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24 Messages
Failing to get or losing IPv6 address
I have been using IPv6 on Comcast for several years with only occasional problems, usually following some disruption of cable service that requires reboots of modem and router.
Recently, and coincident with some apparent changes in Comcast servers, I am sometimes, not always, able to get an IPv6 address (2601::...) after rebooting my router, but then a half hour later it's gone. This is the case with every one of my LAN clients. After the reboot, they will pass the test at test-ipv6.com. After about 30 minutes, the test says they all have no (public) IPv6 address even though the clients still think they do. A DHCP release/renew on the clients produces a local IPv6 address (fe80::...).
My modem is an ARRIS SB8200, and my router is a TP-Link Archer C7. That combination was working until about 1 or 2 weeks ago when this problem began. The router is set for a DHCPv6 WAN connection with a 64-bit prefix designation and IPv6 SLAAC LAN address assignment.
I suspect a problem with DHCPv6 address assignment and/or renewal that has crept in with the apparent Comcast server configuration changes in this area (northeast Massachusetts). Other people have short DHCP renewal times. There was also a problem about 5 years ago that seems similar in which DHCPv6 renewal reply packets had a TTL that was too small so that the reply never reached the DHCPv6 client on the router.
Does anybody else have a similar problem or any insights into this?
flatlander3
Problem Solver
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1.5K Messages
2 years ago
I did have something exactly like that previously with their DHCPv6 servers. You'd get an IPV6 address on the WAN -- /64 prefix is all you can get in my market on residential accounts -- and it would work for a time, clients would be happy for a few days. Then, out of the blue, routing would be boned on the clients. It was the lease renew failing on the WAN.
I could make the problem less frequent by not releasing the address on a renew -- sometimes called "Do not allow PD/Address release", but flipping on debugging showed that sometimes the DHCPv6 server just wasn't there, and the requests timed out. I don't know if their gear upstream was flaking out, or under attack, or what was going on.
Ultimately, I just disabled IPV6 entirely until earlier this year when I tried it again. For whatever reason, it started working and is pretty solid now.
Another thing you might have going on is your router advertisements -- whichever device you have that is handling that. I'm doing it with a firewall, and have bridge mode set for coax so that part is just a modem. I don't know what your gear has for configuration options. SLAAC alone didn't work for me here. Clients would never get address. Revalent Options:
Managed = firewall sends RA packets with dhcpv6 only
Unmanaged = firewall sends RA packets with SLAAC -- disables dhcpv6
Stateless = firewall sends RA packets with SLACC, but dhcpv6 for other things (NTP, DNS)
Assisted = firewall sends RA packets with SLAAC or dhcpv6
The LAN interface is set to "track interface" (Tracks the WAN) instead of dhcpv6.
Now I couldn't tell you why, but the only way I can make it work is in "Assisted" mode, which doesn't make a whole lot of sense. Maybe it does to you. I can't get any information out of Xfinity on their DHCPv6 implementation and all support can do is read me silly marketing nonsense bullet points all about "better and faster" if I use their insecure, crippled firmware rental gateway.
If the leases are short, maybe they're working on stuff in your area. They didn't seem particularly short in my area (3 days or so) when I was having the problem, but who knows. Perhaps they changed their implementation in your area without telling anyone. There's always disabling IPV6 entirely until there isn't an issue in your area as an option (although perhaps not a great one).
Maybe someone has a better idea.
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trsm3
Frequent Visitor
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24 Messages
2 years ago
In addition to LAN clients not having a working IPv6 address, the router did not have an IPv6 WAN address, although it had a delegated prefix on the LAN side.
Today I did a DHCPv6 "renew" on the router. A WAN address snapped in along with a new prefix. The LAN clients all picked up new and working IPv6 addresses. 4 hours later it's still solid.
Interestingly, the location shown by any of those "what's my IP" sites is 60 miles away. In the past few weeks my apparent locations when I had working addresses were at 2 other locations, each about 40 miles away.
It's looking more to me like Comcast is reprovisioning servers in the area and not getting it quite right. We'll see.
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EG
Expert
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110K Messages
1 year ago
@user_7qtgao
Please create a new topic of your own here on this board detailing your issue. Thanks. The original poster has not returned. 5-month-old dead thread is now being closed.
For future reference, it is better to submit your own post as it creates a ticket to get help, and posting on an old thread can delay getting help.
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