HP StorageWorks 2/140 FW 08.01.00 McDATA Products in a SAN Environment Plannin - Page 106

Planning Considerations for Fibre Channel Topologies 3 • Bandwidth - ISL connections can be used to increase the total bandwidth available for data transfer between two directors or switches in a fabric. Increasing the number of ISLs between elements increases the corresponding total ISL bandwidth but decreases the number of port connections available to devices. • Load balancing - Planning consideration must be given to the amount of data traffic expected through the fabric or through a fabric element. Because the fabric automatically determines and uses the least cost (shortest) data transfer path between source and destination ports, some ISL connections may provide insufficient bandwidth while the bandwidth of other connections is unused. To optimize bandwidth use and automatically provide dynamic load balancing across multiple ISLs, consider purchasing and enabling the OpenTrunking feature key. For information about the feature and managing multiple ISLs, refer to OpenTrunking and General Fabric Design Considerations. • Preferred path - Preferred path is an option that allows a user to configure an ISL data path between multiple fabric elements (directors and fabric switches) by configuring the source and exit ports of the origination fabric element and the Domain_ID of the destination fabric element. Each participating director or switch must be configured as part of a desired path. For information about the feature, refer to Preferred Path. ATTENTION ! Activating a preferred path can result in receipt of out-oforder frames if the preferred path differs from the current path, if input and output (I/O) is active from the source port, and if congestion is present on the current path. In general, Fibre Channel frames are routed through fabric paths that implement the minimum possible hop count. For example, in Figure 3-11, all traffic between devices connected to director S1 and director S2 communicate directly through ISLs that connect the directors (one hop). No traffic is routed through director S3 (two hops). If heavy traffic between the devices is expected, multiple ISL connections should be configured to create multiple minimum-hop paths. With multiple paths, the directors balance the load by assigning traffic from different ports to different minimum-hop paths (ISLs). 3-22 McDATA Products in a SAN Environment - Planning Manual