IPv6 not working. Comcast blames the router.

[neil0311]

I have a RT-N66U running Asuswrt-Merlin v.3.0.0.4.270.26. Comcast has recently enabled IPv6 in my area and my cable modem (Moto SB6120) is showing it enabled.

I posted a thread at the Comcast forums, and the Comcast tech is confirming that IPv6 should be available on the CPE side of the modem. I confirmed my settings on the router with him and the error I get in the system log that shows I'm not getting a v6 address.

The tech is blaming the router. The thread is located here:

http://forums.comcast.com/t5/Basic-.../Asus-RT-N66U-router-IPv6-Issues/td-p/1693763

What I see on boot in the router log is the following which the Comcast tech is describing in the thread as "odd":

Dec 31 19:00:11 dnsmasq[536]: using nameserver 75.75.76.76#53
Dec 31 19:00:11 dnsmasq[536]: using nameserver 75.75.75.75#53
Dec 31 19:00:12 dhcp6c[578]: client6_send: transmit failed: Cannot assign requested address

Is there anything else that I can check? I can't call Asus as suggested because the router isn't running stock firmware, and even if I reinstall the stock firmware, and it works, it doesn't really solve the problem of why it's not working with the Merlin build.

Any help is appreciated.

 

[neil0311]

I should probably add that I have run the IPv6 test provided by Comcast and it shows that I am not getting and don't have an IPv6 address.

My computer is running Windows 8 and is configured for IPv6.

 

[cmillar6]

Did you try powercycling the Modem?

 

[RogerSC]

Did you try powercycling the Modem?

Good point, I would turn off both the modem and the router (with IPv6 enabled already), and leave the modem off for a few minutes (10 minutes, maybe). Then turn on the modem, and let it come up, and the lights on it stabilize. Then turn on the router and let it come up. Give the router some time to get going on IPv6...this can happen immediately or may take 15 minutes or even an hour to come up on IPv6.

If you've already tried this, then you could try borrowing a router that is known to work with IPv6, and try it. I'm also on Comcast, and IPv6 works here, same router, same firmware version (different cable modem, though).

 

[RMerlin]

I know Asus has done a lot of work specifically for Comcast's IPv6 support. What Comcast is doing different from other ISPs that requires custom support, I don't know. But I've heard of quite a few users being able to use Asuswrt and Asuswrt-Merlin on their Comcast IPv6 connection without any problem. You could try a different FW version to see if it's a regression, or if it might have been fixed in 354.

 

[RogerSC]

I know Asus has done a lot of work specifically for Comcast's IPv6 support. What Comcast is doing different from other ISPs that requires custom support, I don't know. But I've heard of quite a few users being able to use Asuswrt and Asuswrt-Merlin on their Comcast IPv6 connection without any problem. You could try a different FW version to see if it's a regression, or if it might have been fixed in 354.

As I said, I'm using the same model router and same firmware version that the OP is using, and IPv6 is working well for me. So that doesn't fit the classic definition of regression (or firmware bug, for that matter *smile*). There may be something wrong with the modem and/or router that the OP has, or a bad router firmware flash happened, but the firmware is working fine for IPv6 and Comcast here. There could even be something not quite right on the Comcast end, but I suppose that they have a standard implementation that they use (which doesn't guarantee that they get ir right every time *smile*).

 

[neil0311]

Did you try powercycling the Modem?

Yes. Tried both soft and hard reboot.

 

[neil0311]

You could try a different FW version to see if it's a regression, or if it might have been fixed in 354.

If I do flash back to the stock firmware, there's no issue going back and forth, correct?

From what I've read, .354 is foobar, correct?

 

[RMerlin]

If I do flash back to the stock firmware, there's no issue going back and forth, correct?

Correct. I work on maintaining 100% back and forward compatibility.

From what I've read, .354 is foobar, correct?

The wireless driver for the RT-N66U has issues on the 5 GHz band with 40 MHz band width. Some people are fine with it, others can't even connect - it depends on the wireless card you are using.

 

[neil0311]

Thank you.

So I know, are there any other diagnostics or other events to look for in the logs to try and narrow down the cause of the problem?

To me, it looks like the router connects, does a DHCP request to the server for an address, and gets no reply. The Comcast guy calls that "odd" but not sure what I should see or how I can tell if there is an issue on my premises or on Comcast's.

Any pointers, otherwise it's just a matter of trying out firmware and calling Asus, which I'm not really prepared to waste time doing right now. There is nothing that I specifically need IPv6 for, so no reason to waste cycles on fixing something that doesn't much matter at this point.

 

[neil0311]

One last thing to mention. May be totally a coincidence with no connection to IPv6, but since enabling it earlier today, these errors have been popping up in the system log of the router.

