Speed and Duplex

1. Introduction to Speed and Duplex

Imagine this:

You’ve just installed a brand-new switch, connected to a server and a PC.

Diagram explaining the definition of network interface speed and data transfer rates


Everything is wired correctly but performance is very weak !

That’s where speed and duplex settings come into play.

What Is Interface Speed?

Interface speed defines how fast data travels over a network link.

Common interface speed values include:

  • 10 Mbps
  • 100 Mbps
  • 1 Gbps (1000 Mbps)
  • 10 Gbps

The speed depends on:

  • The hardware (NICs, switches)
  • The cable type (copper vs fiber)
  • The interface type (FastEthernet vs GigabitEthernet)

Faster speeds = better performance, especially in high-traffic environments like servers or storage networks.

Diagram explaining the definition of network interface speed and data transfer rates
  • Server to Switch (1 Gbps)
    The server connects on G0/0 using 1 Gbps, ideal for file transfers, backups, and real-time applications.
  • Switch to PC (100 Mbps)
    The PC connects on F0/0 with 100 Mbps, which is more than enough for basic office tasks.

What About Duplex?

Duplex defines how data flows between two devices:

Half-Duplex

In half-duplex mode:

  • Devices can either send or receive but not both at the same time.
  • Collisions can occur, slowing down communication.

This mode was common with legacy hubs but it’s obsolete in modern networks.

Half-Duplex diagram

Full-Duplex

In full-duplex mode:

  • Devices can send and receive at the same time
  • No collisions = faster, smoother communication

Modern switches and NICs use full-duplex by default.

Diagram illustrating full-duplex communication, allowing simultaneous data transmission and reception on a network

2. Auto-Negotiation

Modern switches are smart.

By default, they use auto-negotiation to determine the best possible speed and duplex between two devices.

This avoids mismatches and simplifies network setup.

How Auto-Negotiation Works

When two devices connect:

  1. They exchange supported capabilities
  2. They agree on the highest common speed and duplex mode

Example

By default, interface settings look like this:

SW1# show running-config | include interface GigabitEthernet0/0
interface GigabitEthernet0/0
 speed auto
 duplex auto

SW2# show running-config | include interface GigabitEthernet0/0
interface GigabitEthernet0/0
 speed auto
 duplex auto

speed auto and duplex auto mean both settings will be negotiated automatically.

Auto-negotiation works well when both sides support it. But sometimes, you’ll need to configure things manually.

3. Configuring Static Speed and Duplex

In some environments, like legacy systems or high-performance networks, you may want to disable auto-negotiation and set the speed/duplex manually.

Step 1: View Available Speed Options

SW1(config)# interface G0/0
SW1(config-if)# speed ?
  10     10 Mbps
  100    100 Mbps
  1000   1 Gbps
  auto   Auto speed configuration

Step 2: View Duplex Options

SW1(config-if)# duplex ?
  auto   Auto duplex configuration
  full   Full duplex
  half   Half duplex

Choose full duplex and the desired speed for best performance.

Configuration Example

Let’s manually set both switches to 1 Gbps full-duplex:

![[Configuring Static Speed and Duplex.png]]

On SW1:

SW1(config)# interface G0/0
SW1(config-if)# speed 1000
SW1(config-if)# duplex full
SW1(config-if)# end

On SW2:

SW2(config)# interface G0/0
SW2(config-if)# speed 1000
SW2(config-if)# duplex full
SW2(config-if)# end

Verifying Configuration

Use show interface to confirm settings:

SW1:

SW1# show interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up 
  Full-duplex, 1Gbps, media type is RJ45

SW2:

SW2# show interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up 
  Full-duplex, 1Gbps, media type is RJ45

Everything looks good both sides are aligned.

Watch Out for Duplex Mismatches

If one device uses auto and the other is manually configured, they may mismatch (e.g., one side full, the other half).

This causes:

  • Slow transfers
  • Packet loss
  • Collisions

Always configure both ends consistently.

Summary: What You Learned

  • Speed defines how fast data moves
  • Duplex controls if devices can send/receive simultaneously
  • Auto-negotiation is the default and usually works well
  • In special cases, manual settings ensure control and performance
  • Always verify your settings with show interface
  • Avoid mismatches by configuring both sides the same way