Complete walk-through of 100 MB full duplex configuration in Cisco switch step by step.

Simplex – In this mode of communication, data is sent only in one direction, and the device at the receiving end will only receive the data. The role of the sender and receiver cannot be changed in this mode of connection.

Example of Simplex – TV is an example of simplex communication, it only receives data and no data is sent back in the opposite direction.

Half-duplex – On this mode switch can only receive or send at the same time hence half duplex is a slow connection and prevent the full-duplex switch to work with full potential. On the other hand, hubs do not have full-duplex functionality so they can only receive or send data at once.

Example of half-duplex – Radio is an example of a half-duplex in which data is sent by one device and received by the other.

Full-duplex – full-duplex functionality allows a switch to transfer and receive data at the same time that makes the bandwidth double therefore, we must check the devices for duplex mismatch because duplex mismatch causes a collision and when a collision takes place then the data is lost so the device has to send the data again which further reduces the speed of the network.

Example of the full-duplex – Telephone is an example of full-duplex communication as data can be sent and received at the same time.

Most of the time Cisco auto-negotiation for duplex selection works perfectly, however in some instances, a Cisco device is unable to detect the duplex mode and it may result in a duplex mismatch. This could result in collisions in the network. Therefore, to fully utilize the bandwidth and the capacity of the switch, it is a good practice to hard-code the speed and duplex on the Cisco devices.

In this lab, we will configure the speed to 100 Mbps and the duplex mode to full. The duplex mode should be configured on both switches. Now, to configure the duplex and speed, we will have to go into the specific interface on which we want to hard-code the duplex setting.

After configuring both switches with full-duplex, we can solve the issue that occasionally occurs due to auto-negotiate settings in the switches and if there is a 100MB connection between two switches then full-duplex allow switches to work with double speed.

Please note that full-duplex configuration must be configured on each interface on which we want to hardcode the full-duplex mode.


Full duplex


Lab Tasks

  1. Hard-code interface fastethernet 0/1 to both switches to full duplex.
  2. Hard-code Speed to 100 Mbps.

Lab Configuration

Task 1

Switch(config)#interface fastethernet 0/1

Switch(config)#duplex full

Task 2

Switch(config)#interface fastethernet 0/1

Switch(config)#speed 100