ucarp

http://forums.gentoo.org/viewtopic-t-936898-start-0.html

failing over multiple interfaces together

  net.inet.carp.preempt     Allow virtual hosts to preempt each other.  It
                            is also used to failover carp interfaces as a
                            group.  When the option is enabled and one of
                            the carp enabled physical interfaces goes down,
                            advskew is changed to 240 on all carp inter-
                            faces.  See also the first example.  Disabled
                            by default.