Flowcontrol is confusing me

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Jul 16th, 2008
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Hi there,

I've read a fair bit about flow control and although on the surface it seems quite obvious it is actually anything but that. I've got a 6500 switch which has 3 Exchange servers connected to 3 of its GigabitEthernet ports. I've set the switch interfaces to:


flowcontrol receive on

flowcontrol send on


Now I thought this would send and receive pause frames if required, regardless of the settings of the remote device. (And I thought setting the flowcontrol to 'off' would not send pause frames or respond to them regardless of the settings on the remote device, and I thought that flowcontrol send and receive 'desired' would send and receive pause frames if the remote device was capable and it was needed).


It appears that desired and on seem to do the same thing according to some of the literature I've read. Do they use some form of autonegotiation or is it just autosensing to see if a remote device sends and responds to pause frames? The servers are set to 'Auto' for flowcontrol.


Even though the flowcontrol is set to on for send and receive on all the switch ports and all 3 servers are set to 'auto' for flow control, when I issue the command 'show flowcontrol' for the switch ports 2 of them state that they're on and 1 of them states 'disagree'. What does this mean?


I can't find any good definitive documentation that actually explains things. For example what's the point in the 'desired' setting if 'on' achieves the same thing?


And what's the best practice - would it be just to set to 'on' on all GigabitEtherent ports connected to servers running at a gig and set the servers to 'Rx/Tx Pause' rather than auto?


Any advice appreciated,

Thanks

Pete.

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Peter.D.Brown Wed, 07/23/2008 - 01:55
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Thanks Leo,

would you happen to know this: if you haven't got any dropped frames on the gig interface then you don't even need to think about implementing flow control as it's not needed. Would this be a correct assumption? Would I only need to start looking into flow control if frames are being dropped?

Thanks

Pete.

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