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collisions threshold

Answered Question
Mar 27th, 2006
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Hi, recently I encountered a port disabled by err-disable due to collision. Anyone know what is the threshold of the collisions (excessive or late collision) that may result in err-disable. How cisco switches know when to disable the port when it detected the collision on a port?

Correct Answer by Bobby Thekkekandam about 11 years 4 months ago

16 collisions in a row implies an "excessive collision"


By definition, the single collision counter increments when frames have been successfully transmitted after one collision. Multiple collisions occur when more than one collision occurs before a frame is transmitted.


So, when a single or multiple collision occurs, a frame is eventually transmitted, so the threshold counter is effectively zeroed out again.


An excessive collision, by definition, occurs if an interface fails to allocate a slot in which it can transmit its frame without another collision 16 times. This is when the interface is errdisabled.


HTH,


Bobby



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Bobby Thekkekandam Mon, 03/27/2006 - 09:12
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Hi,


on the desktop switch platforms, the threshold is 16 collisions in a row within a 2 second interval. Prior to the changes in CSCed92917, this threshold was 2 collisions in a row within a 2 second interval.


You can disable this check altogether with "no errdisable cause loopback"


HTH,


Bobby


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hiilusing Mon, 03/27/2006 - 13:09
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Thanks for the info.


BTW, what is means by "16 collisions in a row"? Does these collisions include single collision, multiple collision, late collision and excessive collison?

Correct Answer
Bobby Thekkekandam Mon, 03/27/2006 - 13:19
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  • Cisco Employee,

16 collisions in a row implies an "excessive collision"


By definition, the single collision counter increments when frames have been successfully transmitted after one collision. Multiple collisions occur when more than one collision occurs before a frame is transmitted.


So, when a single or multiple collision occurs, a frame is eventually transmitted, so the threshold counter is effectively zeroed out again.


An excessive collision, by definition, occurs if an interface fails to allocate a slot in which it can transmit its frame without another collision 16 times. This is when the interface is errdisabled.


HTH,


Bobby



*Please rate helpful posts

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