EM(4) - Link flap when bridged and adding members (em bridge flap)

[Ryan Stone]

>From a quick look at the code, whenever an interface is added to a
bridge, if that interface does not support a feature currently enabled
on the bridge then it has to disable that feature on all member
interfaces of that bridge.  That would re-init the em interface, which
would cause a link flap.

Take a look at the ifconfig output for em0 and epair0 before epair0 is
added to the bridge.  You will probably see that one or more features
that em supports is not supported by epair (I would bet on something
like LRO or TSO)..  The solution would be to disable those features
from em0 before adding it to the bridge (you can persist this in
rc.conf with ifconfig_em0="...")