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RV340W Comcast - IPv6 very unreliable, IPv4 is perfect


I am unfortunately having to use an RV340W router for a time, and am trying to get IPv6 prefix delegation working on Comcast. This was working flawlessly for years with a Ubiquiti EdgeRouter device up until 2 days ago when I installed the Cisco, so I know there isn't any problem with Comcast or the connection.


The problem is that every ~10-20 IPv6 packets are dropped. This is what a continuous IPv6 ping looks like from a Linux machine connected directly to the ethernet ports:

[user@Linux-RPi ~]$ ping
PING (2607:f8b0:4004:808::2005)) 56 data bytes
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=1 ttl=108 time=29.4 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=2 ttl=108 time=34.7 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=3 ttl=108 time=31.2 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=4 ttl=108 time=34.7 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=5 ttl=108 time=32.4 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=6 ttl=108 time=28.9 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=7 ttl=108 time=36.8 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=8 ttl=108 time=28.9 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=9 ttl=108 time=29.0 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=10 ttl=108 time=29.0 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=11 ttl=108 time=28.8 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=12 ttl=108 time=29.9 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=13 ttl=108 time=29.3 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=14 ttl=108 time=32.1 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=15 ttl=108 time=28.9 ms
ping: sendmsg: Network is unreachable
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=17 ttl=108 time=30.0 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=18 ttl=108 time=29.4 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=19 ttl=108 time=29.1 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=20 ttl=108 time=29.4 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=21 ttl=108 time=29.0 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=22 ttl=108 time=31.4 ms
64 bytes from (2607:f8b0:4004:808::2005): icmp_seq=23 ttl=108 time=34.0 ms

This happens on every device, wired or wireless, on Apple and Windows devices. It's not just ping; as you browse around the web, every minute or so devices report that the "route to host" is lost. It's entirely random. In practice, it means that web pages frequently fail to load and must be manually reloaded. Video calls frequently complain about not being able to start.

If I disable IPv6 entirely on the router, everything works perfectly.


Here is the LAN config:

Screen Shot 2021-03-29 at 2.54.50 AM.png


Here is the LAN config:


Screen Shot 2021-03-29 at 2.58.50 AM.png


Here is router advertisement config:


Screen Shot 2021-03-29 at 3.04.46 AM.png


Can someone provide some support? Like I said, a Ubiquiti router was running on this same connection for years without a single IPv6 hiccup. 

8 Replies 8

VIP Mentor VIP Mentor
VIP Mentor


         - Make sure to use  the latest firmware available on the RV.


The device has the latest firmware (

Any other ideas? 


Wow, I just realized I posted about this exact same issue almost 2 years ago


Of course I got zero responses then.


Does anyone actually read these forums? And not respond with worthless "upgrade the firmware" responses?



Hi there,


From what I see in your logs and the config you've done on the RV the connection is working with intermittent interruptions. What we could advise here is to do a WAN packet capture and try to see why that happens (there might be a Comcast issue as well). .There aren't many options here in the forum we could further advise. I would recommend open a ticket with STAC so they can investigate and eventually escalate the case. Contact details are as follows: