> Check ip neigh output. Does the entry for your default gateway go
> STALE after those 20 minutes?

Yes, exactly:

# ip -6 nei
fe80::1 dev eth0 lladdr 0c:86:10:ed:31:ca router STALE

> Also check the lifetime of any SLAAC ip addresses given in ip addr
> output.


> Do you really need to meddle with the fe80::1 route?

I had no plans to do so.  I just learned during debugging that this would 
reestablish IPv6 connectivity without reboot.

> Do you really
> need an explicit route for fe80::1%eth0? Will it work without?

My hosters docs (Hetzner) recommend that.  They don't specify an IPv6 
default gateway.

> Does adding a route for 2000/3 via fe80::1 dev eth0 help,

nope (just tried it)

> or is it
> really necessary to remove the default route and to re-add it?

yep, connectivity is back

> No need, an fe80::/64 IP address is only valid when an interface is
> added:
> Is the other interface connected? eth1 should not play a role here at
> all.

I had no plans to fiddle with link-local addresses, and of course eth1 
settings should not matter.  I just disabled IPv6 on eth1 for debugging, 
and suddenly IPv6 worked >20min.  Maybe coincidence rather than causality. 

I'd like to learn what's going on here, thanks for your comments.


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