gbic loop on catalyst 2960

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Aug 13th, 2009
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I would like to know if there is a way to validate that a SFP type 1000baseLX is working properly on catalyst 2960.

I am looking for the loop feature that is available on routers.... but not on switches.

The topology is 2 catalyst 2960 switches with both SFP type 1000baseLX connected together with a fiber. This was working find but actually, the two interfaces are down/down.

The idea is to be sure that the problem is on the fiber and not on one of the SFP....

Thank You for your help.

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Marvin Rhoads Thu, 08/13/2009 - 07:25
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UniDirectional Link Detection (UDLD) addresses your concern regarding the fiber. See for more information.

For the SFPs themselves, you might want to review for more information.

Hope this helps.

viyuan700 Thu, 08/13/2009 - 08:02
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You can also use hard loop means take a patchcord and connect transmitter to receiver (in LX you can connect directly but if you have a attenuator would be nice)similarly you can check your card at other end.

IF both cards are working, Now only fiber is left , you can loop the 2 fiber at other end and check whether your fiber have problems.

tquero Thu, 08/13/2009 - 08:36
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Thank You for your updates.

That means that there is no way to perform this loop in software mode with the IOS :-(

I thought about the possibility to physically loop with a patchcord but I am afraid to break the Rx LED of the SFP.

I will have a look on the LX specification to check the minimal distance.

The UDLD will not help me in this situation because both interfaces of both sides are in down/down status.

Anyway, thank you for your updates.


Thomas Quero

viyuan700 Thu, 08/13/2009 - 10:43
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Hi Thomas,

U can try LAN switching board to check if anybody else knows loop in software but you can use physical loop for GLC-LH-SM (I think this is the card u have). As Max Transmit power is -3dBm and Max Receive power is also -3dBm so shd not be a problem.


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