The QSFP+ Loopback function modules are used for testing

In addition to the electrical loopback function, the QSFP+ provides an MSA standard 2-wire serial communication interface to digital diagnostics and preloaded EEPROM two-page memory map. The QSFP+ Loopback also contains a two-colored LED indicator that displays the operating status of the module.
The provides QSFP/QSFP+ power Class (standard), or 4 loading using a customer supplied +3.3V voltage supply.QSFP+ Loopback adapter modules are used for testing QSFP ports on system boards and QSFP switches. The electrical loopback provides a cost effective method for QSFP port testing. Transmitting data from the host is electrically routed (internally to the loopback module) to the receiving data outputs and back to the host.