duplex issue

Unanswered Question
Jul 9th, 2007

have a GIGE interface that will only come Up/Up at HALF duplex on a 2821. is there a known IOS issue on this ?

the other end is an ASA5010 -- IOS 7.2.1

when we set the ASA to FULL and the 2800 to FULL = up/dn on 2800 .

2821 ios is: 12.4(13r)T

any ideas please.

I have this problem too.
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glen.grant Mon, 07/09/2007 - 17:12

Are you setting both the speed and duplex ? Most of the time I have found on gig interfaces you are better off leaving the interfaces as auto on both ends.

chris-lawrence Mon, 12/22/2008 - 06:45


Did you ever hear back from this post? I'm having the same issue.

I'm having the same issue but with a Cisco 2821 GE0/0 connecting to a Cisco 2821 GE0/0... Speed 10 works fine but when wanting to lock the duplex to full at both ends, link goes up/down. Only when I move one end to duplex auto, link goes up/up (however duplex mis-match). Only when I go duplex auto on both interfaces does the link go up/up. In the past I've always been told to take speed/duplex negotiation out of the picture when working with router-2-router links. Is this old school thinking with these 2800 ISRs?

chris-lawrence Mon, 12/22/2008 - 07:30


No - Tried different options (cross-over cables and patch cords) I'm thinking that my old-school practices ended when GigE interfaces hit routers.

I also find it funny when testing my 2800 with a 3800 - I lock the duplex to full but when I try to lock the speed to 10, link goes up/down. Only goes back up/up when I set the speed option back to auto.

Is this a serious bug?

chris-lawrence Mon, 12/22/2008 - 07:52


Just in case anyone is wondering... I'm using a 1-meter certified CAT6 cable between the two interfaces. I may open a TAC with Cisco.


scottmac Mon, 12/22/2008 - 07:58

Does your certified Cat6 cable have four pair?

You're not, perchance, using a pair or two for voice or other signaling are you?

GigE uses all four pair. If you have one or two bad pair, the system will downshift to the lower speed / duplex.

Is that "cat 6 certified" cable hand made or store bought?

This problem is nearly always a cabling issue.

Maybe try a longer cable, one meter is the lower limit ... maybe you have a "collective tolerances" issue; everything is within the envelope of operation considered "OK" ... but the difference between the two are out of spec or beyond the limits of compensation.

Good Luck


chris-lawrence Mon, 12/22/2008 - 08:17

Hi Scott,

Thanks for the reply... Its simplier than this.

Two 2800 ISRs on a lab bench with a certified store bought (4-pair) CAT6 cable connected between them running c2800nm-advipservicesk9-mz.124-9.T7.bin.

I just want to lock the speed (10Mb/s) and duplex (full) on both interfaces and keep the link up/up.

No config on these routers other than the inf config .1 and .2 with speed 10 and duplex auto. When moving to duplex full, I go up/down on both interfaces.


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