- The device you are trying to connect to is not on same subnet.
- The switch or another network device blocks
linklooppackets at hardware level.
linkloopsends a “special” (but standard) 802.3 frame type. (XID or perhaps TEST). The IEEE specs say such a frame should elicit a response, but precious few implementations outside of HP (UX, perhaps iX) seem to do it.
- The device you are trying to connect to is bad (faulty NIC).