Ethernet autonegotiation uses a process by which a switch and an Ethernet NIC together determine the best combination of parameters for that particular link. To support autonegotiation, the switch and the NIC must support multiple speeds, and they also probably support both half and full duplex. So, a 10/100 card connected to a switch can negotiate to use full-duplex 100 Mbps. If the next switch port is connected to a 10-Mbps-only card that does not even support autonegotiation, the switch will use 10 Mbps, half duplex.
Interestingly, a minor debate still pops up in the networking trade press occasionally about how reliable the Ethernet autonegotiation process really works. Many people recommend that you set the speed and duplex settings on any switch port for which you know the settings desired by the device on the other end of the cable. You should understand autonegotiation, but in real life, you should consider statically configuring these parameters for switch ports connected to servers, switches, and routers.
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