Still this same phenomenon with Latest Base System 2.3.1.a.20160508.0101.
Due to 2 PID's of dhcp6c [Why?], WAN(igb0) cannot succeed, and/then LAN(igb1) is appointed but not effective.
To get rid of the ISP-IPv6 over igb1, a reboot is necessary.
This time I saw PPPoE(IPv4) was not established. So, is it eventually necessary that IPv6 should wait for IPv4 to finish successfully?