No link over a current working fibre with a different switch

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Nov 7th, 2008
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This is my problem... and I really cant make it work.

I have two buildings in my office that are connected by fibre, multimode 50/125 micron. At present we have Cisco 2950 with SX port (1000 Base SX). The new switches I want to introduce are 2960 (WS-2960 24-TCL with GLC-SX-MM as SFP modules). When I tried to connect these to the fibre, 2950 at one end, and 2960 at the other, the link doesn't go up no matter what. What could be wrong. The link works fine once I have 2950 at both ends. The patch cord with 2950 is MT-RJ/ST and with 2960 is ST/LC reference to the figure . for ST/LC patch cord.

Kind regards


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Tom Randstrom Sat, 11/08/2008 - 08:13
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Some ideas/questions (which you may have already tried):

1. Do you have any test equipment to test the fiber link (power meter) to determine if the ST-LC jumper cables are good?

2. Are you receiving any LED activity when you plug the ST-LC jumper cables into the 2960?

3. Possibly a bad SFP module.

4. Very dirty connectors on jumpers or SFP ports.

6. Per 2960 manual:

Port and Interface Settings

An obvious but sometimes overlooked cause of port connectivity failure is a disabled port. Verify that the port or interface is not disabled or for some reason powered off. If a port or interface is manually shut down on one or the other side of the link, the link does not come up until you re-enable the port. Use the show interfaces privileged EXEC command to verify the port or interface error-disabled, disabled, or shutdown status on both sides of the connection. If necessary, re-enable the port or the interface.

usman_safdar Sun, 11/09/2008 - 21:23
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Thankyou for the reply.

1. The ST-LC jumper cables are working fine. They have been checked.

2. No LED activitiy when i plug the cables into 2960.

3. When i plug the SFP module, it shows a red emitting light. I opened the console and it doesnt give or show any sign of bad link or port.

4. Connectors and SFP modules are clean.

5. The ports are enabled, all of them.

What could be wrong ?

Tom Randstrom Mon, 11/10/2008 - 08:46
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Is there anyway for you to interconnect the 2960 and 2950 using a short jumper cable?

Not sure what the issue is here (grasping at straws)...

Is the SFP in the 2960 fully inserted in the slot?

Are the jumper cables crossed into the 2960's SFP ports?

Is the SFP a Cisco branded product? I would suspect that the CLI would have indicated to you if it wasn't compatible.

Otherwise, I am not sure what to tell you except contact Cisco TAC and see if they can assist.

frank_chen61 Tue, 11/18/2008 - 10:54
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Hi Usman,

check the above post, I have 2 2960 had similar issue, can you firmly press it with one switch, and ask someone to press the other one, see any light come out of it?


usman_safdar Tue, 11/25/2008 - 23:57
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Thanks all for considering and replying.

I have changed the SFP modules, now the laser light is emitting from both of the GLC-SX-MM inserted into two different 2960-24-TC-L switches. Can i connect a patch cord (LC - LC) and check connecitivity ? Will it work ?

Tom Randstrom Wed, 11/26/2008 - 07:17
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You can directly connect them together with a LC-LC patch cord. You can do this with the GLC-SX-MM SFPs because the output power of the SFP (-3 to -9.5 dBm) falls within the receiver's requirements (0 to -17 dBm). For most longer reach SFPs, you need to add an attenuator to the a short patch cord so the input power to the receiver fall within its specifications.

usman_safdar Wed, 11/26/2008 - 23:48
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Thankyou for your reply.

I have connected LC-LC patch cord. One end in each switch, there is no LED or connectivity sign in the Switch. I Unhooked the LC from one of the switch and the laser light was emitting from it, then i plugged it back into sfp and checked the other, laser light was emitting from the other end of LC patch cord as well. What could be wrong ?

Tom Randstrom Thu, 11/27/2008 - 07:54
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At this point, I do not know why it is not functioning.

Could it be the system is configured for "Dual-Purpose Uplink Port". What does the "Show Interface" for the SFP port indicate?

I can only recommend contacting TAC to see if they can assist with troubleshooting.

usman_safdar Sun, 11/30/2008 - 22:20
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Yes the system is configured for dual purpose uplink port, what should i do ?

Tom Randstrom Wed, 12/03/2008 - 08:23
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Were you able to get the 2960 network operational?

usman_safdar Wed, 12/03/2008 - 20:11
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I am a bit novice in this forcing operation which you asked me to perform. Can you be kind enough to tell me step by step if possible?.


viyuan700 Fri, 12/19/2008 - 13:05
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Hi Usman,

I hope your link have come up and you have find the solution.

If your problem is solved i am just posting in case anybody look for same thing in future.

If not i think what is the problem, when i went to cisco website for 2950,

i found that the 1000base SX port are using 1310nm wavelength

where as GLC-SX-MM for 2960 uses 850nm wavelength.

If you can check the paper you have got with the products.

This is not going to work as both products are using different wavelength.

Its not problem with connecter,cleaning , its not sitting properly.

Buy a SFP for 2960 which works at 1310nm and your link will be up in no time.


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