HP StorageWorks 2/16V HP StorageWorks Fabric OS 5.2.x administrator guide (569 - Page 325

18 Administering Extended Fabrics This chapter contains procedures for using the Extended Fabrics licensed feature, which extends the distance that interswitch links (ISLs) can reach over a dark fiber or DWM connection. The Extended Fabrics feature is not used over FCIP connections over IP WANs. To use extended ISL modes, you must first install the Extended Fabrics license. For details on obtaining and installing licensed features, refer to "Maintaining licensed software features" on page 33. About extended link buffer allocation As the distance between switches and the link speed increase, additional buffer-to-buffer credits are required to maintain maximum performance. The number of credits reserved for a port depends on the switch model and on the extended ISL mode for which it is configured. SAN Switch 2/8V, SAN Switch 2/16V, SAN Switch 2/32, SAN Director 2/128, and 4/256 SAN Director (FC2-16 port blades) For the SAN Switch 2/8V, SAN Switch 2/16V, SAN Switch 2/32, SAN Director 2/128, and 4/256 SAN Director using FC2-16 port blades, each port group contains four ports and uses a common pool of credits. Because the number of credits available for use within each port group is limited, configuring ports for extended links on these models might cause other ports to become disabled if there are not enough buffer credits available; for example: • If two 2-Gbit/second ports in a group are configured for L1 mode, each will be allocated enough buffer-to-buffer credits to cause the other two ports in the group to become disabled. • A port connected to a device that is in loopback mode might become disabled for lack of buffers if another port in that group is set to L2 mode. Refer to "Configuring Directors" on page 109 for details about port blade nomenclature. HP StorageWorks SAN Switch 4/32, 4/256 SAN Director, 4/64 SAN Switch, 400 MP Router and (FC4-16, FC4-32 and B-Series MP Router port blades) For the SAN Switch 4/32, 4/256 SAN Director, 4/64 SAN Switch, 400 MP Router using FC4-16, FC4-32, or B-Series MP Router port blades, buffer credits are used by all ports on chip. Buffer-limited port technology allows all ports to remain operational, even when extended links are in use. A buffer-limited port can come online with a minimum of 8 buffer credits, which could be less than its configuration specifies, allowing the port to continue to operate rather being disabled due to a lack of buffers. The 8 buffer minimum should allow 4 Gbit/sec speeds over distances within most data centers. Buffer-limited operation is supported for the L0 and LD extended ISL modes only, and is persistent across reboots, switch disabling and enabling, and port disabling and enabling. Fabric considerations Consider these items that affect the fabric when you configure extended ISLs: • Balance the number of long-distance ISL connections and core-to-edge ISL connections within a switch. Configuring long-distance ISLs between core and edge switches is possible, but is not a recommended practice. • Starting with Fabric OS v4.4.0, VC translation link initialization (an option of the portCfgLongDistance command) is enabled by default for long-distance links. For previous Fabric OS versions that support this option, it was disabled by default. To avoid inconsistency in the fabric, make sure that this value is enabled on both ends of the link. To connect to switches running Fabric OS Fabric OS 5.2.x administrator guide 325