Why Autonegotiate Fails

The IEEE wrote the 802.3u standard that defines auto-negotiation for Ethernet interfaces. The problem is that the standard was optional, and also open for interpretation.

As a result, many devices don’t correctly auto-negotiate and your network ends up with duplex mismatches, 10meg speeds that should be set to 100meg, and collisions when you should have none.

This problem continues to plague network administrators, as it can take many hours to track down exactly where the problem is,  only to discover a duplex mismatch that can be fixed in under 30 seconds.

Thus, finding the “needle in the haystack” in the first place is the difficult part of the process.

The user typically complains about “network slowness” or a “poor quality VoIP call”.  That leads to 4-5 hours of troubleshooting.  In many cases, the problem isn’t evident at the time the engineer checks due to the network being a very dynamic environment — the problem tends to go un-resolved and the user remains dis-satisfied.

What you really need to do is to be proactive and root out the duplex mismatches so they won’t cause problems down the road.


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