Network Warrior

When Auto-Negotiation Fails

When auto-negotiation fails on 10/100 links, the most likely cause is that one side of the link has been set to 100/full, and the other side has been set to auto-negotiation. This results in one side being 100/full, and the other side being 100/half.

Figure 3-1 shows a half-duplex link. In a half-duplex environment, the receiving (RX) line is monitored. If a frame is present on the RX link, no frames are sent until the RX line is clear. If a frame is received on the RX line while a frame is being sent on the transmitting (TX) line, a collision occurs. Collisions cause the collision error counter to be incremented—and the sending frame to be retransmitted—after a random back-off delay.

Figure 3-1. Half duplex

Figure 3-2 shows a full-duplex link. In full-duplex operation, the RX line is not monitored, and the TX line is always considered available. Collisions do not occur in full-duplex mode because the RX and TX lines are completely independent.

Figure 3-2. Full duplex

When one side of the link is full-duplex, and the other side is half-duplex, a large number of collisions will occur on the half-duplex side. Because the full-duplex side sends frames without checking the RX line, if it's a busy device, chances are it will be sending frames constantly. ...