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Some points to remember with regards to speed and duplex negotiation and mismatches.
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Duplex and speed mismatches occur when auto negotiation fails
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or where manual configurations are mismatched.
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So for example one side is configured to use full duplex
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and the other side is configured to use half duplex.
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Auto negotiation may fail because of physical problems in the network
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such as cabling problems or a hub somewhere in the network.
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Duplex mismatch is do cause performance problems, your pings may succeed
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but the movement of large files may be negatively affected
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and UDP may especially have problem with recovering from duplex mismatch issues.
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So a quick bit of history, originally Ethernet was half duplex
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because devices were connected to hubs, so a decision was made that when you can’t
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negotiate with the other side, you would revert back to 10 Meg half duplex
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all the devices weren't designed to auto negotiate
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so the auto negotiating device would revert back to half duplex mode
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if it wasn’t able to negotiate with the other side because it assumed that
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that device must only support half duplex.
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Autonegotiation of speed and duplex didn’t work well in the old days
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and often caused problems with the speed and duplex mismatch.
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If 1 side is using half duplex and the other side is using full duplex.
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The full duplex side thinks that it can transmit it
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any time even when it's receiving traffic, the half duplex side however because it uses
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Carrier Sense Multiple Access Collision Detection or CSMA/SD
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will want the line to be clear before transmitting
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and when it does transmit and the packet is transmitted
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by the other side running a full duplex a collision occurs
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so the half duplex side will back off because of the collision
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and then try to transmit again when the line is clear
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and we'll have this constant issue of a late collisions
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where the full duplex side is transmitting data and can starve the half duplex side
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In other words it can use up all the bandwidth causing a lot of collisions
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and retransmissions on a half duplex side.
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This would cause the frames on a half duplex side to queue up and eventually be drop.
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When looking for problems have a look at the errors which I'll demonstrate
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in the moment, on the full duplex side you will see errors such as your CRC errors
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on half duplex side you typically see late collisions
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and perhaps buffer over loading.
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Management systems from company such as SolarWinds
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and others allow you to see where you’ve got duplex problem on your network
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and help you resolve these issues more easily.
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