duplex
What are the causes of duplex mismatch other than misconfiguration? (power outage? improper cabling? auto negotiation implementation bugs?)
Any thoughts on discovering duplex mismatch? NetMRI seems to use interface error rate. Is it sufficient to just poll all interface error rates on switches/servers? What if this feature is not available on a device?
Edit: Drivers can definitely cause problems with duplex. I have seen Broadcom NIC stuck in half-duplex on FreeBSD.
Full-duplex basically means communication can happen both ways, negating the possibility of a collision. The below diagram should make it a little more clear. One pair is designated a transmit pair, and the other a receive pair. In most environments now, it isn't even necessary to match up transmit/receive pairs, as MDIX handles that for you.
This doesn't eliminate the possibility of collisions on one side as a result of a configuration mistake, such as half-duplex on one side and full-duplex on the other. That's fairly common.
Can two frames going in the same direction crash with each other?
No, frames wait in a buffer until they can be transmitted across a link. One by one.
Actually, TCP and Ethernet are examples of different layers of the OSI model. TCP works in layer 4 (transport layer), which is used for making connections between nodes on a network. TCP is indeed bidirectional, and it's sometimes referred to as connection-oriented.
Ethernet is a layer 2 (data link layer) protocol, which dictates how signals are to be interpreted in the physical layer (layer 1). There are many sublayers to layer 2, but in general, Ethernet is the most popular.
If Ethernet is run in half-duplex, then the node cannot transmit and receive at the same time. But this doesn't mean that the node cannot use TCP. TCP still runs on top of Ethernet, and additionally, on top of IP (Layer 3) to create the bidirectional connection that TCP needs to communicate. So when talking about Ethernet and TCP, they operate as different layers.
TCP still makes the bidirectional connection even though the Ethernet NIC cannot receive and send at the same time.
A good reference to how this works is the classic Stevens book: http://books.google.com/books/about/TCP_IP_Illustrated_Volume_1.html?id=a23OAn5i8R0C
Best Answer
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:
And reports back all interfaces running in half duplex. This would only work for switches. For routers you would need something like:
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.