enc0 without MULTICAST flag

[Luca Di Gregorio]

I configured a site-to-site vpn with ike2,
it works for unicast traffic.

I need to enable ospf on the 2 hosts via enc0, but
ifconfig enc0 shows:

enc0: flags=41<UP,RUNNING>
        index 2 priority 0 llprio 3
        groups: enc
        status: active
        inet .......

So, ospfd shows, in /var/log/daemon:

  ospfd[53563]: if_join_group: error IP_ADD_MEMBERSHIP, interface enc0
address 224.0.0.5: Can't assign requested address

How can I set the flag MULTICAST on enc0?
man ifconfig doesn't say how to to it.