An Alternative to Cross-over Cable -- Auto MDI-X

Before we discuss about Auto MDI-X , Let me first clarify what is MDI and MDI-X

MDI(Media Dependent Interface): An Ethernet port typically used on NIC of a computer.
Examples  : Personal Computers,Workstations,Servers and Routers.

MDI-X(Media Dependent Interface-Cross over): An Ethernet port typically used on network devices. Examples : Hubs and Switches.

There is no hard rule that Computers have MDI interfaces and Switches have MDI-X interfaces. But it is general known convention.

MDI vs MDI-X :

• Straight through cable is used to connect between MDI and MDI-X interfaces. Refer Fig 1
• Cross over cable is used to connect between MDI to MDI (or) MDI-X to MDI-X interfaces. Refer Fig 2

They say that a picture is more than 1000 words:

                              Fig 1: Straight through MDI to MDI-X connection for 10BASE-T

                             Fig 2 :  Cross over MDI to MDI connection

 Enough about the backgroud , lets come back to actual feature

Auto MDI-X:

To connect two ports of same configuration (MDI to MDI or MDI-X to MDI-X),an ethernet cross over cable is needed. But Auto MDI-X automatically detects the required cable connection type and configures the appropriately.

Note: For Auto MDI-X to operate correctly , the speed and duplex settings must be set to "auto".

Got bored with theory , lets jump into configuration

By default Auto-MDIX is enabled on the switches. We can verify this with the following cli:

switch1#show controll ethernet fa0/0 phy | i MD

Auto-MDIX      :  On   [AdminState=1   Flags=0x00052248]

Note: If we hard set the speed/duplex , Auto-MDIX will be turned off automatically. Lets verify this

switch1(config)#int fa0/0
switch1(config-if)#speed 100

switch1#show controll ethernet fa0/0 phy | i MD

Auto-MDIX      :  Off   [AdminState=1   Flags=0x00010A48]

The following table tells us when the link will be up/down.

sw1		sw2
MDIX	Speed/Duplex	MDIX	Speed/Duplex	Link Status
on	auto	on	auto	up
on	auto	on	hard-set	up
on	auto	off	auto	up
on	auto	off	hard-set	up
on	hard-set	on	auto	up
on	hard-set	on	hard-set	down
on	hard-set	off	auto	down
on	hard-set	off	hard-set	down
off	auto	on	auto	up
off	auto	on	hard-set	down
off	auto	off	auto	down
off	auto	off	hard-set	down
off	hard-set	on	auto	up
off	hard-set	on	hard-set	down
off	hard-set	off	auto	up
off	hard-set	off	hard-set	down

Simple condition for the link to be up: at least one side needs to have auto-MDIX enabled along with auto-negotiation of speed and duplex, otherwise the link will be down.

Ethernet Layer1 : Wiring--Speed--Duplex

Wiring:

We have different physical media(Coaxial,STP,UTP and Optical Fiber) to implement Ethernet technology. But nowadays we mostly use UTP and Optical.

In UTP, we use RJ-45 connector and Category 5/5e cable for wiring the devices.

RJ-45 Pinouts with Four-Pair UTP Cabling:

 The most popular standards (10BASE-T and 100BASE-TX) each use 2 twisted pairs (specifically pairs 2 and 3 from above figure),with one pair used for transmission in each direction

Ethernet Cabling Types:

Type Of Cable	Pinouts	Key Pins Connected
Straight – through	T568A (both ends) or T568b (both ends)	1-1;2-2;3-3;6-6
Cross –over	T568A on one end, T568B on the other	1-3;2-6;3-1;6-2

Points to Remember:

1. Eventhough we have 4 pairs, we use only 2 pairs. One pair for transmission and another for reception.
2. For Ex: A PC NIC transmits on pair 1,2 and receives on pair 3,6 ; But switch do the opposite. So a Straight-through cable works well.

Little Bit History of Ether-net

General Meaning of Ether:
 

• 1) The hypothetical medium formerly believed to fill all space and to support the propagation of electromagnetic waves
• 2) Greek myth  the upper regions of the atmosphere; clear sky or heaven

In simple words we can take it as air or free space

Meaning of Ether-net In our World:

A passive broadcast medium with no central control.

Didn't understand, Let me explain you a bit more

Basically it is a shared medium and access to this medium(that is Ether) for packet broadcasts is distributed among the transmitting stations(end-devices) using controlled statistical arbitration.It is nothing but CSMA/CD.

Actual History:

It was developed by DEC(Digital Equipment Corporation),Intel and Xerox in 1970s and was called DIX Ethernet.In the mid-1980s IEEE defined new standards to Ethernet-like networks as 802.3.

First lets start with the topics to be covered for CCIE RSv4

CCIE RSv4 Expanded Blue print

Finally decided to blog

Don't know how to start, but started. Till now i was keep on postponing postponing postponing..... to blog. Tried so many methods to prepare for CCIE. So many times planned but not able to follow the plan consistently.

Finally decided to study by blogging....