Administering Extended Fabrics; About Extended Link Buffer Allocation; Fabric Considerations - HP AA979A - StorageWorks SAN Switch 2/8V Administrator's Manual

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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

About extended link buffer allocation

As the distance between switches and the link speed increase, additional
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- 1 6 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- 1 6 port blades, each
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
HP StorageWorks SAN Switch 4/32, 4
Switch, 400 MP Router and (FC4- 1 6, 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- 1 6, 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
features" on page 33.
port group
Directors" on page 109 for details about port blade nomenclature.
buffer-to-buffer credits
contains four ports and uses a common pool of credits.
/256 SAN Director,
Fabric OS 5.2.x administrator guide 325
are
4/64 SAN