Port configuration and Troubleshooting

Well first of all, let me clarify you, what is full duplex mode and half duplex mode.

Full duplex means the interface can send and receive data at the same time, everybody listens when everybody speaks, though there no collision domain

Half duplex means you will have collisions and slower network performance due to dropped packets, as systems back off and resend their data, you speak I listen when everybody should be silence until my conversation finish.

You need to assign a specific speed to switch ports through following switch configuration commands or else interfaces are being set to default speed as per the table below.

Interfaces always set to default speed as per the table below.

Interface Type	Set speed capability(mbps)	Default
FastEthernet 10/100 ports	10/100/Auto	Auto
Gigabit Ethernet GBIC ports	1000	1000
1000BASE-T	10/100/1000/Auto	Auto

You need to assign a specific speed to switch ports through following switch configuration commands, wherever you see duplex mismatch problem or user complain slow response, which means both ends have different speed and duplex modes. Both ends must be configured same speed and duplex modes.

Switch(config)#  Interface gig 3/1

Switch(config)#  speed auto

Switch(config)#  duplex auto

Switch(config)#  Interface gig 3/2

Switch(config)#  speed 100

Switch(config)#  duplex full

Remember-If speed is set to auto, then the duplex can’t be manually set, duplex should also be set auto

Verification of interfaces speed and duplex settings

Before verifying port settings, you must see the interface capability. If it is fast Ethernet 10/100 port, which is manually set to speed 100, what do you do if other end gigabit Ethernet of default speed 1000mbps? So that verify the interfaces capabilities first and set speed auto-auto, make sure both ends must be  same speed.

To verify the interface capabilities, use the below command

To verify the specific interface speed and duplex setting, use the following commands.

And remember when the speed and duplex setting are being set to default, you will see following symbolic representation, as marked by red pen on below screen shots.

 (1).They display auto-auto, when interface are “not connected” other end.

 (2).They display  a-full and a-100 , when interfaces “connected” to other end.

PC end speed and duplex settings:

Click "Start > Settings > Control Panel".

Select "Network and Sharing Center".

On the right panel select "Change Adapter settings"

Right click on the "Local area Connection" and select "Properties".

In the "Local Area Connection Properties" window select the "Configure" button.

Select the "Advanced" tab.In the scroll list of options find "Speed & Duplex" or "Link Speed & Duplex" and select it.

Auto Negotiation will be the default setting for win 7 computers.