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5-6 Configuring the Network Interface—Auto-Negotiation

Most modern networking internetworking technologies (for example, hubs, switches, bridges, and routers) now incorporate the auto-negotiation protocol. The protocol enables the Ethernet equipment to automate many of the installation steps. This includes automatically configuring the operating speeds (for example, 10/100/1000Mbps) and the selection of full- or half-duplex operation for the data link. The auto-negotiation protocol is defined in the IEEE Ethernet standard 802.3x for FastEthernet.

The auto-negotiation protocol uses a fast link pulse (FLP) to carry the information between each end of a data link. Figure 5-20 shows a data link. The data rate for the fast link pulses is 10Mbps, the same as for 10BASE-T. The link pulses were designed to operate over the limited bandwidth supported by CAT3 cabling. Therefore, even if a link is negotiated, there is no guarantee that the negotiated data rate will work over the link. Other tests on the cable link must be used to certify that the cable can carry the negotiated data link configuration (refer to Chapter 2, “Physical Layer Cabling: Twisted Pair”).


FIGURE 5-20 The two ends of a data link negotiating the operating parameters.

Auto-Negotiation Steps

Each link partner shares or advertises its data link capabilities with the other link partner. The two link partners then use the advertised capabilities to establish the fastest possible data link rate for both links. In the example of the link partners shown in Figure 5-22, computer 1 advertises that its interface supports 10Mbps. The switch advertises that it supports both 10Mbps and 100Mbps. The network interfaces on each link partner are set for auto-negotiation; therefore, the 10Mbps operating mode is selected. This is the fastest data rate that can be used in this data link. The data rate is limited by the 10Mbps capabilities of the computer’s network interface.


Modern network interfaces for computer networks have the capability of running the data over the links in either full- or half-duplex mode. As noted previously, full-duplex means that the communications device can transmit and receive at the same time. Half-duplex means the communications device can transmit or receive, but not at the same time.

In full-duplex operation (10/100Mbps), the media must have separate transmit and receive data paths. This is provided for in CAT6/5e/5 cable with pairs 1–2 (transmit) and pairs 3–6 (receive). Full-duplex with gigabit and 10 gigabit data rates require the use of all four wire pairs (1–2, 3–6, 4–5, 7–8). An important note is that the full-duplex mode in computer network links is only for point-to-point links. This means that there can only be two end stations on the link. The CSMA/CD protocol is turned off; therefore, there can’t be another networking device competing for use of the link. An example of networking devices that can run full-duplex are computers connected to a switch. The switch can be configured to run the full-duplex mode. This also requires that each end station on the link must be configurable to run full-duplex mode.

In half-duplex operation, the link uses the CSMA/CD protocol. This means only one device talks at a time, and while the one device is talking, the other networking devices “listen” to the network traffic. Figure 5-21(a) and (b) shows examples of networks configured for full- and half-duplex mode. In full-duplex operation [Figure 5-21(a)], CSMA/CD is turned off and computers 1, 2, and the switch are transmitting and receiving at the same time. In half-duplex mode [Figure 5-21(b)], CSMA/CD is turned on, computer 1 is transmitting, and computer 2 is “listening” or receiving the data transmission.


FIGURE 5-21 (a) Computer 1 transmits and receives at the same time; (b) computer 1 transmits; others listen.

Figure 5-22(a) and (b) provides an example of the port management features available with the Cisco switch using the Cisco Network Administrator software. The settings for the speed are shown in Figure 5-22(a). An example of setting the switch for auto, half-, and full-duplex are shown in Figure 5-22(b). The auto setting is for auto-negotiate.


FIGURE 5-22 An example of the port management options available with a Cisco switch: (a) 100Mbps auto-negotiation; (b) 10Mbps half-/full-duplex option.

Table 5-4 provides a summary of the advantages and disadvantages of the auto-negotiation protocol.

TABLE 5-4 Summary of the Auto-negotiation Protocol



Useful in LANs that have multiple users with multiple connection capabilities.

Not recommended for fixed data links such as the backbone in a network.

The auto-negotiation feature can maximize the data links’ throughput.

A failed negotiation on a functioning link can cause a link failure.

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