Dell POWERVAULT MD3600F - Manual (Page 40)

40 Planning: RAID Controller Modules
Interoperability of 2 Gbps, 4 Gbps, and 8 Gbps
Devices
The FC standard specifies a procedure for speedy auto-detection. If a 4 Gbps
port on a switch or device is connected to a 2 Gbps port, it must negotiate
down for the link to run at 2 Gbps. If there are two 8 Gbps ports on either end
of a link, the negotiation runs the link at 8 Gbps if the link supports the
required specifications. If a link connects at 4 Gbps in this scenario, possible
causes could be excessive cable length, poor cable quality, loose cables or
SFPs, and so on. Ensure that the cable lengths are not excessive and the
quality of the cable and connector is good.
The same rules apply to 8 Gbps devices relative to 4 Gbps and 2 Gbps
environments. The 8 Gbps devices have the ability to automatically negotiate
down to either 4 Gbps or 2 Gbps, depending on the attached device and link
quality. If the link negotiates down to a speed slower than expected, check
the device.
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