IEEE 802.3ad Link Aggregation defines a protocol called Link Aggregation Control Protocol (LACP) that’s used by OS X Server to aggregate (combine) multiple ports into a link aggregate (a virtual port) that can be used for TCP and UDP connections.
When you define a link aggregate, the nodes on each side of the aggregate (for example, a computer and a switch) use LACP over each physical link to:
Determine whether the link can be aggregated
Maintain and monitor the aggregation
If a node doesn’t receive LACP packets from its peer (the other node in the aggregate) regularly, it assumes the peer is no longer active and removes the port from the aggregate.
In addition to LACP, OS X Server uses a frame distribution algorithm to map a conversation to a specific port. This algorithm sends packets to the computer on the other end of the aggregate only if packet reception is enabled. In other words, the algorithm won’t send packets if the other computer isn’t listening.
Mapping a conversation to a specific port guarantees that packet reordering doesn’t occur.