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.
Everything is wired correctly — but performance feels… uneven.

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

🚀 What Is Interface Speed?

Interface speed defines how fast data travels over a network connection. Common 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: Different Speeds in Action

• 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.

🔄 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.

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 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

🔧 Cisco 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.

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🧠 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