I have recently started seeing repeated logs appearing in my CCR2004, related to the port that’s facing my ISP’s ONT. radvd, warning invalid mtu 1504 on sfpplus1 from fe80::b2a6:51ff:fe6c:d8e0 The log entries started at night after a brief outage, and I know my ISP tends to do maintenance durin…

If that warning message bothers you, increase the MTU value of sfpplus1 to >= 1504 (but still smaller than L2MTU of that interface). Or you can ignore that warning unless it overflows your log with hundreds of entries. As for the need for /ipv6 settings set accept-router-advertisements=yes: If y…

Thank you for your detailed explanation, really helpful indeed. I found your reply in 7.21, will keep an eye out there as well!

zneva, does the thumbs down mean> the 2004 is not the problem? RoS is the problem? Op config is the problem? ipv6 issue ? Thumbs down is easy but why not just state the something else??

According to MikroTik, the RA bug should be fixed soon: [image] V7.21 [stable] is released! Announcements @nogaff So I heard back from Mikrotik and provide the following communications that came in last night my time. First response: Unfortunately, in your …

Wait, so I’ve bitched to my ISP for nothing? They send RAs with M and O unset (the RA contains nothing else) AND they give me a prefix and an address via DHCPv6, obviously no route installed if I uncheck ‘add-default-route’ in the DHCPv6 client. I still think that they need to set at least managed…

Yes, they should set M when they give out address via DHCPv6. But this is not related to the default route. If RA lifetime is positive then the default route should be installed, regardless of whether there is any prefix (in RA message) or DHCPv6 server available.

My ISPs set M but nothing else. 7.22rc1 has improvements that fix my particular error. Curious if yours is addressed as well.

Nope, and I don’t think that it should install a route in this case. If the RA doesn’t contain a prefix and M=0 the router thinks that it has no reason to install a route since there’s no public prefix offered. Even if my ISP does provide one via DHCPv6. Probably MikroTik stops at does RA contain p…

However, I think RouterOS should still install the route even if M=0 and the RA has no prefix. Routes and address/prefix assignment are different things. If we read RFC4861, specifically the section: 6.3.4 . Processing Received Router Advertisements On receipt of a valid Router Advertisement, a …

So you say that MikroTik should install a route even if the RA looks like this? [RA]

Yes, the router lifetime is advertised as 4500s (and has RA Preference Medium), so according to the RFC, fe80::1a5b:ff:feb9:fec9 should be added to the default router list. And a default router by definition provides a default route. The list in this case would also have only one item because no oth…

Is my upstream ISP sending me bad Router Advertisement?

RouterOS General

CGGXANNX February 13, 2026, 10:27am 22

7.21.3 should finally fix this (hopefully)