One day we noticed our Internet just wasn’t what we were used to. Although no-one was complaining we in the IT department were feeling it whenever we downloaded a large file like a Microsoft ISO. Fortunately, we had setup Cacti — a MRTG/RRD monitoring solution. A quick look at Cacti showed we were only getting about 3Mbs of Internet:
Our Internet provider supplies us with a portal in which we can see our usage. Comparing our graph with their graph confirmed a problem existed:
Now, we just had to determine where the problem lie. First, we checked our Internet using speedtest.net on the inside of the firewall. The result was about 3Mbs – just what we expected. Then we ran the same test on the outside of the firewall. The result was 20Mbs — what we should be getting. This meant the problem was in the firewall.
Turns out a port had recently been mis-configured to use half-duplex causing collisions on the connection. Within just a few minutes of discovery our problem had been resolved. If it had not been for the visual graph provided by Cacti we would have been going through logs to determine what and where the problem was easily using up time better spent elsewhere.