To perform autonegotiation, the switch and the NIC must support
multiple speeds, as well as autonegotiation logic. To perform autonegotiation,
the switch and NIC send some messages back and forth. These messages simply
state things like, "I want to use 100 Mbps," with a response like, "Okay, me,
too. Let's do it." Of course, the IEEE specifications aren't so informal, but
that's the gist of the mean.
And if the switch and NIC simply can't agree, they fall back to
10 Mbps, half duplex.
Many of the Ethernet NICs in use today are called 10/100 cards, meaning that they will run at
either speed, and they will negotiate with the switch. In Figure 6-13, Fred has a new 10/100 card, and Wilma has her
same old 10/100 card. Both have negotiated for 100 Mbps, full duplex. Barney,
using an older 10-Mbps only card that does not support autonegotiation, settles
for 10 Mbps, half duplex.
Because GigE products keep falling in price, you can now find
10/100/1000 NICs and switch ports, meaning that the NICs and the switches will
negotiate the use of Gigabit Ethernet as well.
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