Linux Router: TCP slow, UDP fast

Chris chris at
Sun Feb 15 11:18:04 UTC 2009

Thanks for the suggestions everyone.
I've got to the bottom of the problem now (I'm sure there will be a
collective sigh of relief from the list because of the noise this thread
generated :-)).

I installed two brand new, low spec, 3Com switches one at the 'front' of the
network and one 'behind' the routers. They are the same model, same latest
firmware, same config (saved to and then copied off disk) and only their IPs
were different.

The front switch was the problem. As two final tests before removing it we
switched off unicast/multicast broadcast control and flow control
(and simultaneously on the same port on the switch behind the routers)
because there were pause frames showing but not a massive amount in terms of

The switch behind the routers however is serving the same bandwidth equally
well ! We've put an ancient switch in place of the front switch and its been
working perfectly so far.

My lesson learned is too change as little as possible at once ! That said,
recent network changes were spread about a month apart and this very odd
issue was far easier to dismiss than believe due to its bizarre nature.
Especially when providers have changes in their network conditions as
testing is done.

I really appreciate all the input and have learnt loads, possibly just not
in the way I would have liked to :-)

Doubles all round,


More information about the NANOG mailing list