[svlug] suddenly low bandwidth on lan cable

[joel williams]

It was not clear if you were able to isolate the problem to the LAN or 
the wan.

If WAN:
Not sure about the D-Link 320T specifically but there is a classic 
problem with
some of these low end products, including D-Link products; they have memory
leaks that manifest as slow throughput. What happens, is the memory pool 
(eg: skbufs)
gets to a low threshold, and you keep recycling the same 2-6 bufs, which
causes all kinds of problems. For example, the dsl or enet chips require 
a ring buffer
full of bufs, if there are not enough to go around, then data is discarded,
hence lost packets.
Try power cycling the d-link and see if things are better upon reboot, 
and then
degrade again.

Also cable changes could "possibly" trigger this problem as follows:
Amazingly some mis-wired cables may still work, however they may 
auto-negotiate
to half duplex. A correctly wired may auto-negotiate to full-duplex, which
changes the timing and hence buffer management pathology.

You can get adventurous and dig into this; use wireshark and analyze the 
timing
between Ethernet frames, re-tries, corrupted packets, etc.
You will probably see where the problem is occurring.
Start by captururing some pings and traceroutes on a quiescent network.
Ping between hosts on LAN, and add more complex traffic
(to WAN, multiple steams, etc) until you see the problem.
The bottlenecking link or network element may become obvious.
Then you know specifically where to look for the cause.