Therefore I tried running ospf in broadcast mode as well, but then it changed automatically: 

<WARN> myOSPF3: Cannot use interface tun0 as broadcast, forcing ptp

I tried the tap-Interface and it's working (or at least the neighbours were detected) but as said, my system has to use tun and I cannot change it. So there is propably no solution for such settings. I will try bgp instead. Thank you for your help.

2018-04-03 16:18 GMT+02:00 Ondrej Zajicek <santiago@crfreenet.org>:
On Tue, Apr 03, 2018 at 08:05:41AM -0600, Michael McConnell wrote:
> OpenVPN won’t do multicast over TUN, only TAP.

Well, that would be silly from OpenVPN. But tcpdump output from Dawid K
shows that multicast packets are propagated throught TUN:

> 06:59:00.439738 IP (tos 0xc0, ttl 1, id 15270, offset 0, flags [none], proto OSPF (89), length 64)
>     server > 224.0.0.5: OSPFv2, Hello, length 44
>         Router-ID repo.traffic.local, Backbone Area, Authentication Type: none (0)
>         Options [External]
>           Hello Timer 10s, Dead Timer 40s, Mask 0.0.0.0, Priority 1
> 06:59:02.449363 IP (tos 0xc0, ttl 1, id 18875, offset 0, flags [none], proto OSPF (89), length 64)
>     10.29.0.6 > 224.0.0.5: OSPFv2, Hello, length 44
>         Router-ID 192.168.21.17, Backbone Area, Authentication Type: none (0)
>         Options [External]
>           Hello Timer 10s, Dead Timer 40s, Mask 0.0.0.0, Priority 1

--
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago@crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."