Administrator Guide

588 Fabric OS Administrator’s Guide
53-1002920-02
Extended Fabrics device limitations
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• Optimized switch buffering
When Extended Fabrics is installed on gateway switches (with E_Port connectivity from one
switch to another), the ISLs (E_Ports) are configured with a large pool of buffer credits. The
enhanced switch buffers help ensure that data transfer can occur at near-full bandwidth to use
the connection over the extended links efficiently. This efficiency ensures the highest possible
performance on ISLs.
Extended Fabrics device limitations
Brocade recommends that you do not use the FC8-64 port blade for long distance because of its
limited buffers. This blade does not support long-wavelength (LWL) fiber optics and supports
limited distance. However, you can use the portCfgLongDistance command to reserve frame
buffers for the ports intended to be used in long-distance mode through DWDM.
There is a limited number of reserved buffers used for long distance for each blade. If some ports
are configured in long-distance mode and have buffers reserved for them, insufficient buffers may
remain for the other ports. In this case, some of the remaining ports may come up in degraded
mode.
Long-distance link modes
Use the portCfgLongDistance command to support long-distance links and to allocate sufficient
numbers of full-size frame buffers on a specific port. Changes made by this command are
persistent across switch reboots and power cycles.
The portCfgLongDistance command supports the following long-distance link modes:
• Normal Mode (LO) — L0 is the normal (default) mode for an E_Port. It configures the E_Port as
a standard (not long-distance) ISL. A total of 20 full-size frame buffers are reserved for data
traffic, regardless of the E_Port’s operating speed; therefore, the maximum supported link
distance is up to 5 km at 2 Gbps, up to 2 km at 4 Gbps, and up to 1 km at 8, 10, and 16 Gbps.
• Extended Mode (LE) — LE configures the distance for an E_Port when that distance is greater
than 5 km and up to 10 km. LE does not require an Extended Fabrics license. The baseline for
the buffer credit calculation is one buffer credit per km at 2 Gbps. This allocation yields the
following values for 10 km:
- 10 buffer credits per port at 2 Gbps
- 20 buffer credits per port at 4 Gbps
- 40 buffer credits per port at 8 Gbps
- 50 buffer credits per port at 10 Gbps
- 80 buffer credits per port at 16 Gbps
• Dynamic Mode (LD) — LD calculates buffer credits based on the distance measured during
port initialization. Brocade switches use a proprietary algorithm to estimate distance across an
ISL. The estimated distance is used to determine the buffer credits required in LD (dynamic)
extended link mode based on a maximum Fibre Channel payload size of 2,112 bytes. You can
place an upper limit on the calculation by providing a desired_distance value. Fabric OS
confines user entries to no larger than what it has estimated the distance to be. When the
measured distance is more than the specified desired distance, the desired distance (the
smaller value) is used in the calculation.