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Posted by Rich Seifert on November 12, 2007, 11:51 am
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>
> 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?
>
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.
> 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.
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.
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX
Send replies to: usenet at richseifert dot com
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