lagg(4) question

[Brett Glass]

I'm interested in using lagg(4) to provide failover from a main 
wireless link to a backup link. Unfortunately, one of the common 
failure modes, when a link fails, is for the Ethernet port on the 
radio to stay up but not pass any traffic. Does lagg(4) test the 
link, by actually trying to send packets, to determine if the link 
has failed? Or does it rely upon the Ethernet link losing "carrier"?

--Brett Glass