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Cable MODEM "Married" to an Ethernet MAC Address
Last updated: 5/9/02

Q.  Why won't my broadband router communicate with my cable MODEM?  The MODEM worked fine when it was connected to Ethernet network adapter in my PC.

A.  The MODEM is probably still married to the PC network adapter’s Ethernet MAC address (http://duxcw.com/faq/network/mac.htm).  Many cable MODEMs do that.  Many routers have capability to clone the MAC address of a PC connected to it through an option in the router administrative interface.  My Motorola Surfboard SB2100D cable MODEM can be coached into marrying itself to a new MAC address by connecting it to the new network adapter/router, disconnecting the MODEM AC to DC power converter where plugs into the MODEM, letting it sit for a couple of minutes, reconnecting power, and waiting several minutes for the MODEM to initialize.

Once the MODEM is initialized, you may have to release and renew the IP address assigned to your network adapter or router.  This is usually done for the router with the router's browser or telnet administrative interface.  Most routers will do it automatically if you power cycle/reset them (which you may have to do anyway).  To do it for a network adapter in a PC from Windows 9x/Me Start, Run, enter winipcfg, select the network adapter going to the MODEM, and click Release followed by Renew.  For Windows NT, 2000, and XP open a DOS Window and type C:\>ipconfig /? for instructions.  If the resulting IP address starts with a 192,10, or 172 (a private IP address and not an Internet IP address; see http://duxcw.com/faq/network/privip.htm)  the MODEM has not completed the initialization process, is still married to the old MAC address, or cannot connect to your service provider.  My MODEM usually assigns a local IP to establish a connection until the service providers DHCP server (http://duxcw.com/faq/network/dhcp.htm) assigns an Internet IP address.  If the IP address starts with a 169 it is an Automatic Private IP Address (see http://duxcw.com/faq/network/autoip.htm) which was assigned by Windows because it could not find a DHCP server (in either the router or MODEM).  This usually happens when the local network leg to the PC is not working.

Other MODEMs apparently will reset if left unpowered for a longer period of time.  One reader let his MODEM sit overnight with the power unplugged.  The next morning it married itself to the router after power was applied and everything worked.  Finally, the service provider should be able to reset everything from his end.


SB3500 User Guide: "You must allow 5 to 30 minutes to power up the first time because the SB3500 must find and lock on the appropriate channels for communications."

SB3100 User Guide: "You must call your service provider to activate your service. You need to provide the media access control (MAC) address. This address is found on the barcode label marked HFC MAC ID on the rear panel. The address format is 00:20:40:xx:xx:xx."

"Cannot receive or send data. Check the lights on the front-panel. Note the first light from top to bottom that is off. This light indicates where the error occurred.  If the first light that is off is:

Receive During normal operation, the downstream channel is lost. During startup, the downstream channel is not acquired.
Send During normal operation, the upstream channel is lost. During startup, the upstream channel is not acquired.
Online During normal operations, the IP registration is lost. During startup, the IP registration was not successful.

Check that your TV is working if you have cable TV and you have a clear TV picture. If you aren’t receiving your regular TV channels, your data service will not function.

Check the coaxial cable at the rear panel and outlet and hand-tighten if necessary.

Check the IP address (follow the steps on page 13); call your service provider if you need an IP address."


"Surfboard 3100/4100 - When working the Power, Send, Receive and Online lights should be on solid and the Activity light should flash sporadically. If either the Send or the Receive lights are flashing than a connection problem exists to our server. If the lights are indicating a proper connection and the computer is still unable to get online then a problem with the computer could exist. Try resetting the computer and the cable modem. To reset the modem simply disconnect the power supply and leave it unplugged for 1 to 2 minutes. Once you plug the modem back in and the lights come back on then reset the computer."

2/6/03 Alan Spicer sent me  the following info this date...

Married to the MAC ID of the CPE.

This is a function of the DOCSIS configuration downloaded from the TFTP SERVER (MAX_CPE). The default is 1 unless you pay for additional ip addresses. Cloning the MAC id of the original PC may get around this but (most?) ISP will raise it to 2 for you anyway if you call their support. Just tell them you got a new computer (if they don't like routers) to replace the old one. They don't need the MAC ID contrary to popular belief. They just need to know there is a NEW cpe ethernet device.

Newer version of Cisco IOS on the CMTS lets ISP's set a configuration item which allows users to swap CPE's as long as they don't exceed the maximum connected directly (by hub or ethernet switch) to the Cable Modem.

If your ISP uses this CMTS feature all you have to do is swap CPE devices and power cycle the modem. And you're good to go. If they are smart they will limit the length of DHCP leases in their DHCP server configuration. Usually the ISC open source DHCP server. Unless they are a lame Windows shop.

P.S. You can make a NAT router with Windows XP and 2 ethernet cards. You'll want a hub or a switch on the inside to connect the multiple PC's. You can also do the same thing with RedHat Linux (or other distro. of Linux) using ipchains or iptables. It takes a little more work, but has a fantastic suite of ip filtering for firewalling.


Abbreviations decoded…

CPE - Customer Premises Equipment (CPE) - provides integrated services, such as multi-media, voice, and data services.  Or, as he used it a PC, etc. connected to a cable MODEM.

DOCSIS - Data Over Cable Service Interface Specification - defines interface requirements for cable modems involved in high-speed data distribution over cable television system networks.

TFTP - Trivial File Transfer Protocol - a simpler form of FTP (File Transfer Protocol).

Cisco IOS - Available on an extensive range of Cisco platforms, Cisco IOSŪ Software is a network systems software that provides a common IP fabric, functionality and command-line interface (CLI) across a network.

CMTS - cable modem termination system - Systems located in a cable company’s facility to provide Internet access to cable subscribers.

DHCP = Dynamic Host Configuration Protocol

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