llisions, while a full-duplex interface does not. Since full duplex means never having to test for a clear-to-send condition, a full-duplex interface will not record any errors in this situation. The problem should present itself as exces- sive collisions, but only on the half-duplex side. Figure 3-3 shows a link where autonegotiation 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 autonegotiation failure scenario In the real world, if you see that an interface that is set to autonegotiation 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 autonegotiation ports should almost never end up config- ured for half duplex. Chapter 3: Autonegotiation Autonegotiation Best Practices Using autonegotiation 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 autonegotiation, make sure the other side is also set to autonegotiation. 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 autonegotiation mechanism than the one described in this chapter. Gigabit Ethernet should thus always be set to auto- negotiation, unless there is a compelling reason not to do SO (such as an interface that will not properly negotiate). Even then, this should be considered a temporary work- around until the misbehaving part can be replaced. Configuring Autonegotiation For Cisco switches, autonegotiation is enabled by default. You can configure the speed and duplex mode manually with the speed and duplex interface commands in IOS. You cannot set the duplex mode without first setting the speed. The switch wil