Diagnosing a slow network

[Jerry Feldman]

On Tue, 10 Aug 2004 08:38:15 -0400
Brendan <mailinglist at endosquid.com> wrote:

> On Tuesday 10 August 2004 07:45, Jerry Feldman wrote:
> > card. Also, as David mentioned, your network cards should be
> > configured for full-duplex. Most will automatically come up with
> > full-duplex, but I have seen some cards that somehow get stuck in
> > half-duplex.
> 
> I usually use mii-tool for this.
> mii-tool --force 100baseTx-FD
This is an excellent tool and is included in most distros. 
To diagnose a network problem, on each machine first check to see if the
card is actually at full or half:
gaf at gaf:~> mii-tool eth0
eth0: negotiated 100baseTx-FD, link ok

If it is not, then you need to find out why. If the card is not
autonegotiating FD, then the problem could be with the switch. I
actually had this problem at Compaq where the card on my workstation
would somehow think it is in FD and the switch would think it was in HD.
In this particular case, the problem was both with the card and the
switch where we had to manually set the card to FD as well as the switch
(which was programmable). 

The first thing you need to do in diagnosing is to measure the
throughput between each machine. Once you find out where the problem
lies, then you can diagnose why. Being in Half Duplex will certainly
reduce your throughput, but it could be indicative of another problem.
as I mentioned earlier, a flaky switch can mess up an entire subnet.
Additionally, while the original poster mentioned that he was using a
switch, remember that a hub will generally cause all the systems on a
subnet to operate at the lowest common denominator, so if you have a
10Mbps HD card on the subnet, then the entire subnet will be 10Mbps.
Another issue could be cables, but if you are using Cat V cables, then
that should not be an issue. 

-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix user group
http://www.blu.org PGP key id:C5061EA9
PGP Key fingerprint:053C 73EC 3AC1 5C44 3E14 9245 FB00 3ED5 C506 1EA9
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