Loopback

The TX datapath can be looped back to the RX datapath in two places:

MII Local Loopback—Data provided to the MII transmit interface passes through a single sampling register and then loops back and outputs on the MII receive interface.
Post Scrambler Loopback—The loopback occurs just before the TX SerDes gearbox. The data is fed back into the TX datapath just after the receive SerDes gearbox. This loopback allows you to exercise a very large portion of the datapaths.