[c-nsp] The myths of autonegotiate vs forced (was: full duplex mismatch speed - dynamips)

[Heath Jones]

Cheers Peter!

I agree for sure that forcing should be a temporary workaround and that
there could still be underlying issues. Disabling certainly isnt the long
term solution!!

I started reading wikipedia about autoneg and the best I could come up with
is that there is sufficient attenuation and then noise so the receiver
doesnt see the fast link pulses (not sure how many it must miss to change
state though), either that or there is something causing enough delay
/ temporary phase shift that the pll at the receiver is out of whack and
doesn't decode the capability correctly on occasion.

That's the thing that gets me - the links where it only happens very
occasionally. If it happens all the time or not at all, its obvious.

 Older fibre still does autoneg, anything >1G doesn't as far as I'm aware..







On 20 August 2010 08:12, Peter Rathlev <peter at rathlev.dk> wrote:

> On Fri, 2010-08-20 at 07:33 +0100, Heath Jones wrote:
> > I'm really curious as to why there are many people here saying forcing
> > ports is a bad thing though. I was pretty surprised to be reading that
> > actually, its good to have another perspective on the idea.
>
> IMO forcing ports isn't "bad" per se, but it's error prone and
> complicates the network. Sometimes you need this manual configuration,
> most times you don't.
>
> > I've seen countless issues where inter switch links, inter router
> > links and also links between servers and switches have cause so many
> > issues. On almost all of these occasions, forcing will solve the
> > problem.
>
> If these are Gigabit links, then you should return the equipment to the
> manufacturer. It doesn't follow 802.3 correctly and thus isn't "Gigabit
> Ethernet".
>
> > The link is actually going down while the renegotiation happens. This
> > causes a L2 topology change, so frames will be dropped. In a service
> > provider environment, there will be a L3 topology change - IGP does
> > its thing and this may take some time (especially on a heavily loaded
> > router). The end result is customers start calling wondering where
> > their traffic went.
>
> If auto-negotiation gives problems like that (and we're talking a
> "modern network) you would just be hiding the problem by disabling
> auto-neg, not solving it.
>
> Actually 802.3 auto-neg can sometimes help to discover bad cabling.
>
> > It sounds like this is a matter of opinion and the opinion depends on
> > the environment in which it is being applied, no ??
>
> Technically I guess every argument is based on opinion. But as a funny
> man once could have said: I'll advise all my competitors against using
> auto-negotiation. :-)
>
> > I'll be honest here, I've never truely understood the cause of speed
> > duplex mismatches. Noise would be the obvious one, but does noise
> > actually play a big part on relatively short cat5 links? Dodgy
> > connectors? Problems with the PLL decoder getting out of sync (noise
> > again?)? Faulty clock?? Someone jumping on the cable??
>
> The duplex thing is about Ethernet legacy; you don't have the problem on
> fiber links, since these can't be simplex (AFAIK, please correct me if
> I'm wrong). But any copper port _might_ be connected to a hub from 1993
> some day, and the standard tries to make that work.
>
> --
> Peter
>
>
>