Brocade FICON Administrator’s Guide (Supporting Fabric OS v7.3.0) - Manual (Page 33)

The following figure represents components of link and FC addresses that are explained in this section.
FIGURE 15 Link and FC address components
Domain ID
Although you enter the domain ID in decimal format when configuring a switch, it is represented in
hexadecimal formal in the FC address.
For single-byte addressing, the domain area returned from the switch where the channel logs in is used
for the FC address. Therefore, the channel and control unit must be in the same logical switch. With
two-byte link addressing, the most significant byte of the link address is used for the domain area of the
FC address. Once two-byte link addressing is defined for a channel, all link addressing for that channel
must use two-byte link addressing.
Port area
The port area address is the single-byte link address or the least significant byte of a two-byte link
address. The link address is entered in the HCD in hexadecimal format and is represented in
hexadecimal in the FC address.
ALPA
The Arbitrated Loop Physical Address (ALPA) was originally used in fibre channel for loop devices.
Currently, the ALPA is used for N_Port ID Virtualization (NPIV), which allows multiple WWNs to log in to
the same switch port. The ALPA determines the logical entity to which frames belong. This is how
virtual servers using zLinux or zVM can share the same channel. Because the DCX and DCX 8510-8
Backbones can have up to 512 ports, the upper two bits of the ALPA are used in certain addressing
modes.
FICON does not use the ALPA byte. However, the ALPA is a required byte in the FC address. The
channel completes the FC address for a control unit link address by acquiring the ALPA that the switch
returned to the channel when the channel logged in. This is why the ALPA must be the same for both
the channel port and the control unit port. This is also the reason why 10-bit addressing mode cannot be
used for FICON (refer to
Addressing modes on page 44).
Domain ID
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