egotiation 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 workaround until the misbehaving part can be replaced. Chapter 3: Auto-Negotiation Configuring Auto-Negotiation For Cisco switches, auto-negotiation 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 will com- plain if you attempt to do SO: 2950(config-if)# duplex half Duplex can not be set until speed is set to non-auto value To set the speed of the interface, use the speed command. If the interface has previ- ously been configured, you can return it to auto-negotiation with the auto keyword: 2950(config-if)# speed ? Force 10 Mbps operation Force 100 Mbps operation auto Enable AUTO speed configuration Once you've set the speed, you can set the duplex mode to auto, full, or half: 2950(config-if)# duplex ? auto Enable AUTO duplex configuration full Force full duplex operation half Force half-duplex operation Configuring Auto-Negotiation CHAPTER 4 VLANs Virtual LANs, or VLANs, are virtual separations within a switch that provide dis- tinct logical LANs that each behave as if they were configured on a separate physical switch. Before the introduction of VLANs, one switch could serve only one LAN. VLANs enabled a single switch to serve multiple LANs. Assuming no vulnerabilities exist in the switch's operating system, there is no way for a frame that originates on one VLAN to make its way to another. Connecting VLANs Figure 4-1 shows a switch with multiple VLANs. The VLANs have been numbered 10, 20, 30, and 40. In general, VLANs can be named or numbered; Cisco's imple- mentation uses numbers to identify VLANs by default. The default VLAN is numbered 1. If you plug a number of devices into a s