Last week, I visited a client to test their Internet speed. The company is paying for a 35Mb/s down and a 5Mb/s upload speed. However, when the company ran speed tests, the results revealed they were only getting 4Mb/s down and 3Mb/s up. To verify that the LAN side of the network wasn’t causing the problems, I connected my laptop directly to the Ubee DDW3600 series modem provided by Time Warner Cable, and ran a speed test. To my surprise, when I directly connected my laptop – BOOM: 35Mb/s down and 5Mb/s up. The modem was receiving the full bandwidth, but the connection between the LAN and WAN had a serious bottleneck.
The connection to the WAN was the “outside” interface on a Cisco ASA 5505. The interface was hard-coded to 100Mb/s, full-duplex.
“show int ethernet 0/0” provided some very interesting output!
647803 CRC errors! Cyclic redundancy errors are almost ALWAYS due to speed/duplex mismatch. After waiting on hold with TWC for a while, tech support checked the modem, and what do you know…their Ubee modem auto-negotiated to 1000Mb/s instead of 100Mb/s! This drove me crazy after being sure to check that the firewall port was set to not auto-negotiate to prevent this exact problem! After TWC changed the port speed of their modem, the internet speed jumped to full 35Mb/s down and a 5Mb/s up.
The office I work at has an identical modem, and the office also has had slow internet speed. I checked the interface on the firewall at the office, and it had 6 million CRC errors! Another call into TWC. 20 minutes later, tech support had found the SAME problem on TWC’s modem. The tech I spoke with at TWC said that they see this all the time, and it is a known issue. The Ubee modem will see the other side as a 100Mb/s connection, but still set its local port speed to 1000!
I am posting this as a warning to always be sure to have the ISP define their port speed on the modem/router which connects to the outermost part of the network. It could save you a lot of hassle!