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.

Today I found out another interesting bit of information about the MAC address, actually its not just 1 bit its 2 bits.

The most significant byte is the first byte in the address and in that byte the last bit is the most significant bit.

Also the most significant bit is called the Individual/Group (I/G) bit and the next most significant bit is the Universal/Local (U/L) bit.

If the I/G bit is set to 0 it defines that the address is unicast and conversely if it is set to 1 it is multicast or broadcast.

If the U/L bit is set to 0 it means the address is vendor assigned but it if it set to 1 it is administratively assigned, its noted tha many devices do not enforce the rule of the U/L bit.