cable modem giving private IP

hello, i have been trying to connect Cisco 1720 router to time-warner cable modem for home network. the router has two ethernet ports, fa0 and eth0. i want eth0 to be internal and fa0 to connect to modem. i enabled dhcp on fa0, but the cable modem always gives a private IP instead of public. time warner has no idea how to fix this. their response was that for some reason the modem thinks that the router is one of their network's nodes.

i would really appreciate your response.

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You may also wish to "post" on the cable modem newsgroup:


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Try changing the MAC address on the FA 0 interface so that it does not have a Cisco vendor ID in the first 6 characters

int fa 0 mac-address EEEE.EE00.0001 end

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How old is your IOS. There was a bug in the DHCP client some time ago that did exactly that. One of the option fields was inappropriately filled in and made the router look like a cable modem to the DHCP server.

It seems Google really is our friend:

The bug ID is CSCdx67972:

------------ A Cisco router running 12.1T or 12.2 IOS will incorrectly set the Class ID (Option 60) of a DHCP Discover Packet.

This only causes a problem when the following is true:

1) Router is setup to be a DHCP Client. 2) DHCP Server is looking at Option 60 to assign IP addresses 3) Router is connected behind a Cable Modem.

This is a rare problem that does not effect every customer running the router as a DHCP Client.


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Martin Gallagher

I updated the IOS, and it works fine now. Thanks for all your help.


Mart> On Thu, 10 Aug 2006 12:28:43 -0700, wrote: >

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