I have a Sparklight Certified cable modem, but registration says it is not supported. Why?

Sparklight certification is based upon cable modem specifications provided by the manufacturer, as well as required Sparklight standardizations. To be allowed through registration, Sparklight validates a cable modem's compatibility with our network by testing various settings on the cable modem. If we detect that a cable modem is incompatible for any reason, no matter the model, registration will not occur. In rare cases, certified cable modems may exhibit deviations from the specs we were given and may in fact be incompatible with our network. For security reasons, the specifications validated are not provided.

For more information on supported cable modems, see this support article HERE.