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Collisions on a Full-Duplex Interface?

Unanswered Question
Aug 29th, 2002
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Hello All,

I saw something interesting a month or two ago, on a 100BaseTX interface between a firewall and Cisco 3600. A show interface on the cisco showed a high number or collision, but the connection was configured full duplex on both ends and the speeds matched. The cable was typed and certified Cat 5 UTP. The cables used were 6 ft - 15ft in length. There was not power lines or external noise near the cable run. When the interface is changed to half duplex the problem deceases drastically.

Any ideas????


Greg @ Arlington

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umarzoli Fri, 08/30/2002 - 06:06
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you should check if the interface is really working in full-duplex mode with

the "show interfaces ..." command. I have had once the problem that, even

the interface was configured to use full-duplex, it runs in half-duplex mode.

Under normal conditions a full-duplex interface shouldn't check for collisions,

so this could also be related to an IOS problem.

Tip: A full duplex analyser if available can be plugged in between to check

if there are problems on the cable.

Due to the interface change to half-duplex: For sure do you have changed

both sites to half-duplex? And remember : collisions are quite normal on

half-duplex connections.


Ulrich Marzoli


aybra Mon, 09/02/2002 - 02:04
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you have to verify that really your connexion is in full duplex mode.

try to make them in autonegociation and inspect the process of negociation, normaly if all things go right so you will have full duplex mode.



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