Testing fibre cables from a switch

[cosimotodaro]

Hi everyone,

I have two new Cisco switches connected to each other with a fibre cable.

Does anyone know any CLI commands to test the fibre cable from any of the two switches?

(I know there is the command "test cable-diagnostics ... etc".  But, this only works with copper)

Thank you

[rayframe1 · Accepted solution]

There's nothing to test the fiber directly, other than a separate fiber tester device like a light meter or an OTDR

What is usually done is to compare the optical TX and RX levels on both sides, to ensure the signal is within range, if its within range you should be OK. The optical levels are usually found in the show interface transceiver or show controllers interface or show interface detail, you may have to search to find this, don't be confused by the TX and RX thresholds

If for example on the first side the optical levels show TX is 1 and RX is -5 and the facing side has TX 1 and RX -22 then the facing side is RX'ing a lot worse signal than the first side. The loss across in one direction is 6 dBm and the other way is 23 which would indicate a connection problem. Note; a -40 or -50 or -80 means no signal

Depending on what you see you troubleshot from there

[cosimotodaro]

Hi,

Thank you for the information.  This would be a good test if I could do it.  Last Friday, I very quickly searched for the commands to see what the optical level were, but I couldn't find them.  I'll have to search for these commands properly next time.

Thank you

[cosimotodaro]

By the way, I have a 6509 at one end and a 3560 at the other end of the fibre cable.  Thanks

[Tom Randstrom · Accepted solution]

Whether you will be able to view the optical transmit and receive levels will depend on the optical transceivers that are installed in the equipment.  The following document outlines the transceivers that support the DOM capabilities.

http://www.cisco.com/en/US/docs/interfaces_modules/transceiver_modules/compatibility/matrix/OL_8031.html

Without DOM enabled transceivers, the only way to test the fibers is as described by rayframe1; with a light source and power meter.

http://www.exfo.com/Documents/TechDocuments/Specification_Sheets/Fiberbasix-50-angHR.pdf

[Tom Randstrom]

If you purchased SFPs with DOM (digital optical monitoring) capabilities, you can use the command SHOW INTERFACE TRANSCEIVER to find the each device's transmit and receive levels:.    

http://www.cisco.com/en/US/docs/ios/interface/command/reference/ir_s4.html#wp1184714

The Fiber Optic Association (thefoa.org) has good general information on fiber testing and design.

[cosimotodaro]

Hi Tom,

The SFPs don't have DOM capabilities.  I have a 6509 at one end and a 3560 at the other end of the fibre cable.  Do you know which command I can use to see the transmit and receive levels?

Thank you

[rayframe1]

The commands to see the optical levels are above, show interface transceiver, sometimes show controllers interface has optical info, but the optical info may not show up if the SFPs are not recognized properly, a possible problem with non-Cisco SFPs . If you are looking at Cisco SFPs or you know the vendor of the SFPs you can see if they can do anything about the lack of diagnoistics. Generic SFP's can be programmed to provide such info by the vendor

As mentioned the main question is whether the optical signal getting to the far side is within the thresholds. Here are some ways to do this;

• Obtain or borrow a power meter (or second option if available an optical tracer)
  
• You can test the fiber by looping it at the near and far end and see if the other side comes up.
  
• Otherwise you could bring an SFP (with a switch it works in (if needed)) with working diagnostics to use as an improvised 'light meter'. Do any of your SFPs show optical RX level?
  

We didnt mention how long is the fiber or who installed it. Both may affect the troubleshooting options. Another possibility if the optical signal is OK is that the SPFs are not compatible, with the switch, interface or each other. Is the fiber the right type? (Sorry if this is too obvious a question)

Do you have the info on the SFPs?

[cosimotodaro]

Hi,

The SFPs we are using are the traditional ones.  They are 1000BASE-SX.  They don't have an DOM capabilities.  I can't see any Trasmit or Receive levels.  That's it.  There is nothing I can do at this point.  In any case thank you very much for your time in explaining.  It's been very useful.

[Leo Laohoo · Accepted solution]

The SFPs don't have DOM capabilities.

Yes and no.

"Traditional" SFP such as the GLC-SX-MM or GLC-LH-SM are not DOM-capable.  Cisco has released the End-of-Sale announcement for the non-DOM SFP and the replacement parts are the DOM-capable SFP.

End-of-Sale and End-of-Life Announcement for the Select Cisco Gigabit Ethernet SFP Modules

[cosimotodaro]

Hi, thank you for informing me that these SFPs are end of sale.

[Leo Laohoo]

Thanks for the ratings.  Glad to be of some form or assistance.