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Synica1's avatar
Synica1
Guide
Mar 08, 2019
Solved

CM1100 IPv6 Problem (Xfinity)

Hi everyone, I replaced our C7100V with a new CM1100 yesterday. After a bit of wrangling on the phone with Comcast, it's activated and working... but only on IPv4. Firmware version after the Comcas...
Troubleshooting
  • vkdelta's avatar
    vkdelta
    Mar 11, 2019

    CM is just a bridge modem. It is not do anything for CPE side (LAN side) Ipv6 other than just mere forwarding it.

     

    Comcast does DHCPv6 for the routers and provides a delegated Prefix (PD) along with it. 

     

    Not sure what firewall you are using, but most linux ones support DHCPv6 with PD. 

     

     

Synica1's avatar
Synica1
Guide
Mar 08, 2019

Update 1: 

After working with a customer expert on the Comcast forums (not an employee), I've got some more data.  Plugging a laptop directly into the port I used for the firewall and power-cycling the CM1100 resulted in the laptop pulling an IPv6 address (2001::x) and Class A IPv4 address.  Plugged the firewall back in, power-cycled again, and it still didn't get an IPv6 address. 

 

Thought 1:  Perhaps Comcast only issues addresses to its gateways via DHCPv6 and since the firewall only supports SLAAC/RA, that's the issue?

Thought 2:  Could I take the IPv6 address the laptop pulled when I connected it directly to the CM1100, and set that as a static address on the external firewall interface?

 

Thanks, and happy Friday!