A couple of notes on the Ethernet Basics chapter from the CCIE Routing and Switching Exam Certification Guide

Wiring – On Routers & PCs transmit pair is wires 1 & 2 and the receive pair is 3 & 6, switches are the opposite way around this is so the receive pair will communicate with the transmit pair between devices. If two PCs (or routers or switches) are directly connected, to ensure the transmit and receive pairs match up a crossover cable should be used which swaps wire 1-3 and 2-6, alternatively the port will need to support Auto MDIX which can resolve the problem automatically.

Auto-negotiation – Fast Link Pulses are used to detect the speed of a connected interface with 17  100ns pulses with informs the  device on the other end of the wire the speed/duplex settings. Fiber does not use Fast Link Pulses but the negotiation works in a similar way.

Preamble - The preamble consists of 62 alternating 1s and 0s ending with a pair of 1s, this 8 bytes of signalling information is sometimes referenced as a 7byte preamble and 1 byte start of frame delimiter.

I/G, U/L MAC Bits – I wrote a post a while ago regarding a couple of bits in the MAC, its important to note that the second from last bit in the first byte of the MAC is the Individual/Group (I/G) bit which if set to 1 signifies that its a multicast or broadcast MAC. Also the last bit is Universal/Local (U/L) bit which indicates where it is set to a 1 indicates it has been administratively as opposed to vendor assigned, but many devices and drivers do not enforce the U/L bit.

Automatic Medium-Dependant Interface Crossover is a licenced technology from HP. According to their website at http://www.hp.com/hpinfo/abouthp/iplicensing/automdix.html 200million ports supporting the technology will ship this year.

Its not supported on all Cisco switches, & it prevents the wrong cable from being installed, eg a striaight through between two switches, by simply swapping the transmit and receive pairs if the wrong cable has been used.