Flow Control & QoS

[andrew.butterworth]

With Catalyst 3550's when enabling QoS the documentation explicity states that 802.3x flow control MUST be disabled on all ports. There is no explanation why this must be configured (although I can see issues with links pausing etc). What are the technical limitations of this?

I have also been looking for any other information regarding whether this is applicable for Catalyst 6500's as well but I can't find anything. The 6500 QoS documentation does not state anything about flow control.

Slightly confused.....

Andy

[Prashanth Krishnappa]

The recommendation in a 3550 to turn off flowcontrol with QoS is because when flowcontrol is turned on, and a pause frame is received/transmitted, packets would be discarded irrespective of the packet priority. This can result in higher delays/drops for even the priority packets. It is possible that the expected QoS behavior will not be seen in this case.

I think it is different in a 6500 because each port in 6500 has its own packet buffer on the line card while 3550 has a shared memory in the switching engine.

Hope this helps.

[andrew.butterworth]

Can anyone point me in the direction of some links on CCO to explain the architechture and the limitations regarding QoS on the 3550 & 2950. Also any links as to why the difference on the 6500 with QoS.

I would have thought that flow-control should be disabled on all switches due to the fact delays could be introduced into the network if a switch 'pauses' on a link?

Thanks

Andy