[wellylug] Network cards running at 10mb/s instead of 100 - w hy?

Damon Lynch damon at photo.geek.nz
Mon Oct 20 19:12:25 NZDT 2003


On Tue, 2003-10-21 at 07:55, Ewen McNeill wrote:
> In message <1066628275.3089.29.camel at damon.asianreflection.com>, Damon Lynch wri
> tes:
> >On Tue, 2003-10-21 at 07:10, Ewen McNeill wrote:
> >> Given the mii-tool output you list I'd be looking at what the card is
> >> plugged into.
> >
> >[description involving cross over cables and PCI 4-port "hub card"]
> 
> FWIW, I've always been fairly dubious about the "NIC plus hub" cards.
> Perhaps they're convenient for some people's networks, but just as I
> like external modems, I like external hubs/switches. 

I'll keep that in mind next time I have to buy networking gear.

> 
> Having puzzled out your network a bit it appears to look like this:
> 
>   ['net]
>      |
> +---------+
> | gateway |---------[DMZ machine]
> +---------+
>   | | | | 
>         [Internal]
> 

Yes, that's it exactly.

> and you seem to have various links where one end is claimed as 10Mbps
> and the other is claimed as 100Mbps (eg, DMZ machine has a 3c59x at
> 100Mbps, FD, connected to dmfe on the gateway at 10Mbps half duplex).

Again yes, that's it exactly.

> I'm surprised that works at all.

Oh!

> 
> Crossover connections are prone to failures in negotitation, leading to
> one end thinking the connection is at one speed, and the other at the
> other speed.

I had no idea about this class of problem.  Speaking as a general
principle, it does seem to be a pain to have to get a switch / hub just
to connect one DMZ machine to a gateway.

>   This can cause all sorts of weirdnesses, particularly if
> there is a duplex mismatch (full/half), that only show up under load.
> (A crossover with a mix of half and full duplex links will work okay
> under a small load, and really fall apart under high load, especially
> with fast machines.)  Sometimes this means you have to force/lock the link
> to a particular combination of settings (at BOTH ends) in a crossover
> cable to get reliable operation.

Do you mean force down to 10, or force up to 100?

> 
> In the "autonegotiate" state, it should pretty much be the hardware
> determining everything, and the software just reporting it.  So if it's
> not negotiating the right thing it's more likely to be a hardware issue
> (or poor implementation of the negotiation). 

Perhaps I am beginning to understand why people pay $$$ for networking
gear instead of going for the cheap stuff like I do :)

> 
> I'd be inclined to test things into a known-good (external) switch
> before looking for software issues.  Especially given the "no link"
> reports (which indicate that it's not getting the autonegotatiate/link
> alive polls).

Thanks for your advice.  I also note from the syslogs that the kernel is
trying for 100mbps, so I guess it must be a hardware issue.

Damon

> 
> Ewen




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