[SWLUG] Sniffing for dud packets

[Foeh Mannay]

Hi, all.

Sorry this isn't a strictly Linux question, and also for the preamble 
rambling...

In the school where I work there is a 100 meg fibre link from the server room 
off to a computer lab at the other end. I suspect it's had it in some way or 
another since the network that spurs off it is very slow, and when I was 
installing a fairly big app the other day loads of the machines were crashing 
out (Wintendo for you) and giving unavailable resource messages, not to 
mention being a load slower than elsewhere in the complex.

Anyway, I started doing some pings etc and noticed it was losing about 2 to 4% 
when the network wasn't loaded very much, when I bumped up the packet size to 
about 13k and did it with some traffic on the network it started dropping 
more like 40% - I'm not sure if I'm being paranoid but that seems pretty dire 
because 13k is one segment, and in any case you shouldn't just lose that much 
stuff on a switched network, right?

I thought I might try sniffing some traffic but it didn't turn up an awful lot 
(except a load of spanning tree spam from a cisco switch owned by county) but 
I thought maybe it wouldn't since either the switch or the stack would be 
dropping corrupt stuff before it got to my sniffer.

I could be wrong in any / all of these assumptions, but what I'm basically 
asking is whether there are any apps I could use to check / diagnose a dud 
fibre link? I stole a junk machine and sneakily installed BSD on it, so 
that's happily within reach of the one end's media convertor... plus my 
laptop will reach the other in a cab at the opposite end of school, so I can 
run *nix on either end, just not sure what!

Any advice would be very welcome.

PS if anyone could shed light on this:

The networks guy said you can have a maximum of 2 devices (switches) between 
any 2 PCs on 100 meg, any more and it won't work. I thought that was just 
hubs, but I could be wrong.

Interestingly enough he's the guy that fitted the fibre link which goes:

server -> switch -> media conv -> media conv -> switch -> switch -> switch -> 
PC

Not 100% certain the last switch is actually there, but there may be an extra 
one at the start so for argument's sake we'll call it an average - that's 
more than 2 switches between 2 computers!

Cheers chaps!

Foeh.