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Old 02-02-2010, 05:11 AM
 
Default link-up link-down ethernet switch tale of woe

Thought I would post this to debian as someone on the list or out there
in the webiverse might find it useful.

My story begins with an Intel MB with Realtek R8169 ethernet device, an
MPC55 nvidia ethernet device on a different PC and, finally, what turns
out to be a piece of sh*t linksys ethernet switch. Everything is
supposed to be operating 1000.

I had a heck of a time with the Intel MB because Debian stable's kernel
was too old to support it properly but the unstable install did the
trick. The other PC goes smoothly, once I give 'noapic' as a boot
param. *sigh*.

Now everything is "working" except both computers are giving me
link-up and link-down messages in /var/syslog. NFS is going away for a
minute at time and iperf says I'm getting about 70MB/s. Well that ain't
1000 MB/s.

After looking around a bit I discover ethtool and use it to attempt to
turn auto-negotation off. Well it seems to work on the realtek but the
forcedeth driver gives me messages that are totally inconsistent with
what it's telling me should be happening. I upgrade the install and it
becomes MORE broken. Now it's giving me errors that it didn't give me
before. So much for ethtool, it's a great idea, if it would only
work. The distressing part is that the forcedeth is on a 3 year old
pc. So much for the linux and old hardware meme. Although in
fairness, I think that forcedeth is a relatively recent driver which
allows the driver to be genuine free software instead of nvidia
binary-only. A virtual beer to the device driver writer if that's the
case.

Finally I give-up and plug in both ethernet connections into my
wrt54g. I figure reliable 100 is better than sh*t 1000.

Well that fixes the problem completely. I'm getting reliable 100MB/s
connection (iperf says 90MB/s) without ethtool or link-up/link-down
messages.

So I order up a new switch (HP procurve 1400-8G). It arrives, I cross
my fingers and plug in the ethernet cables. 1000 lights come on and
iperf tells me I'm getting 700MB/s, solidly. Yeah for HP, and
supposedly I've got a lifetime warranty (sorry for the commercial
message, but hey, it worked !).

I take the linksys apart, and here's where things get kind of
interesting. Those of you who have any hardware experience with
ethernet phy's are probably aware that they typically use a 25MHz clock.
This crazy piece of dung has a 25.0006 MHz crystal. That is not a
typo.

I've heard rumors of manufacturers overseas grabbing anything in the
way of parts so they can ship it out the door, and I _know_ they don't
test. Too expensive, it's cheaper for you to test it. Probably happens
in the US too - I mean it would if we actually made anything anymore.

I'm really interested if anybody with some hardware experience can shed
light on the crystal I found. I even measured it in the lab and it's
25.0006MHz alright. I'm 99% sure that it's the wrong frequency for
proper ethernet operation, but the level of incompetence that it implies
is amazing.

So morals of the story:

iperf is your friend !
Good hardware is STILL hard to find.
Give the linux drivers the benefit of the doubt (unless it's video).

HTH some poor soul who finds themself in a similar situation.

Brian


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Old 02-02-2010, 07:23 AM
Stan Hoeppner
 
Default link-up link-down ethernet switch tale of woe

briand@aracnet.com put forth on 2/2/2010 12:11 AM:

> I take the linksys apart, and here's where things get kind of
> interesting. Those of you who have any hardware experience with
> ethernet phy's are probably aware that they typically use a 25MHz clock.
> This crazy piece of dung has a 25.0006 MHz crystal. That is not a
> typo.
>
> I've heard rumors of manufacturers overseas grabbing anything in the
> way of parts so they can ship it out the door, and I _know_ they don't
> test. Too expensive, it's cheaper for you to test it. Probably happens
> in the US too - I mean it would if we actually made anything anymore.
>
> I'm really interested if anybody with some hardware experience can shed
> light on the crystal I found. I even measured it in the lab and it's
> 25.0006MHz alright. I'm 99% sure that it's the wrong frequency for
> proper ethernet operation, but the level of incompetence that it implies
> is amazing.

You're barking up the wrong tree Brian. That crystal isn't the source of your
problem. It's well within spec. I'm sure the system design tolerance is much
greater than 0.0006%. PHYs use PLL circuits to clean up the clock anyway, so
even if that clock crystal chip was running at 25.05 MHz it wouldn't cause
problems. BTW, what "lab" equipment did you use to "test" this crystal and
verify its frequency? /laughs

I'd say you probably just got a defective switch. It happens. And probably
more frequently with the cheap stuff than the more expensive stuff. No surprise
there.

Here's a somewhat reverse example. I recently purchased a $10 Rosewill 8 port
10/100 desktop switch from Newegg just to have some extra bench testing ports.
The uplink to the closet switch works fine, and it works great using full auto
negotiation with my main desktop. But if I force 100FDX in the driver the link
won't come up *period*. Some gear will *only* work with auto negotiation.
Speaking of which...

What's interesting in your case is that you're dealing with 1000BaseTX. IEEE
802.3ab *requires* auto negotiation for all copper gigabit ethernet; manual
settings aren't allowed, period.

Again, I'd say you just got a bad switch. And it has nothing to do with the
clock crystal being 0.0006% off target. Check the crystals in some of your
other electronics and see how far off they are. I bet most are off by more than
0.0006% The problem with the Linky could be the power supply, the transformer,
the switch chip, a marginal or cracked trace on the board, a defective capacitor
(one of those itty bitty suckers you can barely see soldered to the board),
could be any number of things. But it ain't that crystal.

--
Stan


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Old 02-02-2010, 11:01 PM
Tony Nelson
 
Default link-up link-down ethernet switch tale of woe

On 10-02-02 03:23:31, Stan Hoeppner wrote:
> briand@aracnet.com put forth on 2/2/2010 12:11 AM:
>
...
> > ...25.0006 MHz crystal. ...
...

> ... I'm sure the system design tolerance is much greater than
> 0.0006%. ...

Me too, but (25.0006/25.0 - 1.0)*100.0 = .0024%. Googling shows that
the IEEE 802.3 spec is 50ppm (.000050 or .0050%), so, with the
crystal's own tolerance (whatever it is) it may be OOT.

--
__________________________________________________ __________________
TonyN.:' <mailto:tonynelson@georgeanelson.com>
' <http://www.georgeanelson.com/>


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