Recently, I noticed the following messages in the system log of a Ubuntu 14.04 LTS host that is running radvd:
Jun 28 13:15:33 myhost radvd: do you need to add the UnicastOnly flag? Jun 28 13:15:33 myhost radvd: interface ovsbr0v20p0 does not support multicast
At first, I was surprised, but after writing a small program, that checks for the IFF_MULTICAST flag in the interfaces attributes, I realized that the interface in fact does not support multicasts (or at least says so).
As it turns out, virtual interfaces added to an Open vSwitch bridge do not support multicasts in older versions of Open vSwitch (Ubuntu 14.04 LTS ships with Open vSwitch 2.0.2). I cannot tell for sure, in which version muticast support has been added. Looking at the changelog, it looks like this is present since Open vSwitch 2.4.0. Anyway, the version of Open vSwitch shipped with Ubuntu 16.04 LTS (Open vSwitch 2.5.0) supports multicasts on virtual interfaces and the IFF_MULTICAST flag is set for those interfaces.
This means that radvd should not have any problems when using an Open vSwitch virtual interface on Ubuntu 16.04 LTS.