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

[Peter Rathlev]

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