FW V06.XX/HAFM SW V08.02.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DD-TE, July 2004)

Planning Considerations for Fibre Channel Topologies
63SAN High Availability Planning Guide
Characteristics of Arbitrated Loop Operation
When implementing Fibre Channel arbitrated loop topology, consideration must
be given to switch operating mode, device connectivity, and loop configuration.
This section describes the characteristics of arbitrated loop operation, including:
■ Shared Mode Versus Switched Mode
■ Public Versus Private Devices
■ Public Versus Private Loops
Shared Mode Versus Switched Mode
Arbitrated loop switches operate in shared or switched mode as follows:
■ Shared mode — When set to shared mode, the switch acts as a hub that
implements arbitrated loop topology (although the loop has the physical
appearance of a star configuration). When a loop circuit is initialized and
established, arbitration protocol ensures that only one device attached to a hub
port (H_Port) owns the loop at a time.
The port establishes communication with another device attached to an
H_Port (or the B_Port), and half-duplex or full-duplex operation (the default
is half duplex) allows the devices to transmit or receive frames at 1.0625
Gbps. During frame transmission between these devices, the full bandwidth
of the switch is used and no other H_Ports or devices are available for
connection. When frame transmission completes, the loop circuit closes and
other devices are able to contend for operation (using standard loop
arbitration).
Shared mode operation is illustrated in Figure 21. Part A shows a server S
2
connected to device D
2
and communicating at 1.0625 Gbps. The B_Port and
six remaining H_Ports are inactive. Subsequently, part B shows public device
D
1
connected to fabric-attached server S
1
, also communicating at 1.0625
Gbps (through the B_Port). The seven remaining H_Ports are inactive.
Note: A Director 2/64 is shown in Figure 21 and other figures as an example. Any
HP director or edge switch can be used.