Intel Ethernet X520 10GbE Dual Port KX4 Mezz User Manual page 133

Set Up as Responder
This allows the user to set up the adapter as a responder so a connected system can perform the network test portion
of the diagnostics tests. Although you can use a variety of adapters as the responder and connect directly or through a
switch, the best results are obtained with a cross-over cable and a same-type adapter.
When you press <Esc>, the responder operation is canceled and control is immediately returned to the Networking
menu.
Detect Spanning Tree
Spanning trees can be troublesome in a networking configuration. The Detect Spanning Tree option attempts to detect
if a spanning tree exists on the network. This is done by resetting the link and listening for spanning tree packets.