1 HV Installation and Operations Guide

Chapter 2 Fibre Channel Arbitrated Loop Addressing 35

Chapter 2

Fibre Channel Arbitrated Loop Addressing

On a Fibre Channel arbitrated loop (FC-AL), each device appears as an

Arbitrated Loop Physical Address (AL_PA). To obtain an AL_PA, two methods

called ‘soft’ and ‘hard’ addressing can be used by the bridge. The bridge can

be configured to use either method. For hard addressing, the user specifies the

AL_PA of the bridge.

Hard addressing is recommended for FC-AL environments where it is

important that the FC device addresses do not change. Device address

changes can affect the mapping represented by the host operating system to

the application. An example of such an environment would be a tape library

installation, where the application configuration requires fixed device

identification for proper operation. Hard addressing ensures that the device

identification to the application remains constant.

Hard Addressing

When acquiring a hard address, the bridge attempts to acquire the AL_PA

value that was specified by user configuration. If the desired address is not

available at loop initialization time, the bridge comes up on the FC loop using

an available soft address. This allows both the loop and the bridge to continue

to operate. This situation would occur when another device on the arbitrated

loop has acquired the same address as that configured on the bridge.

When connected to a Fibre Channel switch, the bridge is identified to the

switch as a unique device by the factory programmed World Wide Name

(WWN).

Soft Addressing

When acquiring a soft address, the bridge acquires the first available loop

address starting from address 01 and moving up the list of available AL_PAs

from 01 to EF. In this mode, the bridge automatically obtains an available

address and participates on the FC loop, as long as there is at least one

address available on the loop that is connected to the bridge. Fibre Channel

supports up to 126 devices on an arbitrated loop.