BladeSystem Virtual Connect
Showing results for 
Search instead for 
Did you mean: 

Understanding "Preferred" speed on VC FCoE server profile

Trusted Contributor

Understanding "Preferred" speed on VC FCoE server profile

A question from John on VC FlexNIC speed settings:





I’ve got a blade enclosure running VC 4.10, and has two FlexFab modules. BL460c Gen8blades. The uplinks show that 3 of the four are at 8Gb, while one is at 4Gb: ( not sure when this happened, but originally all were at 8gb)




The servers that use X1 above have their FCoE server profile speed set to “Preferred”.




Manual says:

Preferred—The speed of the network is the same as the preferred speed of the

network to which the connection is associated. If no preferred speed is

configured for a network, it behaves like "Auto".


So, in this instance, what speed would the blades corresponding FCoE FlexFabric port be operating at ?

Does this mismatch generally cause issues ( ok, I realize this is a very non-specific question)….




Reply from Dave:




The SAN Fabrics are showing Uplink speeds, external SAN switch.


The FCoE HBA… is showing Downlink speeds to the server NIC/HBA.


The two are independent of each other. Downlink FCoE speeds default to 4Gb to leave 6Gb for the Ethernet NICs.

These can be changed as needed.


The Uplink speeds will depend on the VC setting and the external connected switch.

If set to ‘Auto’ for speed, then it will sink to the external switch set speed.

It can also be set to 4Gb, 8Gb to lock in a specific speed.


I would edit the top FCOE_A line to verify that it shows ‘Auto’ under the Speed column.

I would leave it at ‘Auto’ and have the SAN team check the SAN switch port speed.

Also verify that the SFP is rated at 8Gb. Have not seen a 4Gb SFP for a long time.




Reply from John:





All those items check out ( Uplink ports set to “auto”, the switch does indeed show this port at 4gb, while showing the others at 8gb, and the part number on the SFP in the switch shows it to be an 8Gb SFP).  When we can get a change control approved, we are going to down the port at the switch end, and up it, in hopes it will re-negotiate to 8gb.


The reason for the question on the downlink side, was that I never quite understood the difference between what “Preferred” means, vs “Auto” ?




And from Vincent:




"Preferred" means take the preferred speed that was configured for the fabric, but you need to check Advanced Settings on the Fabric to get to that setting, see screenshot below. If that preferred speed is not set on the fabric, Preferred behaves like Auto.