HP Surestore 64 FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availabil - Page 67

Planning Considerations for Fibre Channel Topologies 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. A Loop Switch Logical Equivalent B H_Ports D1 D2 S1 D1 D2 S1 Figure 27: Shared Mode operation and logical equivalent Part (A) of Figure 27 shows device D1 connected to server S1 through a pair of H_Ports, and communicating at 1.0625 Gbps. Although the remaining switch H_Ports (six ports) and device D2 are unavailable for connection, frame traffic between device D1 and server S1 travels through a loop that consists of all eight H_Ports, device D1, device D2, and server S1. Each H_Port not participating in the communication pair and the NL_Port on device D2 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 D1 and server S1 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. SAN High Availability Planning Guide 67