[libvirt] macvtap - no incoming ipv6 traffic processed on kvm host unless i start tcpdump on interface

Dear folks, I'm using for the first time macvtap interface for my virtual machines in bridged mode. VM -> HOST -> Router -> INTERNET This works fine for ipv4 connectivity. For ipv6 my virtual machines receive appropriate v6 address from radvd but are not able to receive answer packages from outside (ping -t -6 google.de was started inside VM). I see the ping request/response on my router: 14:10:52.147834 IP6 2a01:198:200:8350:dc8b:cd82:144e:14eb > 2a00:1450:4001:806::1018: ICMP6, echo request, seq 108, length 40 14:10:52.182073 IP6 2a00:1450:4001:806::1018 > 2a01:198:200:8350:dc8b:cd82:144e:14eb: ICMP6, echo reply, seq 108, length 40 14:10:55.179874 IP6 2a01:198:200:350::2 > 2a00:1450:4001:806::1018: ICMP6, destination unreachable, unreachable address 2a01:198:200:8350:dc8b:cd82:144e:14eb, length 88 But i do not receive the reply on the VM. However on the KVM host - when i start a tcpdump on the macvtap interface with root@s1:~# tcpdump -ni macvtap0 ip6 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on macvtap0, link-type EN10MB (Ethernet), capture size 262144 bytes 14:12:37.134516 IP6 2a01:198:200:8350:dc8b:cd82:144e:14eb > 2a00:1450:4001:806::1018: ICMP6, echo request, seq 129, length 40 14:12:37.188529 IP6 fe80::12fe:edff:fee6:cfa > ff02::1:ff4e:14eb: ICMP6, neighbor solicitation, who has 2a01:198:200:8350:dc8b:cd82:144e:14eb, length 32 14:12:37.189040 IP6 2a01:198:200:8350:dc8b:cd82:144e:14eb > fe80::12fe:edff:fee6:cfa: ICMP6, neighbor advertisement, tgt is 2a01:198:200:8350:dc8b:cd82:144e:14eb, length 32 14:12:37.189202 IP6 2a00:1450:4001:806::1018 > 2a01:198:200:8350:dc8b:cd82:144e:14eb: ICMP6, echo reply, seq 129, length 40 packages starting to get processed and VM receives replies. Any idea what is happening here? Cheers, Stefan