Hello -
I recently replaced a hap ac with a RB5009 running 7.10. I also enabled ipv6 which seems to function as expected. The only thing is I am seeing this message in the log every 2 seconds or so.


That MAC addr associated with that ipv6 addr shows up as the only router in ipv6/neighbours. That interface is connected to a cable modem->ISP. I've tried setting a 1500 MTU in ipv6/ND for 'all' interfaces with no difference. Is the log message anything to be concerned with besides being an annoyance?

That is your ISP's BNG router. They forgot to suppress IPv6 RAs from their side, and it seems they are using jumbo frames on their side. Which of course won't work for you "9192" as cable modems can't vary that kind of packet size.

You can try contacting the ISP and ask them to stop sending you RAs with MTU flag, but it's a long shot.

This actually appears to be new behavior as of 7.9ish, see here

Not sure whether this is just new visibility into an existing problem, or what, but it sure does saturate my logs. I had to drop radvd warnings from being logged at all

MikroTik cannot be held guilty if it highlights improvised ISPs that don't know how to do their job...

Thanks for reporting this issue as I have been seeing as well from Xfinity/Comcast. I had also been seeing it till recent updates of ROS.

This actually appears to be new behavior as of 7.9ish, see here

Not sure whether this is just new visibility into an existing problem, or what, but it sure does saturate my logs. I had to drop radvd warnings from being logged at all

What was your Logging configuration to drop the warnings ??

under /system/logging open the warning entry (on most default configs it's #2 in the list) and add !radvd.

This does blow out ANY radvd warnings but at least my logs aren't burying important stuff in like an hour

MikroTik cannot be held guilty if it highlights improvised ISPs that don't know how to do their job...

Also, this is entirely fair. all my searching has definitely shown this is a comcast problem, and I am not trying to gripe about it as though it's MT's fault

I see it too.

On an RB4011 I can set the MTU of the WAN interface that large, and the log warning stops. As you said, not all RBs can MTU that large.

Alternatively, you might make the default route static and turn off "accept router advertisements". Just copy the route that RA automatically creates leaving out the MTU setting.

Something like:
This is working for me.

On an RB4011 I can set the MTU of the WAN interface that large, and the log warning stops.

I can confirm this.
I have a RB4011 and I did it like you post and also this is working for me.