I was asked to diagnose network connectivity problems within a LAN, specifically from a client to a server on the same subnet but not on the same switch. The user was convinced that it was a bandwidth congestion issue. However, I suspected a physical problem or a link issue, as switch statistics did not indicate traffic patterns deviating from baseline measurements.

I first ran a ping test to the switch the server was on from the client's machine. The result was a loss of about 5 percent of the packets. However, the switches employed on this particular network had the "feature" of assigning a low priority to Internet Control Messaging Protocol echo packets. It was not uncommon for a ping test to this type of switch in a stand-alone configuration to drop a low percentage of packets. Therefore, this test produced inconclusive results, and I realized I had to dig deeper.

I placed the mobile NDT server on the same switch that the client's server was attached to and went to the client's machine to run an NDT test. NDT reported a mismatched duplex in the path between the client and server. Finding the mismatch and correcting it resolved the issue.

Case 2: The ISP

I was working on transitioning a site's Internet link from a DS3 to a Gigabit Metro Ethernet throttled to 100Mbit/sec. My preparation for the cutover included placing the mobile NDT server on a switch where the Metro Ethernet terminated.