Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) - Manual (Page 528)

Extended Fabrics device limitations
Brocade recommends that you do not use the FC8-64 and FC16-64 port blades for long distance
because of their limited buffers. These blades do not support long-wavelength (LWL) fiber optics and
only support 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.
• Static Mode (LS) -- LS calculates a static number of buffer credits based only on a user-defined
desired_distance value. LS mode also assumes that all FC payloads are 2,112 bytes. Specify LS
mode to configure a static long-distance link with a fixed buffer allocation greater than 10 km.
Extended Fabrics device limitations
528 Fabric OS Administrators Guide
53-1003130-01