networkd RA being ignored unless promiscous

[Ede Wolf <listac@xxxxxxxxxxxxxxxx>]

Hello,

I am having two machines that receive their ipv6 address and their 
default route via RA, both on the same physikal link, as is the router.

While one works as expected, the other machine behaves in a way, that it 
gets it's default route after restarting networkd, but once that has 
expired, it get's deleted - despite quite a lot RA having been send way 
beforehand by the router, confirmed with tcpdump (and seen on the other 
machine as well, where the expire counter gets frequently reset 
accordingly).

Wether I am waiting 30 minutes or thee hours, it seems, subsequent RA 
are completely being ignored.
Unless, now for the strage part, I set that corresponding interface into 
promiscous mode.
Being promiscous, suddenly RA are being honored again, the default route 
gets reinstalled and the expire timer gets reset with each subsequent RA.
Removing the promiscous mode and again the route lifetime counts down to 
zero and gets deleted.

So in short, expect the inital RA after restarting networkd subsequent 
RA seem to be ignored unless the interface is in promiscous mode. I've 
compared the ipv6 sysctl of the interface of the machine that works with 
that spooky one, no real differences.

This certainly is not a  general networkd issue, but maybe someone has 
an idea, what network configuration parameters I might have to recheck. 
And since restarting networkd does temporarily fix the issue, I've 
thought, maybe this might be the right place to start.

Thanks

Ede