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Archive for December, 2010

Did you know that there is a fiber tester inside your SFPs?

by on Dec.07, 2010, under Tutorials

Cisco calls it DOM – Digital Optical Monitoring – and it’s built into some of their SFP, XenPak, and X2 transceivers:

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

Unfortunately, but not surprisingly, the feature isn’t built into any of the common SFPs that most network engineers use on a day to day basis, such as the GLC-LX or GLC-SX units.  Cisco thinks that DOM functionality is worth an extra $300 a pop, putting the cost of a DOM-enabled single mode SFP close to $800.

I have found, however, that some third-party SFPs include the DOM functionality.  I’ve been using the single-fiber SFPs from Champion One for many years.  They work great, only use a single fiber (instead of a pair) and give you DOM functionality for free.

Here’s how to get started with DOM:

1)  Enable support for non-Cisco SFPs:

PSKL_6509(config)#service unsupported-transceiver

2)  Enable DOM Monitoring :

PSKL_6509(config)#transceiver type all

3)  Install some DOM-compatible transceivers.

4)  Take some light measurements!  In this example, I’m using a 1000SFP31B20L Single Fiber SFP in slot 2/9/22:

PSKL_6509#sh interfaces gigabitEthernet 2/9/22 transceiver 

ITU Channel not available (Wavelength not available),
Transceiver is internally calibrated.
If device is externally calibrated, only calibrated values are printed.
++ : high alarm, +  : high warning, -  : low warning, -- : low alarm.
NA or N/A: not applicable, Tx: transmit, Rx: receive.
mA: milliamperes, dBm: decibels (milliwatts).

                                  Optical   Optical
            Temperature  Voltage  Current   Tx Power  Rx Power
Port        (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
----------  -----------  -------  --------  --------  --------
Gi2/9/22      44.1       3.26      22.2      -2.5      -5.1

This feature is incredibly handy when troubleshooting fiber issues.  A low value in the Rx Power column indicates that you have a bad fiber, or more commonly, a dirty jumper somewhere.    You can even use MRTG or Cacti to log and graph your optical health over time.

~Eric

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