Cisco 2955S questions

Unanswered Question
Sep 22nd, 2008

Hello everybody,

I'm trying to connect two 2955S switches using SM fiber, I'm able get the connection up for a couple of hours and then it goes down. Sometimes goes up and down several times during the day; however I'm not able to get a stable connection. The switches are located in buildings that are located less than a mile apart.

Is there a minimal distance for connecting those devices using single mode fiber? If that is the case, what is that distance?

Should I use attenuators? If that is the case how do I calculate the required attenuation?

Thank you for your help

I have this problem too.
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Giuseppe Larosa Fri, 09/26/2008 - 12:34

Hello Armando,

the distance is not a problem and you shouldn't need to use an attenuator.

Are there any log messages that could be related to to the troubled link ?

try to disable the interface keepalive some IOS versions have issues with them that lead to errdisable.

Do you see incrementing errors in the interfaces during the day ?


show interface gi0/x

If available have someone test attenuation with an optical TDR when a loop is made on the other side.

Verify also to use GBICs/SFPs that are of the same vendor because using different can sometimes lead to problems.

Hope to help



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