Ethernet – Full-Duplex and Speed negotiation

By far the most common issue would be misconfiguration. Some people still hold on to old beliefs that auto negotation does not work reliably.

If you have different vendors there can be issues but I don't see it very often. I would much rather rely on auto negotiation as my first solution and if that fails fall back to hard coding it.

It is quite common to see ports operating in 100/half because if one side has auto and other side has 100/full then the speed can be detected but the duplex must be assumed to be half because it can't be negotiated.

Your NMS should be able to detect this but you could do a script that logins and does:

sh int status | i half

And reports back all interfaces running in half duplex. This would only work for switches. For routers you would need something like:

sh interfaces | i Fast|Giga|half

Then you would need to parse it in some way. It's probably nicer to use SNMP though. Browsing Ciscos SNMP page SNMP Object Navigator indicates that you should be able to use OID 1.3.6.1.4.1.9.5.1.4.1.1.10 which is PortDuplex.

Shared physical media: only one device on the shared media can access it at any given time, the rest has to wait.

Halfduplex point to point links, uses the same physical media in both directions. It is the same frequency and point to point electromagnetic wave/channel. It is halfduplex as a radio can only transmit or receive at a given point in time. The two sides of the point to point link can not transmit at the same time they share the physical media

Multiplexing: is the interleaving of multiple data streams, the resulting multiplexed stream can then be sent over a full or half duplex link.