FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DC-TE, June 2003)

Planning Considerations for Fibre Channel Topologies

67SAN High Availability Planning Guide

Shared Mode Operation

When set to shared mode, a loop switch implements standard Fibre Channel

arbitrated loop topology, and distributes the frame routing function through each

loop port. Shared mode operation and its simplified logical equivalent are

illustrated in Figure 27.

Figure 27: Shared Mode operation and logical equivalent

Part (A) of Figure 27 shows device D

connected to server S

through a pair of

H_Ports, and communicating at 1.0625 Gbps. Although the remaining switch

H_Ports (six ports) and device D

are unavailable for connection, frame traffic

between device D

and server S

travels through a loop that consists of all eight

H_Ports, device D

, device D

, and server S

Each H_Port not participating in the

communication pair and the NL_Port on device D

provide a repeater function

that allows frames to pass around the loop at the full switch bandwidth.

Part (B) of Figure 27 shows the logical equivalent of this arbitrated loop. When

frame transmission between device D

and server S

completes, the loop circuit

closes and other ports attached to initiating devices arbitrate for loop access.

When operating in shared mode, the switch is a serially reusable resource that

provides service access to all ports on the loop. Access is granted by successful

arbitration. When arbitration is won by a device, the loop is busy and other

arbitrating devices must wait for loop access.

Device attachment and loop construction are not limited to the eight switch

H_Ports. Through the use of cascaded unmanaged hubs, the Fibre Channel

architectural limit of 125 FC-AL devices can attach to the switch. For example,

Figure 28 shows a private loop composed of a loop switch, 20 FC-AL devices, and

two unmanaged hubs.

H_Ports

Logical Equivalent

Loop Switch