HP Surestore 6.140 Tape Library - Manual (Page 192)

C-8 Appendix C
Introduction to Fibre Channel
Fibre Channel Technology
Addressing
Device Addresses on a Fibre Channel Loop
Each device on a Fibre Channel Arbitrated Loop (FC-AL) has a unique
address known as the Arbitrated Loop Physical Address (AL_PA). There
are 126 valid AL_PAs that correspond to the 127 devices allowed on a
Fibre Channel loop with values between 0-255 (not all values are valid).
The “address index” also represents the device’s address. Address
indexes are used on devices where external switch settings determine
the AL_PA. Each number between 0-126 corresponds to one physical
address.
Address Assignments
Soft addressing and hard addressing are used to assign a unique AL_PA
to each device. A device using soft addressing will take the first available
AL_PA as its own. If the device is later power-cycled, it might select a
different AL_PA. Although soft addressing has an easy loop setup, many
host systems (including HP-UX
™ and Windows NT ™ ) lack the ability to
track such dynamic address changes.
Hard addressing solves the problem of dynamically changing AL_PAs,
but requires more user intervention to set up. The user must select an
AL_PA for each device. When the device is connected to a loop, the device
will attempt to use the user-specified AL_PA. If the device is later
power-cycled, it will attempt to use the same AL_PA. This results in
more stable addressing over time.
Address Conflicts
When all devices use soft addressing, address conflicts will not occur.
However, if more than 127 devices are connected to the same loop, those
extra devices will fall into a non-participating state in which they cannot
communicate without reconfiguring the loop.
When using hard addressing, address conflicts may occur if more than
one device is trying to use the same AL_PA. When this occurs, one device
will take the specified AL_PA. The other will either take the first
available AL_PA or fall into a non-participating state, depending on the
hardware.