Using auto-MDIX on Cisco Switches (mdix auto)

For those of you who don’t know what MDIX is, it stands for Media Dependant Interface Crossover.  In other words, it’s the feature on switches that allows you to use a patch (straight through) cable rather than a crossover cable to interconnect switches.  It’s a great feature to have but there is some debate in regards to whether or not it should be used.

Personally I never use it.  Why?  Since I started Cisco it has been beaten into my head that trunks use crossover cables.  That’s just how it was.  Truthfully, most trunk links these days are going to be fiber but if we do run across a copper trunk we’ll use a crossover cable.

So why would we still mess around with using crossover cables when managed switches can flip the pairs for us?  Because there are a few things that you might not know about the auto-mdix feature on Cisco switches that can leave you perplexed if you don’t fully understand it.

The one big problem with auto-mdix is that you HAVE to use auto duplex and auto speed settings on the trunk ports.  Let’s take a look at an example of an auto-mdix configuration.

2940# config t
Enter configuration commands, one per line.  End with CNTL/Z.
2940(config)#
int faste0/4
2940(config-if)#
mdix auto
2940(config-if)#
1w6d: %LINK-3-UPDOWN: Interface FastEthernet0/4, changed state to up
1w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/4, changed state to up
2940(config-if)#
duplex full
1w6d: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/4, changed state to down
1w6d: %LINK-3-UPDOWN: Interface FastEthernet0/4, changed state to down
2940(config-if)#

I start by configuring the port for auto-mdix.  As you can see, the instant I configure the option, the interface loads and comes up.  However, the instant I hard code the port to a duplex of full, the interface goes down.  A documented requirement of the auto-mdix feature is that you have to let both sides do auto duplex and speed negotiation.  So, if your company standard is to hard code speed and duplex on trunk ports, then you’ll be using a cross over cable.

I will admit that it’s a great feature to use in a pinch.  Sometime I just don’t have a crossover cable with me and in those cases I’ll use it temporarily.  But I always go back and put a crossover cable in its place.  That’s just me though.  I know some people that use it religiously, and other that won’t touch it.  I see it as an unnecessary complication that can cause issues down the road.  If an engineer doesn’t see the auto-mdix configuration and sets the speed or duplex they can end up stumped for days.  You should be aware that its an option, but be aware of its limitations.

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