To flow or not to flow

Hi all,

It's well known that there a problems when one end of an Ethernet link wants to auto-negotiate and the other doesn't. But what happens when both sides *do* negotiate, but one end wants flow-control on and the other end doesn't want it? Is the link then completely without flow-control? Or is it only flow-controlling in one direction?

Background: I ran into an issue with a crashed Windows PC (blue screen). Apparently the PC's networkcard is still running, but since Windows isn't, it doesn't empty the receive buffers anymore. Apparently the network cards then sends out PAUSE messages to the switch. So far so good, but then when the internal memory of the switch fills, it started to send out PAUSE messages to all other connected PC's. In the end all PC's came to a network-standstill. Traffic flowed again as soon as I disconnected the cable to the blue-screening PC (apparently the switch then realized that it could flush its buffers). Nice way to hang up a complete network. That's why I want to disable flow control on the PC, but on an unmanaged switch I can't.


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If both ends of the link are *capable* of flow control, but only one end is willing to receive (and react to) PAUSE frames, then the link will be flow-controlled in one direction. If only one end of the link is capable of flow-control, there will be no PAUSE frames sent in either direction, i.e., the link will run open-loop.

That's an example of a poor policy decision in the switch; i.e., throttling back all input ports when there is congestion on a single output port.

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