Some points to remember with regards to speed and duplex negotiation and mismatches. Duplex and speed
mismatches occur when auto-negotiation fails or when 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 mismatches do cause performance problems.
Your pings may succeed but the movement of large files may be negatively affected and UDP may especially
have problems with recovering from duplex mismatch issues so a quick bit of history.
Originally ethernet was half duplex because devices were connected to hubs.
So a decision was made that when you can't negotiate with the other side you would revert back to 10
meg half-duplex older devices weren't designed to auto- negotiate.
So the auto-negotiating device would revert back to half-duplex mode if it wasn't able to negotiate
with the other side because it assumed that that device must only support half-duplex. Auto-negotiation
of speed and duplex didn't work well in the old days and often caused problems with the speed and duplex
mismatch.
If one side is using half-duplex and the other side is using full-duplex, the full-duplex side thinks
that it can transmit it any time even when it's receiving traffic.
The half-duplex side however because it uses carrier sense multiple access collision detection or CSMA
slash CD will want the line to be clear before transmitting.
And when it does transmit and the packet is transmitted by the other side running at full-duplex a collision
occurs.
So the half duplex side will back off because of the collision and then try and transmit again when
the line is clear and we'll have the constant issue of late collisions with the full-duplex side is
transmitting data and can starve the half duplex side.
In other words, it can use up all the bandwidth causing a lot of collisions and retransmissions on the
half-duplex side this would cause the frames on the half-duplex side to que up and eventually be dropped.
When looking for problems
have a look at the areas which I'll demonstrate in a moment on the full-duplex side you will see errors
such as CRC errors or runts, on the half-duplex side you typically see late collisions and perhaps
buffer overloading, management systems from companies such as solar winds and others allow you to see
where you've got Duplex problems on your network and help you resolve these issues more easily.