HP AA979A - StorageWorks SAN Switch 2/8V, Administrator'S Manual, Page: 340 / 465

352 Administering Extended Fabrics
See Configuring Directors , page 203 for details about port blade nomenclature.
For the following switches, buffer credits are used by all ports on the chip. Buffer-limited port technology
allows all ports to remain operational, even when extended links are in use.
• SAN Switch 4/32
• 4/64 SAN Switch
• 4/32B SAN Switch
• 400 Multi-protocol Router
• Brocade 7600
• 4/256 SAN Director using the following port blades:
• FC4-16
• FC4-16IP
• FC4-32
• FC4-48
• FR4-18i
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 than being disabled due to a lack of
buffers. The 8 buffer minimum should allow 4 Gbps 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.
• VC translation link initialization (an option of the
portCfgLongDistance
command) is enabled by
default for long-distance links. To avoid inconsistency in the fabric, make sure that this value is enabled
on both ends of the link.
Choosing an extended ISL mode
The following extended ISL modes are supported:
• L0—Level 0 static mode; the default.
• LE—Level E static mode; supports links beyond 5 km.
• LD—Dynamic mod;: uses automatic distance detection for a user-specified distance.
• LS—Static long distance mode; uses the user-specified link distance.
The following extended ISL modes are supported, but cannot be configured on a switch running Fabric OS
version 5.2.0 and later.
• L0.5—Level 0.5 static mode (designated LM when listed with the
portcfgshow
command).
• L1—Level 1 static mode.
• L2—Level 2 static mode.
For dynamic long distance links, you can approximate the number of buffer credits using the following
formula:
Buffer credits = [(distance in km) * (data rate) * 1000] / 2112
The data rate is 1.0625 for 1 Gbps, 2.125 for 2 Gbps, 4.25 for 4 Gbps, 10.2 for 10 Gbps.