CWDM LAB! - How to test two MUX CWDM-8A and GBICs/SFP

Andres Franco · ‎10-24-2012

Hello there!

We are having a situation in our DataCenter with some CWDM GBICs that stopped working. Some did enter in err-disabled and some are showing as not connected. One of them did show link-flap as the err-disabled reason. We did troubleshoot testing the following:

1. Shut/no shut of the interface.

2. Removing and placing the modules again.

3. Changing the CWDM SFP to another port in the switch.

4. Changing the fiber patch cord.

5. We did also check the media type for the interface.

But the at the end, all of them continued to show as not connected (including those that were previously in error disabled state). So we are starting to think that it might be a problem with de MUX. Thats why we would like to set a kind of lab with 2 additional MUX in a point to point scheme in order to discard de the CDWM GBICs and MUX. The question is, if i connect two switches with the GBICS through the MUX using fiber patch cords, will i have issues with CWDM power?, do i need a minimum length for this test? what would you suggest? what are your comments about this sceneario?. Thanks in advance.

chris-hart · ‎10-25-2012

Can the CWDM GBIC/SFPs be tested by looping the RX and TX with a fibre patch lead? You would need to put an attenuator in the loop as well because the TX power level is quite high, I think the TX Level is typically around +3db and the Max Rx level is -7dB.

If you loop an interface on the Catalyst Switch is will go into err-disabled because it sees the same keepalive coming back, when I'm testing I enter the "no errdisable detect cause loopback" command to stop this.

Going a bit further is it possible to connect two CWDM GBIC/SFPs of the same frequency in a pair of back to back switches, with appropriate attenuators, the data sheet seems to suggest that the RX optical input frequency should be between 1450nm & 1620nm so it might work.

Andres Franco · ‎10-25-2012

Hello Chris!

Many thanks for your quick reply to my answer. I found your comments very usefuls. I got one more question, it might be a silly one. How could I loop an interface on the Catalyst switch? using two CWDM SFP modules of the same color and connecting them with a fiber patch cord? If so, is there any chance to damage de SFPs by means of a power issue?. Thanks a lot.

Andrés.

Andres Franco · ‎10-25-2012

An additional question and sorry for that.

It is possible to use the an OTDR in the monitor port of the CWDM-MUX-8A in a production environment without affecting service? Thats something i´m not sure about and i would like to clarify as an alternative for the assessment of this issue. Thanks.

Andrés.

Tom Randstrom · ‎10-25-2012

"The question is, if i connect two switches with the GBICS through the MUX using fiber patch cords, will i have issues with CWDM power?"

You didn't mention which model mux/demux you are using (maybe CWDM-MUX8-A). If you connect MUXes back to back, you should install 10dB attenuators between the MUXes.

The max transmit output of the CWDM transceiver is +5db and the maximum receiver input power is -7dB; which indicates that you need at least 12dB (+5 -(-7)) of light loss to prevent over-powering the receiver. Two MUXes (CWDM-MUX8-A) will add 4.4dB of loss (2 x 2.2dB); leaving the need for at least 7.6 dB of attenuation (round to 10dB). I've shown the calculation below.

Tx Max Output = +5 dB

- Rx Max Input = -7 dB

=======

Needed loss = 12 dB

- CWDM Mux loss = 4.4dB

=======

Min. Attenuator = 7.6 dB

The receivers in the CWDM transceivers are wideband, meaning any of the CWDM wavelengths can be connected to the receiver of a transceiver (does not need to match its transmit frequency).

I hope this helps!

Andres Franco · ‎10-25-2012

Hello Tom, thanks for your answer.

Yes, i´m using CWDM-MUX-8A. Taking into account what you have said about CWDM transceivers at the end of your post, It is possible to make a loop with two different CWDM SFPs on the same switch?, Where you suggesting that for testing purposes i could try to connect a CWDM GBIC to a different color on the MUX just to verify if it is working (I imagine that the interface will turn to the up state but it will not exchange data because of the wavelength)? is there a command or something i could use to determine if the GBIC is damaged? I was thinking that with a show int status, it will show me that the GBIC is not present in case it was damaged, but i´m not sure. Thanks in advance.

Andrés.

Tom Randstrom · ‎10-25-2012

I am not sure what would happen if you transmitted from one switch SFP and received on the same switch on a second SFP receive port. I would suspect the second SFP would show link but not sure what the switch will say. Won't hurt the SFP receive port.

SFP Troubleshooting Guide

http://www.cisco.com/en/US/products/hw/modules/ps4999/products_tech_note09186a00807a30d6.shtml

An OTDR into the monitor port will probably not be a good idea on an active network. My assumption is that the OTDR's output may be a wideband 1550nm signal, that may transmit across some of the CWDM wavelength frequencies in use. I don't have experience with this.

viyuan700 · ‎10-28-2012

"Taking into account what you have said about CWDM transceivers at the end of your post, It is possible to make a loop with two different CWDM SFPs on the same switch?"

How can you loop two DIfferent CWDM SFP?

You have GBIC-MUX-DEMUX-GBIC. First you have to check whether these GBIC are working for that. First you can check GBIC on both end by looping TX to Rx but with an attenuator. Once you have confirmed your GBIC are fine at both ends. I think if you have SFP to FC type of patchchord will help you here to loop easily.

Then you need to check MUX DEMUX. To loop MUX demux you can loop the Network port (With Attenuator) You don't need 2 SFP to test whether a channel is working or not. Suppose you connect one SFP at 1470 and then loop the network port then the same 1470 wavelength will be comming at 1470 receive. If you are thinking you can loop 1470 to some other wavelength it is not going to work optical MUX/DEMUX works in a different way.

Once you have tesetd one mUX then you can conenct 2 MUX/DEMUX in Point to Point link and if evrythig worked before and not working now will help you to identify the not working box.

chris-hart · ‎10-25-2012

Hi Andres,

Yes you can use the monitor ports on the MUX CWDM-8A to monitor the power levels of the channels on the trunk of a production in-service network but you have to use an OSA (Optical Spectrum Analyser) or power meter not an OTDR (Optical Time-Domain Reflectometer).

Theoretically connecting two CWDM GBIC/SFPs with different channel frequencies back to back should work, just make sure you put attenuators on both the TX an RX legs, 12dB should be OK but I would use 15 dB just to be on the safe side.

Let us know how you get on.

Chris