RJ 45 pinouts

10-BASE-T and 100BASE-TX uses pairs two and three, gigabit Ethernet uses all four pairs.
Pinout for straight cable: 1-1;2-2;3-3;6-6
Pinout for crossover cable: 1-3;2-6;3-1;6-2

A standard PC transmits on pair one and two and receives on three and six. A switchport is
the opposite. If two alike devices are connected a crossover cable should be used although
MDI-X is a standard today.

Cisco switches can detect the speed of a link through Fast Link Pulses (FLP) even if autonegotiation is disabled but the duplex can not be detected and this means that half duplex must be assumed. This is true for 10BASE-T and 100BASE-TX. Gigabit Ethernet uses all four pairs in the cable and can only use full duplex mode of operation. Also note that for gigabit Ethernet autonegotiation is mandatory although it is possible to hardcode speed and duplex .

Ethernet uses Carrier Sense Multiple Acess/Collision Detection (CSMA/CD). Before a client can send a frame it listens to the wire to see that it is not busy. It sends the frame and listens to ensure a collision has not occured. If a collision occurs all stations that sent a frame send a jamming signal to ensure that all stations recognized the collision. The senders of the original collided frames wait for a random amount of time before sending again.

Deferred frames

Frames that were meant to be sent but were paused because frames were being received at the moment. If in half duplex sending and receiving can not occur at the same time.


Collisions that are detected while the first 64 bytes are being transmitted are called collisions and collisions detected after the first 64 bytes are called late collisions.


Provides synchronization and signal transitions to allow proper clocking of the transmitted signal. Consists of 62 alternating one and zeroes and then ends with a pair of ones.

I/G bit and U/L bit

The I/G bit is placed in the most significant byte and the most significant bit of the MAC address. If set to zero it is an Individual (I) address and if set to one it is a Group (G) address. Multicast at layer two always sends to 01.00.5E which means that the G bit is set. The bit before the I/G bit is the U/L bit, this indicates if it is an Universally (U) administerad address or an Locally (L) assigned address. If it is an MAC address set by a manufacturer this should be set to zero.


SPAN and RSPAN are used to mirror traffic. The source of traffic can be a VLAN or a switchport or a routed port. Traffic can be mirrored from both rx and tx or just one of them. SPAN sends the traffic to a local destination port, RSPAN sends the traffic to a RSPAN VLAN which is used to transfer the traffic to its destination. Note that some layer two frames are not sent by default including CDP, VTP, DTP, BPDU and PagP, to include these use the command encapsulation replicate. SPAN is configured with the monitor session command.

Ethernet – notes
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