Original subtitles

Some points to remember with regards to speed and duplex negotiation and mismatches.

Duplex and speed mismatches occur when auto negotiation fails

or where manual configurations are mismatched.

So for example one side is configured to use full duplex

and the other side is configured to use half duplex.

Auto negotiation may fail because of physical problems in the network

such as cabling problems or a hub somewhere in the network.

Duplex mismatch is do cause performance problems, your pings may succeed

but the movement of large files may be negatively affected 10

and UDP may especially have problem with recovering from duplex mismatch issues. 11

So a quick bit of history, originally Ethernet was half duplex 12

because devices were connected to hubs, so a decision was made that when you can’t 13

negotiate with the other side, you would revert back to 10 Meg half duplex 14

all the devices weren't designed to auto negotiate 15

so the auto negotiating device would revert back to half duplex mode 16

if it wasn’t able to negotiate with the other side because it assumed that 17

that device must only support half duplex. 18

Autonegotiation of speed and duplex didn’t work well in the old days 19

and often caused problems with the speed and duplex mismatch. 20

If 1 side is using half duplex and the other side is using full duplex. 21

The full duplex side thinks that it can transmit it 22

any time even when it's receiving traffic, the half duplex side however because it uses 23

Carrier Sense Multiple Access Collision Detection or CSMA/SD 24

will want the line to be clear before transmitting 25

and when it does transmit and the packet is transmitted 26

by the other side running a full duplex a collision occurs 27

so the half duplex side will back off because of the collision 28

and then try to transmit again when the line is clear 29

and we'll have this constant issue of a late collisions 30

where the full duplex side is transmitting data and can starve the half duplex side 31

In other words it can use up all the bandwidth causing a lot of collisions 32

and retransmissions on a half duplex side. 33

This would cause the frames on a half duplex side to queue up and eventually be drop. 34

When looking for problems have a look at the errors which I'll demonstrate 35

in the moment, on the full duplex side you will see errors such as your CRC errors 36

on half duplex side you typically see late collisions 37

and perhaps buffer over loading. 38

Management systems from company such as SolarWinds 39

and others allow you to see where you’ve got duplex problem on your network 40

and help you resolve these issues more easily.