Problem of duplex - Cisco 3560

Unanswered Question
Jul 15th, 2010

Dear All,

I have a problem of duplex negociation in my switch. I have two 3560 switchs connected via Gi0/0 port using fibr. when i launch in the second SW the sh mac-address-table i see two mac-addresses connected to my fa0/9 :

SW2#sh mac-address-table

mac1 (HSRP)     fa0/9


mac2 (device)     fa0/9

SW2#sh spaningtree root

I get fa0/9

SW#sh int fa0/9

i get hal-duplex and 100MB/s

SW2#sh spaningtree

i get fa0/9 connected using Shr Type

Is someone have an idee why i get the HSRP + 2 mac for the same port ? why is in half-duplex ?

if the type of cable used to connect a device to fa0/9 is crossover, thus the duplex is automaticaly half ?

is there any possibility to have a Hub behind fa0/9 ? how can i detect that ?

Special question : i have been connected using telnet to my two switch with ter mon option, and when the fa0/9 of the second switch has been disconnected , i get the message that the interface gone down in both screens ? what does means ? it's normal ? i think the message would be displayed only in the second SW.

Manay thanks for your help and assistance.

Best regards,


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Viral Bhutta Sun, 07/18/2010 - 14:48

what versios are you running.

Can I get the configuration from both ports in the switches.

Have you tried to either hard code the speed and duplex(if copeer) or do a nonegotiate (if copper).

Also please paste the entire output of show spanning-tree for a particular vlan.

How is fa0/9 configured, is it an access port.

My guess is that you have a hub connected to fa0/9 which will put the port as 100mbps and half duplex and under spanning-tree you would see Shr (which you are already seeing.)

Duplex does not depend on the cable but it depends on the negotiation done between the connected devices. Hub would negotiate for 100mbps half duplex.

For your special question,

you should see the port going down only on one SW2 if its not connected to SW1 through that port fa0/9

Since you are seeing it on both screen, I am assuming that fa0/9 on sw2 is connected in someway to Sw1.

Check for show cdp ne fa0/9

Let me know if this helps.

Ayad Mon, 07/19/2010 - 01:59

Hi Bhutta,

I could have some additionals informataion :

in the port Fa0/9: it's connected a 3G cisco router and all port configurations in the router are to auto. Acoording to a local technician, the problem of duplex may be caused by crossover cable, used to connect the router to switch.

if the cable is crossover, it's normal to have halp-duplex ?

SW and SW 2 are connected via fibr (Gi0/1).

for Special question:

SWs are connected via Gi0/1 port. it's normal to see information and alerts in both screen when there is a change only in one SW  (i.e : fa0/9 of SW2)?

Please help me to understand connection between cisco /hub and crossover cables.

Thanks you.

Best regards,

cabrademonte Mon, 07/19/2010 - 08:55
Usually you want to hardcore the speed and duplex on one side at least if not both. That is a known problem , and the solution is to hard code.


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