On Mon, 25 Jan 2016 20:12:12 -0500, Retired wrote:

These 2 webpages may help you.
http://www.practicallynetworked.com/...addr_clone.htm
http://www.tp-link.us/faq-68.html

Basically, it seems that reference implies Comcast, even though
they might not even know it themselves, is requiring *two* MAC
addresses for authentication:
1. The MAC address of the docsis3 Costco cable modem, and,
2. Some other MAC address (which seems to be the router, by default).

Had I known that when we set it up, I would have made the
router DE:AD:BE:EF:CA:FE, which at least I can remember.

There was one disturbing sentence in your reference:
http://www.practicallynetworked.com/...addr_clone.htm

"some ISPs have started to recognize the MAC addresses of
popular routers and disconnect service if a router is
detected."

A router is a requirement, not an option; so if they are doing
that, they stink as ISPs.