Re: Fix for kern/20700

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Tue, Dec 06, 2011 at 09:12:47AM -0500, Thor Lancelot Simon wrote:
> On Tue, Dec 06, 2011 at 10:41:01AM +0100, Manuel Bouyer wrote:
> > 
> > But with old devices, hard setting both ends is often the only
> > way to get the desired media.
> 
> I think this is largely a myth.  I believe it is caused by the unpleasant
> fact that there's a race condition in the interaction between Nway and
> parallel autodetection, such that when one side is set to Nway and
> the other's set to no autonegotiation, the Nway side can fall back to
> parallel autodetection and end up with the wrong duplex setting.

I don't think so. I think it was really a bug in the Nway implementation
of these cisco devices (for the 6000 chassis, I suspect it
wasn't even capable of doing Nway, only autodetect, but I've
not managed such a device directly so I'm not sure).


> 
> The result is that people will set one side to hardwired speed and
> duplex (no autonegotiation) and *that* will then cause the link partner
> to "autonegotiate" the wrong duplex setting as it fails at Nway and tries
> to do parallel detection.

No, to get things to work I had to set both ends to 100TX-FD. Otherwise
there would be a duplex mismatch, of course.

> 
> This problem has become less common as vendors have stopped making it
> possible to turn off Nway, since turning off Nway is what causes it.

On cisco it's still possible to turn off Nway. But for some
time, cisco devices have been doing Nway properly.

> 
> Using the user's selections to restrict the set of what modes will be
> offered is a good thing.  Turning off Nway entirely just because the
> user has picked a particular mode is a very bad thing.

I still think it's required in some case. I think I still have
cisco 1900 swicthes around, I can do some tests if it's
usefull.

> 
> At the very most, it should perhaps be possible to turn off autonegotiation
> by specifying an additional "force" media option but I think even that is

I like the idea of a "force" mediaopt. This would require a big
warning in release notes though, as this would be a behavior change for
some setups.

> a bad idea.  Apple, for example, doesn't make it possible...

Typical apple users won't try to connect an apple device to
a 15-years old switch.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--