The attenuation of a 10km link of SMF @1310nm is .35/km = 3.5dB (vs 28.57dB in your spreadsheet). Most splics will be 0.05db each and connectors of <0.25ea. 3dB margin on a 10km link is probably excessive.  ... View more

The same thing happens to me by reviewing the SFP DWDM's datasheet.: https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/dwdm-transceiver-modules/product_data_sheet0900aecd80582763.html It indicates: "When shorter distances of single-mode fiber are used, it might be necessary to insert an optical attenuator in the link". Shorter than how much? For long reach optical transceivers, the transmitters optical output power is greater than the receiver's input power range. To use the DWDM transceiver you linked to, this module has a transmit output power of +4dBm (max) which the receiver's input range is -9 to -28dBm.    To calculate the minimum attenuation required for this link to operate correctly: Transmit power (+4dB)  minus  Receiver input power (-9dBm) = Minimum link attenuation (+13dBm). Fiber loss is ~ 0.25dB/km (@1550nm). So, 13db / 0.25dB/km =  52km minimum link length. So if you are doing bench testing, it would be necessary to add a minimum of 13dB of attenuation to have the received power level to fall within the receiver's operating range (-9dBm to -28dBm).     Let us know if you have additional questions. ... View more

OM1 and OM4 are both multi-mode fibers; OM1 has a 62.5um core and OM4 has a 50um core.    What type of 10G SFP+ is installed in the switches?  The  SFP-10G-LRM transceiver module will operate over OM1 fiber when a mode conditioning patch cord is used on each end of the link. This mode conditioning cable may be the fiber you are seeing coming off the 10G SFP.    ... View more