Problem with autonegotiation on 3750 Electrical port

amit.das · ‎12-06-2017

Hi All,

I am facing a weird issue with auto-negotiation in CISCO 3750 switches. I have configured one switch port as duplex full and speed 1000 in both the switches. Now when I am connecting these two switch ports with RJ45 cable the link is showing down. If I change the duplex mode to auto in any one of the switches or both are kept in auto mode the link do shows up with the same cable. This issue is not faced if I use the optical ports instead of electrical port. Is this the correct behavior?

Any hints would be greatly appreciated!

Thanks in advance.

kayukovvalera · ‎12-06-2017

Hi,

Try to perform switchport nonegotiate, udld port on ports.

amit.das · ‎12-06-2017

speed nonegotiate can be done in optical ports. In electrical ports no such option is available.

Leo Laohoo · ‎12-06-2017

@amit.das wrote:

speed nonegotiate can be done in optical ports. In electrical ports no such option is available.

Don't muddle the topic.

You're talking about hard-coding speed/duplex on a copper port. You won't be able to hard-code speed/duplex on a fibre optic port.

amit.das · ‎12-07-2017

Agreed. So does this mean that with CISCO 3750 switch, full duplex on both side will not work?

Leo Laohoo · ‎12-06-2017

@amit.das wrote:

I have configured one switch port as duplex full and speed 1000 in both the switches.

This not only means auto-negotiation is disabled but MDI/MDI-X is also disabled.

@amit.das wrote:

Now when I am connecting these two switch ports with RJ45 cable the link is showing down.

Are you using cross-over cable or straight-through?

amit.das · ‎12-07-2017

This not only means auto-negotiation is disabled but MDI/MDI-X is also disabled- Is this normal? Can we enable this forcefully?

Are you using cross-over cable or straight-through?- Straight-through.

Leo Laohoo · ‎12-07-2017

@amit.das wrote:

This not only means auto-negotiation is disabled but MDI/MDI-X is also disabled- Is this normal? Can we enable this forcefully?

No. If you disable speed and duplex auto-negotiation, MDI/MDI-X is disabled. There's no way around it.

@amit.das wrote:

Are you using cross-over cable or straight-through?- Straight-through.

And this the the reason of your initial problem: When you disable speed/duplex auto-negotiation you also disabled MDI/MDI-X. When this happens the link between two switches are down because you're using the wrong cable. The cable must be a cross-over cable.