Cisco 2851 router with IOS 15.1(4)M12a
SB6141 has been running for years doing IPv4 & IPv6, with /60 PD, and no problems.
I bought a new CM1100 (fw 2.01.02), plugged a Macbook directly into it, accessed the Walled Garden, and registered it. It worked fine and had both IPv4 & IPv6 addresses.
Then I put the CM1100 in front of the Cisco, and all I got was event logs filling up with TFTP failure errors. No connectivity, just one channel locked. SNR, downstream power, upstream power, all excellent. Plugged the Macbook on it, and again it worked. Plugged a Mac Mini into it, and that worked as well. Then I started pruning the router config and discovered that removing IPv6 from the interface config enables it to work (on IPv4-only). Just re-inserting the "ipv6 enable" statement creates the problem w/o any of the other IPv6 statements.
At this point I am baffled.
How can a router *behind* the CM prevent the CM from connecting with the CMTS???
I've put the SB6141 back into service.
ip address dhcp
ip nat outside
ip virtual-reassembly in
zone-member security ZONE_EXT
ipv6 address dhcp rapid-commit
ipv6 nd autoconfig default-route
ipv6 dhcp client pd hint ::/60
ipv6 dhcp client pd CCPFX rapid-commit
ipv6 dhcp client request vendor
Are you power cycling the CM1100 modem each and every time that you change the device that is being connected to it ?
Then I put the CM1100 in front of the Cisco, and all I got was event logs filling up with TFTP failure errors. No connectivity, just one channel locked. SNR, downstream power, upstream power, all excellent. Plugged the Macbook on it, and again it worked.
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.