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New Member

interconnecting two 3750s over gigabit ethernet

I happen to connect gigabitethernet ports of two 3750 switches using a Cat5e cable which crossed only two of the four pairs. To my surpise these ports did connect at 1000Mb/full duplex. Next I connected the two ports with an all four pairs crossed cable and found that the switches again interconnected at 1000Mb/Full duplex.

Does it mean that I can interconnect 3750s with either a 2 pair or a 4 pair cross-over cable without any performance problems?

4 REPLIES

Re: interconnecting two 3750s over gigabit ethernet

I would not recommend doing it with anything other than a fully wired cable, although the switch is quite amazing at figuring out what you have and making the best of it.

Newer versions of the IOS for 3750 switches also do auto-MDIX, so the need to use a crossover cable goes away. I'd recommend upgrading and using a normal cat5e cable...

New Member

Re: interconnecting two 3750s over gigabit ethernet

I would like to know what does Auto-MDIX stands for?

Also more information about the technology would be appreciated.

Gold

Re: interconnecting two 3750s over gigabit ethernet

Auto-MDIX is "Automatic Media-Dependent Interface Crossover". Derives from MDI, which is the kind of port that provides the physical connection to a cable for computers, network-attached printers, and routers.

Switches and hubs have MDIX ports. When you connect an MDI device to an MDIX device, the cable uses a straight-through pinout. This is the more common type of patch cable.

When you connect MDI to MDI (such as a direct, back-to-back hubless/switchless cable connection between two computers) or MDIX to MDIX, you need a cable with a cross-over pinout. These patch cables are less common.

Because crossover cables are sometimes hard to come by, some hub and switch manufacturers have made their devices with one port configured as an MDI rather than MDIX. That way, you can use a straight-through cable to connect two hubs or switches.

Other manufacturers use a sliding or toggle switch to manually select the pinout of a particular port to be either MDIX or MDI.

Auto-MDIX is the electronic version of the sliding or toggle switch. It determines what pinout configuration the port should take, automatically. But how it does that, I have no idea.

I never really needed to know HOW auto-MDIX worked before, just that it did, and which ports were capable of doing it. Now I've got something else to look into...

New Member

Re: interconnecting two 3750s over gigabit ethernet

It was really helpful for a novice like me. I really appreciate the clear and understandable languauge used in the reply. I have started the studies for CCNA exam and got a motivation with the kind of reply I got. Thank you very much for the information.

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