CM1100 won't do IPv6 with Cisco router (on Xfinity)

I went back and reviewed the Wireshark packet captures I had made, both on the standalone Mac, and on the vlan capture made via running the CM<->router traffic running through a monitored VLAN on a switch. It finally occurred to me that the router sends out IPv6 routing advertisements, (expected of any IPv6-capable router), but the Mac does not (as expected of a standalone host). 

Some research of Cisco docs pulled up these interface options:

ipv6 nd ra suppress  (to suppress unsolicited RAs)

ipv6 nd ra suppress all  (to suppress all RAs)

I suspected the RAs on the interface toward the modem were not needed in this topology, so I added the "ipv6 nd ra suppress" to the interface, then power-cycled the SB6141. Everything worked as normal. (Solicited RAs are still available if requested.)

Then I crossed my fingers, swapped in the CM1100, and registered with the Walled Garden.

It works. So the problem is a bug in the firmware of the CM1100 that causes TFTP failures if RAs are seen on the LAN interface, but at least there is a workaround.