Duplex setting on Trunk Ports

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Jan 15th, 2008
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Hello all -

I have two 3550's and I have trunking enabled on the fast ethernet ports. I have tried to set both ends to 100/full, however they do not come up. If I go back and set them both to auto/auto they come up. Can I not set trunk ports to 100/full?



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Kevin Dorrell Tue, 01/15/2008 - 05:50
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Interesting. I don't see why not. Maybe it has some strange interaction with DTP. How are you forming your trunks, desirable-desirable, desirable-auto, or what? Did the ports come up in access mode, or did they not come up at all?

You could try debug dtp all

Kevin Dorrell


mcechman Tue, 01/15/2008 - 05:56
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They are desirable on both ends. If I do a debug dtp all, what would I be looking for? Trunk status?

Edison Ortiz Tue, 01/15/2008 - 06:29
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It sounds like you are using a straight-thru cable vs a cross-over cable. Once you configure the speed and duplex you are automatically disabling Auto-MDIX (http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml#appc)

Change your cable to cross-over cable and then you should be able to change the speed and duplex.




glen.grant Tue, 01/15/2008 - 07:02
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Edison did they have auto-mdix back on the 3550's ? Not sure they did .

Edison Ortiz Wed, 01/16/2008 - 11:53
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Glen, you are right.

glen.grant Wed, 01/16/2008 - 15:41
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I have never seen something like this . I would make sure the trunking parameters are exactly alike on both ends . Usuaully though you should get at least a link light , are the patches ok ? When you set it as auto does it autonegotiate 100/full ?


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