Monthly Archives: March 2010

Too Simple to Fail?

I’m doing some low-level Ethernet work (TCP/IP), with a device connected through two plain vanilla network switches. (I’m testing breaking the connection with neither side detecting the link-lost condition.)

I made a change to my software, and suddenly the Ethernet is dead. It seems to send and receive nothing.

I back out my change, and it still doesn’t work.

I back out a few more: no effect.

As it turns out, one of the Ethernet switches got confused and quit routing correctly (even through this simple configuration).

You can tell that the switch has some smarts:

  • When you look at the traffic with WireShark, you don’t see what’s exchanged between the switch’s other nodes.
  • When you have a mix of GB devices and 10MB devices on the same switch, the gigabit devices transfer data at full speed, so you know the switch isn’t exposing every packet to every (10MB) connection.

Still, I was astonished that this could happen.

For the record, the switches were:

  • Netgear ProSafe 5 Port Gigabit Switch (Model GS105)
  • Netgear 4-Port 10/100 Dual Speed Hub (Model DS104)

The older DS104 was the prime suspect. I simplified the route to just one switch (the GS105), which got it working. So I cycled power on the DS104, reconnected the original configuration, and everything worked again.

Update 4/8: They say you never learn anything from a mule’s second kick. I unceremoniously decommissioned the DS104 after it did the same thing again, this time with no provocation.