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Ethernet Collisions and Troubleshooting

Author: gokhankosem, on 08 Jul 12 - 1 Comment
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Ethernet Collisions and Troubleshooting

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.

The possible causes of the late collisions are incorrect cabling, non-compliant number of hubs or bad NICs(Network Interface Cards).

The maximum retries of the interface to allocate a slot without a collision is 16. Interface retries 16 times before giving up. If it gives up, the frame is not send. And it is called “excessive collision”.

Here, there a re two parameters that is used with excesssive collisions. These are, TRC(Transmit Retry Count) and TDR(Time Delay Reflectometer). TRC is the number of the transmit retries of the associated packet. TDR is the count of the time from the start of a transmission to the collision time.

You can check the excessive collisions with the command “show controller interface”.

The common causes of the excessive collisions are, full-duplex on a shared ethernet, broken NICs or too many stations on the shared medium.

Lasty, I would like to point out something. On Cisco Catalyst switches, the every excessive collision is reforted, because of the fact that the “service internal mode” is “on”. If you close this by “no service internal” command, then it reported this only the excessive collisions reach a certain threshold.

In this article, I have tried to show you the important ethernet collision types that we can come across. I hope this can be useful for you ;)

About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

One comment for “Ethernet Collisions and Troubleshooting”

Mehmet Onur

I especially need advice on Sing , Mult and Late collisions.

Could you explain it more via Alcatel cases ?
Thanks in advance ,this was the second link on google

November 12th, 2012 at 10:13

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