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:
||During normal operation, the downstream
channel is lost. During startup, the downstream channel is not acquired.
||During normal operation, the upstream
channel is lost. During startup, the upstream channel is not acquired.
||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
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
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.
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
= Dynamic Host Configuration Protocol