May 16 14:17:38 kernel: Neighbour table overflow.
May 16 14:17:38 kernel: Neighbour table overflow.
May 16 14:17:38 kernel: Neighbour table overflow.
May 16 14:17:38 kernel: Neighbour table overflow.
May 16 14:17:38 kernel: Neighbour table overflow.
May 16 14:17:38 kernel: Neighbour table overflow.
May 16 14:17:39 kernel: Neighbour table overflow.
May 16 14:17:39 kernel: Neighbour table overflow.
May 16 14:17:39 kernel: Neighbour table overflow.
May 16 14:17:39 kernel: Neighbour table overflow.
May 16 14:18:07 kernel: printk: 650 messages suppressed.
May 16 14:18:07 kernel: Neighbour table overflow.
May 16 14:18:07 kernel: Neighbour table overflow.
May 16 14:18:07 kernel: Neighbour table overflow.
May 16 14:18:07 kernel: Neighbour table overflow.
May 16 14:18:07 kernel: Neighbour table overflow.

 

[RocketJSquirrel]

Comcast Tuska is one of the most responsive employees on the Comcast forum. Stay with her and she will help you if she can.

The Neighbour Table overflow is also common. Search this subforum for easy workarounds. In my case, after several months of these errors, they ceased.

In other words, both of these issues are Comcast's, not Asus's.

 

[cmillar6]

Merlin's latest build with Comcast and ipv6 works just fine, I have Comcast and a AC66u and ipv6 works great although I get flooded with neighbor table overflow errors but that's a different issue.
A quick google search points to compatibility issues for select users with your particular modem and Comcast. You could perhaps have them come out with one of their rental modems to troubleshoot further.

 

[RogerSC]

Yes, Chris Tuska was the Comcast person that told me that IPv6 was provisioned in my area, and should work. Very helpful person.

 

[saintdev]

I have Comcast and IPv6 working. I needed a new modem (Ubee model). After that all I had to do was to set IPv6 to "Native with DHCP-PD", then reboot the router. You MUST reboot the router after changing the IPv6 setting, otherwise it will not acquire an IPv6 address. Other than that, it just worked for me.

Also, other actions will cause it to lose the address.

 

[steelman10]

I have a RT-N66U running Asuswrt-Merlin v.3.0.0.4.270.26. Comcast has recently enabled IPv6 in my area and my cable modem (Moto SB6120) is showing it enabled.

I have the same setup as you including the cable modem and router firmware. My IPv6 is working well with Comcast. I had no issues getting it to work.

The only issues I have had are 1. the neighbor table overflow warnings (which did not seem to impact connectivity) and 2. open ports showing up in a security scan. I am using an init-start script to address the neighbor overflow errors and a firewall-start script to take care of the security issue. So far the issues seem to be resolved.

 

[cmillar6]

I have the same setup as you including the cable modem and router firmware. My IPv6 is working well with Comcast. I had no issues getting it to work.

The only issues I have had are 1. the neighbor table overflow warnings (which did not seem to impact connectivity) and 2. open ports showing up in a security scan. I am using an init-start script to address the neighbor overflow errors and a firewall-start script to take care of the security issue. So far the issues seem to be resolved.

What did you set your ARP cache limits at to resolve the neighbor table overflow errors? I've set mine to 4096 and still see them, I just live with it now.

 

[steelman10]

What did you set your ARP cache limits at to resolve the neighbor table overflow errors? I've set mine to 4096 and still see them, I just live with it now.

I set the same limits for IPv4 as IPv6. I do not know if it makes a difference but it does not hurt. I have not had any errors since running the script. Here are the values:

echo 512 > /proc/sys/net/ipv6/neigh/default/gc_thresh1
echo 2048 > /proc/sys/net/ipv6/neigh/default/gc_thresh2
echo 4096 > /proc/sys/net/ipv6/neigh/default/gc_thresh3
echo 512 > /proc/sys/net/ipv4/neigh/default/gc_thresh1
echo 2048 > /proc/sys/net/ipv4/neigh/default/gc_thresh2
echo 4096 > /proc/sys/net/ipv4/neigh/default/gc_thresh3

 

[RocketJSquirrel]

open ports showing up in a security scan. I am using an init-start script to address the neighbor overflow errors and a firewall-start script to take care of the security issue. So far the issues seem to be resolved.

Can you tell us more about the open port security issue? What's the problem and how did you fix it? Thanks!

 

[RMerlin]

Can you tell us more about the open port security issue? What's the problem and how did you fix it? Thanks!

Basically, all IPv6 traffic gets routed to your LAN, with no firewalling. See this ticket for a discussion on the matter.