uplex Server Switch Transmitting is always allowed Ethernet interface Ethernet interface There is no collision detection in full-duplex operation 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. The other end of the link, being half-duplex, will listen the RX line, and will not transmit unless the RX line is available. It will have a hard time getting a chance to transmit, and will record a high number of collisions, resulting in the device appearing to be slow on the network. The issue may not be When Auto-Negotiation Fails obvious because a half-duplex interface normally shows collisions. The problem should present itself as excessive collisions. Figure 3-3 shows a link where auto-negotiation has failed. Full Duplex Half Duplex Server Switch Transmitting is always allowed RX line monitored for packets Ethernet interface Ethernet interface Only transmit if no RX activity Figure 3-3. Common auto-negotiation failure scenario In the real world, if you see that an interface that is set to auto- negotiation has negotiated to 100/half, chances are the other side is set to 100/full. 100-Mbps interfaces that do not support full duplex are rare, SO properly configured auto-negotiation ports should almost never end up configured for half duplex. Auto-Negotiation Best Practices Using auto-negotiation to your advantage is as easy as remembering one simple rule: Make sure that both sides of the link are configured the same way. If one side of the link is set to auto-negotiation, make sure the other side is also set to auto-negotiation. If one side is set to 100/full, make sure the other side is also set to 100/full. Be careful about using 10/full, as full duplex is not supported on all 10Base-T Ethernet devices. Gigabit Ethernet uses a substantially more robust auto-n