Check < \- frames that are deferred can't be sent out the destination port right away and get queued. _Excess_ deferred frames are those dropped when the output queue is overloaded.
There are several possible causes:
* the output port is linking at a slower speed than expected
* a stream from a faster link is funneled through a trunk group (usually, you can't successfully run a fullspeed 10G link through a 10x 1G trunk)
* your network architecture has a bottleneck and might need rethinking