Switch interface status and duplex/speed synchronization

Troubleshooting is a very important in a career of a network engineer. If a network of the company in which you are working goes down you must have the ability to perform troubleshoot and fix problems so your network quickly comes back online and the company resume it's work. In this post we will talk about troubleshooting in switches.There are many features you need to analyze on switches in which some are shown below :

• Checking Interface status and statistics
• Pattern of forwarding frames
• Synchronization of speed and duplex
• Examining port security
• Configuration of VLAN and VLAN trunking

When you use the command of show interface status in switches, The command shows two types of status which is the Line status and the Protocol status. The line status refers to the layer 1 (Physical Layer) status condition and the protocol layer gives us the layer 2 (Data Link Layer) status condition. The line and protocol status can be in an up state, down state or administratively down
(manually done by the host) state. Below are the combination status and causes made by both the status

Line Status         Protocol status         Interface                 Causes

1. up                                      up                   connected                 The interface is working
2. down                                 down                disabled                    Disabled by port security
3. up                                     down                not connected            Not expected on switch
4. down                                 down                not connected            Bad cable or wrong pinouts
5. administratively down          down                disabled                    Done by the user manually

                                                                                                              
Duplex and Speed 
We all know what is speed of a  network but most of you are not familiar with duplex .There are two types of duplex:

• Full duplex : The switch can send and receive at the same time.
• Half duplex : The switch cannot send and receive at the same time.

Now remember, the speed and duplex of the switch and the user end devices must be same. Suppose the pc is sending data at 100mbps but the switch speed is just 10 mbps. The link won't work because the speed must be the same. Same isthe condition for duplex that both the devices must have the same duplex mode.There is a command called "Auto-negotiation" which helps us to solve this problem.

In any case,if the auto-negotiation setting fails on a switch or a user end device, it then uses default settings which are

• Speed :Use the slowest supported speed (mostly 10mbps).
• Duplex: If 10 or 100 mbps, use half duplex otherwise it will use full duplex.

In the next post, we will learn about the port security which is an important part of a switch in terms of security and troubleshooting.