Pablo Averbuj

Comcast IPv6 redux

August 3, 2016 | Comments Off

TL;DR: IPv6 router configuration can impact cablemodem provisioning on Comcast. Don’t let programmers run networks.

Follow up to my previous post on getting IPv6 configured on a Cisco router on Comcast.

I recently realized I was an idiot (for many reason) but in this case because I was still paying Comcast $10 a month for a cablemodem. Previously we had our home telephone service with Comcast and my research had indicated that you couldn’t buy a retail modem in that case (but I’m not so sure now). But in either case we moved back to Vonage and so after it while it dawned on me that we could save the rental fee.

After truly minimal research I ordered an Arris SB6183 from Amazon. I received it and activated it but I had a lot of trouble getting IPv4 reliably. In fact, as long as it was plugged into the Cisco it would not get sync with Comcast. If I unplugged the ethernet it would get sync and afterwards I could plug it in and sometimes it would even work.

That wasn’t a sustainable solution so I then ordered a TP-Link TC7160. I got the modem, it would get sync but I still wouldn’t get IPv4 assignments and when I got IPv6 assignments they wouldn’t route.

This was fairly strange, and with both modems I could plug them into a laptop and have them work normally. I could even have the laptop spoof the MAC address of the router interface, plug it into the router. The router would get DHCP but then not be able to route traffic. But the same router/configuration had worked with the old Comcast-rented modem without issue. I pondered this for a good while and it most likely had to be something IPv6 related.

I started turning off IPv6 settings in the router until it would work. The key configuration was “ipv6 unicast-routing”. When that was disabled, it would get IPv4 and work great. Enabled (even after IPv4 was assigned and working) and it would instantly stop routing traffic.

I’ll spare you the details of the fiddling that ensued but this is what I found: I needed to suppress IPv6 route announcements on the WAN interface.

The fix:

conf t
 int GigabitEthernet8
  ipv6 nd ra suppress

Analysis/Speculation:

I believe what happens, and what changed, is that the newer cable modems are getting their provisioning over IPv6 while the old modem was provisioning over IPv4 (or something else that’s not IPv6). The route announcements were confusing the cablemodem and causing it to prefer sending its provisioning traffic to my router instead of the Comcast next-hop. When the route announcements stop it works as intended.

Bonus #1:

Even though I saw recommendations for the Arris SB6183, the TP-Link TC-7160 boots faster and runs cooler than the Arris. The amount of heat from the Arris was very surprising, the coax connector was too hot to touch, and it heats up very quick. The morning after I switched I felt the coax connector on the TP-Link and it was normal as I would have expected.

Bonus #2:

Comcast doesn’t care about your IPv6 DHCP hints.

! doesn't work.
ipv6 dhcp client pd hint ::/56