Ethernet Collisions and Troubleshooting ⋆ IpCisco

In this article, we will focus on ethernet collisions and how to troubleshoot these common situations.

Let’s start with, what is collision. In computer networking Collision is the situation that occurs while the two or more device try to use the shared medium at the same time. They both send but because of the fact that the medium is half-duplex, the transmission is not realize. If this medium is ethernet, then we call this collision ethernet collision.

Ethernet uses CSMA/CD(Carrier Sense Multiple Access/Collision Detect) mechanism for detection of the ethernet collisions. Let’s check this mechanism for two devices, A and B, on the same ethernet medium.

When I decided to send a frame, it chechs the medium. If the medium is not available, it waits, or if the medium is available, then it sends the frame. If A and B thinks that the medum is available at the same time, then they both send the frame, and the collision is detected. They both waits for a random time to try again. The time after the collision is divided into time slots, they both get a random slot.If the same situation occurs one more time, they extend the number of the slots.

About the collisions, there are two counters. These counters are, “deferred counter” and “collision counter”. In Cisco devices , when we use the command “show ethernet” command with the related interface, we see these two counter value.

“Deferred counter” is the counter that counts how many times the interface has tried to send a frame, but each time the medium was busy.

“Collision counter” is the counter that counts the number of frames for which one or more collision occured. There are also two sub counters of collision counter. These are “single collisions” and “multiple collisions”. We can vies this information with the command “show controller”.

The collision detection time is important. For example the ethernet collisions can be detected up to 51.2 microseconds after the transmission start. Or up to the 512th bit of the frame. If one collition is detected after this time, then it is called “late collision”. You can check this with the command “show interface interface” command.