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Network Warrior, 2nd Edition by Gary A. Donahue

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Configuring Autonegotiation

For Cisco switches, autonegotiation is enabled by default. You can configure the speed and duplex mode manually with the speed and duplex interface commands in IOS.

You cannot set the duplex mode without first setting the speed. The switch will complain if you attempt to do so:

2950(config-if)#duplex half
Duplex can not be set until speed is set to non-auto value

Additionally, gigabit switches may give you further warnings about trying to configure half duplex on gigabit ports:

SW-3750(config-if)#duplex half

% Duplex cannot be set to half when speed autonegotiation subset contains 1Gbps

To set the speed of the interface, use the speed command. If the interface has previously been configured, you can return it to autonegotiation with the auto keyword:

SW-3750(config-if)#speed ?
  10    Force 10 Mbps operation
  100   Force 100 Mbps operation
  1000  Force 1000 Mbps operation
  auto  Enable AUTO speed configuration

Once you’ve set the speed, you can set the duplex mode to auto, full, or half:

SW-3750(config-if)#duplex ?
  auto  Enable AUTO duplex configuration
  full  Force full duplex operation
  half  Force half-duplex operation

On NX-OS, the commands are the same as the IOS equivalent:

NX-5K(config)#int e1/5
NX-5K(config-if)#speed ?
  10    Force 10 Mbps operation
  100   Force 100 Mbps operation
  1000  Force 1000 Mbps operation
  auto  Enable AUTO speed configuration
NX-5K(config-if)#
NX-5K(config-if)#speed 1000
NX-5K(config-if)#duplex full

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