a non-participating
NL_Port.
One more reason exists for an L_Port being in non-participating mode; you can place more than 127 L_Ports
on a loop, and, if this occurs, loop initialization permits only the first 126 NL_Ports and the first FL_Port to
acquire a valid AL_PA. The remaining L_Ports must wait for a position to open up to obtain a valid AL_PA.
An L_Port can get a valid AL_PA in four ways:
1. Assigned by the fabric (if present)
2. Use the previously-assigned address
3. Hard assigned by the backpanel
4. Soft assigned and acquired through the loop initialization process
The most common method that disc drives use is to have the backpanel provide a hard assigned address. This
is similar to setting a SCSI ID on a drive, but with Fibre Channel, the ID is set on the backpanel rather than the
drive. If, for some reason, a duplicate address is encountered when hard-assigned addresses are processed,
the drive will revert to acquiring a soft-assigned AL_PA during initialization. This insures that every node gets a
unique AL_PA.
A more detailed explanation of loop initialization is provided in the following section.
8.2 Loop initialization
Loop initialization is the process used to obtain or verify AL_PAs. Loop initialization occurs at power-up, when
a new node is inserted in the loop, or for error recovery.
During loop initialization, each NL_Port discovers that it is connected in an arbitrated loop topology and that
there may be multiple NL_Ports on the arbitrated loop. The procedure used for this discovery is called the loop
port state machine (LPSM). The FC-2 protocol is also used.
Table 13: AL_PA addressing
Values (hex) Use
00 Reserved for the FL_Port (if present)
01-EF Contains 127 valid addresses
F0 Reserved for fairness
F1-F6 These values do not have neutral running disparity
F7-F8 Reserved for loop initialization
FB, FD, and FE Reserved for future use
FF Reserved to address all ports in broadcast mode
Table 14: AL_PA value priorities
AL_PA value Priority
00 Highest priority (assigned to the FL_Port if present)
01 Highest priority NL_Port (on a public loop)
EF Lowest priority NL_Port address
F0 Used only by the fairness algorithm and has no priority