TROUBLESHOOT CABLE MODEM TELECOMMUTERS ETHERNET CONFIGURATIONS
With the rapid spread of broadband access through cable modems,
companies are finding that many employees want to telecommute at least
part-time. Telecommuting can be a money-saving proposition for
employers as well, and it's easy to set up with Windows' built-in VPN
Alas, many network administrators find they are stuck in the starting
gate, unable to get telecommuter's computers to talk reliably to cable
modems over Ethernet. The problems are subtle, but solvable. Here's a
checklist to follow when troubleshooting cable modem connections:
1. Verify that the Ethernet cable between the computer and modem is
not too short -- it should be at least six feet long. Many cable
modems today have 100BaseT ports, and 100BaseT requires a minimum
cable length of six feet to avoid signal reflections and cross-talk
that can impair performance.
2. Switch your user's Ethernet driver from autonegotiated speed and
duplex mode to fixed values. Often port-speed autonegotiation is
unreliable, resulting in speeds that switch randomly or don't synch
up at all. Start by manually configuring the Ethernet driver to 10
Mbps and half-duplex. In Win98/ME, the values are under Control Panel,
Network, Selected Ethernet Adapter, Properties, Advanced. In Win2K/XP,
they are under Control Panel, Network and Dial-Up Connections, Local
Area Connection, Properties, Configure, Advanced.
3. Verify IP settings. Usually cable modems require you to configure
Ethernet for DHCP. However, if you have a static IP address assigned
by your provider, verify that it, the subnet mask, and the gateway
address are correct. The most frequently forgotten or misconfigured IP
setting is the gateway address (which is used only with static IP
configurations), so be sure to double-check it.
4. Open a command (DOS) window and use the ping command to
progressively ping outward from the problem computer to the Internet.
Ping the documented gateway address first, followed by the name server
addresses, and then Internet site addresses. You can determine DHCP-
assigned gateway and name server addresses by running the ipconfig
5. Verify that the assigned name servers are functioning by using the
built-in Windows command nslookup. You should be able to type nslookup
followed by any public domain and receive the corresponding IP address
as a response.
6. If your user can surf the Web but can't establish a VPN session,
verify that your upstream provider isn't blocking the VPN protocol.
Many cable providers block VPN for basic Internet access accounts,
requiring telecommuters to pay for a more-expensive connection plan to
have VPN enabled.