Excessive CSMA/CD Collisions

I was concerned about the excessive amount of CSMA/CD on the office LAN today. I’ve been trying to get the office staff to register their MAC address with my server so that I could issue licences accordingly. If we all did this then it wouldn’t be so much of a problem.

I logged into the switch to attempt to track down the unregistered users by following their DHCP trail. This is what I found:

Ch3atSW1>sh csmacd collision summary

% insufficient majesty

Ch3atSW1>en
Password:

Ch3atSW1#sh csmacd collision summary

=====================================================
Displaying frame collision summary for interface: all
=====================================================

Normal collisions: 2228401
Late collisions: 156
Near collisions: 56880
Fatal collisions: 1357640
--------------------------------
TOTAL COLLISIONS: 3642987

As you can see the number of collisions was very high and a significant portion of these collisions were fatal. I checked the vendor’s release notes for any clues. It had this to say about collisions.

“Collisions occur on a LAN when the network hub looses control of a frame and two or more frames then collide with each other. Collisions are sometimes inevitable but can be minimised by the following practices.

1. Never allow frames to speed on your LAN. If you do not set the speed limit on all hub ports some frames will speed up. While speeding frames make your network faster, it also increases the chance of a frame loosing control and causing a collision. Excessive speed also increases the chance of a fatal collision.

2. Some users like to modify their frames for better performance. Most modified frames are easy to detect as the "turbo" flag has been set in the header. Most states have outlawed modified frames from participating on a LAN. It is not recommended that you allow modified frames on your LAN as they increase the chance of collisions.

3. PC’s entering sleep mode will often flood the LAN with lethargic packets. If these packets do not reach their destination before they go to sleep they will cause a collision.

Under extreme circumstances a collision can be fatal. Fatal collisions jam up the network and cause a backlog of traffic. Speeding frames, tired frames and poorly maintained frames are the most likely cause of fatal collisions.

It is also important to register your frames. If an unregistered frame causes a collision then you may be held liable for the loss of data it causes.”

More on the CSMA/CD next week...