[c-nsp] full duplex mismatch speed - dynamips

[Heath Jones]

Thats an interesting point! I had that problem yesterday with a ethernet
extension service CPE connecting to 2800. The CPE didn't like no auto.

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.

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.
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.

It sounds like this is a matter of opinion and the opinion depends on the
environment in which it is being applied, no ??


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??


Is there someone here that has actually gone to town on this one - CRO etc??



Cheers
Heath



On 20 August 2010 03:52, John Neiberger <jneiberger at gmail.com> wrote:

>  >>>> Adam, you are my new best friend. I've been saying this for the past
> >>>> few years and people still think I'm crazy. I flat out refuse to
> >>>> manually configure speed and duplex for someone unless it is
> >>>> demonstrated (or I can verify) that a duplex mismatch is actually
> >>>> happening or there is some other extenuating circumstance that
> >>>> requires it.
> >>>
> >>> JFTR, count me in that camp as well (and that has been discussed here
> >>> on c-nsp just a few months ago as well).
> >>>
> >>> The PA-FE-TX is really the only thing left in our network that needs
> >>> force-duplex.
> >>>
> >>
> >>
> >> I, believe it or not, still have a 3640 floating around with a NM-4E set
> >> to 10/full on all interfaces. It hasn't died yet and does its intended
> >> job perfectly.
> >>
> >> I'd tried to turn up a fast ethernet with Verizon a while back and their
> >> policy was to set the ports on the overture to 100/full, no auto.
> >>
> >> But these are both fringe cases these days. Other than that it's autoneg
> >> all the way. It's not the 90's anymore. There's absolutely no reason to
> >> not use it.
> >
> > I think the nailing issue is still more of a problem in "telcos" as
> opposed
> > to "isps". Telcos being in general larger, slower and more fond of
> process
> > and procedure which once instituted are impossible to remove :)
> >
> > I've worked for a telco which insisted on forcing ports both internally
> and
> > customer facing. They went so far as to force 100/full on ports facing
> > phones and printers.
> >
> > It took a while to remove the practice. I found challenging people to
> tell
> > me when the last time they saw an issue caused by an autonegotiating port
> > where the other side wasn't forced helped quite a lot, as no one could
> think
> > of any, it was just something they'd always done, for fear of the autoneg
> > boogeyman!
> >
> > adam.
>
> Even more insidious is the fact that hard-setting both sides to
> 100/full can actually cause a duplex mismatch. Many NIC drivers still
> expect to see an autonegotiating link partner even when hard set. If
> they don't detect a partner participating in Nway, they
> will--according to spec--assume they are connected to a hub and fall
> back to half duplex even when configured for full duplex. This is
> important because most Cisco switches made in the last eight years or
> so completely disable Nway if you hard set them. If you connect a
> PC/server that expects an Nway partner, you'll get a duplex mismatch.
> I've personally corrected this on a few hundred occasions.
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