Fast Ethernet

Fast Ethernet, as defined in IEEE 802.3u, retains many familiar features of 10-Mbps IEEE 802.3 Ethernet variants. The age-old CSMA/CD logic still exists, but it can be disabled for full-duplex point-to-point topologies in which no collisions can occur. The 802.3u specification calls for the use of the same old IEEE 802.3 MAC and 802.2 LLC framing for the LAN headers and trailers. A variety of cabling options is allowed—unshielded and shielded copper cabling as well as multimode and single-mode fiber. Both Fast Ethernet shared hubs and switches can be deployed.

Two of the key features of Fast Ethernet, as compared to 10-Mbps Ethernet, are higher bandwidth and autonegotiation. Fast Ethernet operates at 100 Mbps—enough said. The other key difference, autonegotiation, allows an Ethernet card or switch to negotiate dynamically to discover whether it should use either 10 or 100 Mbps. So, many Ethernet cards and switch ports are called 10/100 cards or ports today because they can autonegotiate the speed. The endpoints autonegotiate whether to use half duplex or full duplex as well. If autonegotiation fails, it settles for half-duplex operation at 10 Mbps.

The autonegotiation process has been known to fail. Cisco recommends that, for devices that seldom move, such as servers and switches, you should configure the LAN switch and the device to use the identical desired setting instead of depending on autonegotiation. Cisco recommends using autonegotiation for switch ports connected to end-user devices because these devices are moved frequently relative to servers or other network devices, such as routers.

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