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Chapter 4 4-21
Library Administration
Configuring the Library for Fibre Channel (for firmware revisions
greater than 1.14)
Library Administration
Library Administration
Additional Fibre Channel Information
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.