So I’m curious your configuration – you say you are running ND on the WAN interface. That means your wan is Advertising itself as a router – i don’t believe that’s where your default route is coming from.
I thought it was best practice NOT to advertise out on the WAN, but only to advertise to your internal segments.
My router advertising itself on WAN was a concern. But sniffing did not confirm that. Somehow RouterOS doesn’t do this. In addition, I set ra-lifetime=0 (ra-lifetime=none, RouterOS replaces 0 for none) which excludes the router from the Default Router List selection on receivers if ever such advertisement takes place. I set ra-preference=low just in case, although I don’t think it does anything because the router is not in the Default Router List.
RAs are not the only ICMPv6 messages related to Neighbor Discovery. This is the reason for running ND on WAN. In particular it’s important for RouterOS to learn via neighbor solicitation that upstream gateway is a Router as the appropriate flag will be set in the advertisement sent in reply.
I wish RouterOS was more specific and allowed control over whether I want to run RAs in addition to other Neighbor Discovery facilities.
I suspect you are getting the default route because you enabled “accept router advertisements” in IPv6 settings. That’s about inbound RA. Is that what you have configured? Or has this been off the whole time?
Correct, I rely on RAs from the upstream for IPv6 configuration. These RAs set flags that require me set up a DHCPv6 client for address and prefix.
The default route is coming to you inbound from Xfinity’s RA. Its Xfinity that is declaring that high MTU. The radvd daemon that is running in the mikrotik is the thing that’s complaining about it to the log. (just clarifying)
Correct, this is the behavior that I observe.
(also, you mentioned your cable modem can’t pass jumbo packets, my modem doesn’t indicate anywhere in its specs. Why do you say there is no way? Not that it matters much, but i’m curious).
My Arris SB6141 (of which Comcast is fully aware of) uses DOCSIS 3.0 which supports MTU of up to 1500 per various sources I Googled.