How to configure macvtap to let it pass multicast packet correctly?

libvirt's macvlan has gained support for multicast. Unfortunately it is disabled by the default setting trustGuestRxFilters="no", and the documentation is not explicit that this breaks multicast. As you observed, breaking multicast also breaks IPv6.

https://bugzilla.redhat.com/show_bug.cgi?id=1035253#c15

You may be able to work around this by manually setting trustGuestRxFilters="yes". There is a limitation: "support depends on the guest network device model, as well as the type of connection on the host". "Currently it is only supported for the virtio device model, and for macvtap connections on the host".

https://libvirt.org/formatdomain.html#elementsNICS

IMO the natural model would be to allow multicast by default. Blocking multicast reception, on a network you think you're directly connected to, is an unpleasant surprise. Especially since macvtap still seems to allow sending multicast packets (as well as spoofed MAC source addresses!).

The answer by sourcejedi contains the solution, but maybe not explicitly enough. Using "virsh edit", set the trustGuestRxFilters attribute on the network device:

<interface type='direct' trustGuestRxFilters='yes'>

See libvirt docs. The same can be done for all interfaces on a libvirt network.