Comcast recently made network upgrade in my area. Since then, every 2-3 seconds, my log shows the message: invalid mtu 9192 on ether1_WAN from fe80::21c:73ff:fe00:99. To make the Winbox log useful, I have to manually filter out the message.
Is there a way to suppress this message?
Does the busy log file cause problems? It is annoying!
Should I disable IPv6?
Comcast has been worthless with helping. The said my old DOCSIS 3.0 modem was the problem. They sent their monstrous Gateways, which, as expected made no difference. It also has two wifi networks which cannot be disabled, which clutters my spectrum.
I doubt Comcast will solve the problem by changing their MTU broadcast.
It seems that many ISPs suffer from same problem. Solution so far is to disable logging that message.
Default logging setup is to keep logs in memory. So excessive logging doesn't harm hardware (e.g. due to excessive wear of flash storage). But it does clutter logs with useless entries making it much less readable and uses up log line limits so there's less space for (potentially) useful entries.
And to filter out those messages, add "!radvd" to logging action for topic "warning". Like this:
/system logging
set [find topics="warning"] topics=warning,!radvd
And no, you don't have to disable IPv6 if it otherwise works for you. After all, the message is a warning, not error (or critical).
Does this actually work? I.e. is ISP's router configured with jumbo MTU as well?
Because if egress L3 interface has large MTU, then TCP stack will try to utilize it (in this particular case only traffic originating from router itself will likely be affected). And transmission of jumbo packets will fail (I can think of at least two different failing cases) if receiver (ISP's router in this case) is not configured with matching MRU.
The log noise immediately stopped when I made this change. (I re-checked before posting.)
is ISP's router configured with jumbo MTU as well?
The router is a Chateau PRO ax. The piece you are referring to is an Arris SB6190 in bridge mode; not 100% pure modem since I can still access it by its hard-coded IP on the inside, but close enough. The letters "mtu" don't appear on any of its status screens.
That will be what I need as it limits suppression to the MTU.
In the meantime, I will use the message suppression rule @mkx suggested, which is working.
I wasted over an hour with Claude and ChatGPT. They gave instructions for settings that don’t exist or didn’t work. When I put in @mkx ‘s rule, Claude said it was a better solution than what it was providing.
That is problem with that all Chat's ... suggest the best fitting answer, not the right one. When you provide them the right one, "they" are "so sorry and promise to be better".
