Auto-negotiation, Speed, and Duplex
By default, each Cisco switch port uses Ethernet auto-negotiation to determine the speed and
duplex setting (half or full). The switches can also set their duplex setting with the duplex
interface subcommand, and their speed with—you guessed it—the speed interface subcommand.
Switches can dynamically detect the speed setting on a particular Ethernet segment by using a few
different methods. Cisco switches (and many other devices) can sense the speed using the Fast
Link Pulses (FLP) of the auto-negotiation process. However, if auto-negotiation is disabled on
either end of the cable, the switch detects the speed anyway based on the incoming electrical
signal. You can force a speed mismatch by statically configuring different speeds on either end of
the cable, causing the link to no longer function.
Switches detect duplex settings through auto-negotiation only. If both ends have autonegotiation
enabled, the duplex is negotiated. However, if either device on the cable disables
auto-negotiation, the devices without a configured duplex setting must assume a default. Cisco
switches use a default duplex setting of half duplex (HDX) (for 10-Mbps and 100-Mbps
interfaces) or full duplex (FDX) (for 1000-Mbps interfaces). To disable auto-negotiation on a
Cisco switch port, you simply need to statically configure the speed and the duplex settings.
Ethernet devices can use FDX only when collisions cannot occur on the attached cable; a
collision-free link can be guaranteed only when a shared hub is not in use. The next few topics
review how Ethernet deals with collisions when they do occur, as well as what is different with
Ethernet logic in cases where collisions cannot occur and FDX is allowed.