Speed and Duplex Settings on 3750

[johnilu]

I have had problems with speed and duplex settings on some devices. I will set them static on both host and switch sides, say to 100 full. The ports will not come up, or display some other problem till I set the switch side to auto negotiat. I can then set the port back to 100 full, and everything works fine.

This has been happening on Catalyst 3750 switches mostly, but I have noticed it on at least one other platform.

Can someone explane what is happening.

 

[ADB100]

The 3560 & 3750 support auto-MDIX and I have seen strange behaviour when leaving auto-MDIX enabled but hard-coding the speed/duplex. Try turning off auto-MDIX and see if it makes a difference:

interface fastethernet1/0/1
no mdix auto

HTH

Andy

 

[vipergg]

What AB100 said is correct more than likely it is auto mdix . If auto mdix is enabled this will only work if the port settinsg are set as auto , if you try to hardcode the ports it will kill them . Turn off auto mdix unless you need and don't have the correct cables .

 

[johnilu]

Thanks for you input. auto-MDIX sounds like it is probably the problem.

Does anyone else have an idea.

 

[jneiberger]

Always set your switchports to AUTO. There is almost never a good reason to manually set the speed and duplex if you're using equipment manufactured in the last four or five years. Manually configuring your speed and duplex settings is just asking for problems.

 

[Kwazzy]

jneilberger - I would agree only if the remote/host device is also at autonegotiate. If you must hard set the ports, then both sides must be hard set.

Both sides must match, auto to auto or hard set to hard set. Setting the switch to auto, and thinking hardsetting the device is the preferred method, will rain havok to your communications. This goes for Cisco, Nortel, 3com...ect.

Autonegotation will use the lowest denomiator if it does not sence the autonegotation on the remote end. Which is half duplex. So, if one side is Hard Set (100 Full), and the other is auto, then you will have the 100 Full and 100 Half.