PM-4-ERR_DISABLE link flap error detected when plugging in fibre on cisco switch

I was trying to connect two switches on different floors

lvl1 sw -> lvl2 sw

When I plugged in the fibre I could see the light was making it to the upstairs but when plugging the fibre cable into the switch on lvl2 the port was going into err-disable with the reason as link flap.

The cause of the issue was the wrong type of fibre cable was used in the fibre patch. Single mode cable was used. When my switch transceiver (10Gig-SR multimode) and fibre patches leads were OM3 multimode.

The core of single mode fibre is much narrower than multimode so they can't work together too much light (signal) is lost.

There are two fixes and both will probably cost you:

1 - re-run the fibre patch cables with multimode cable (OM3 or higher). Range is 300m so this is what is used inside most buildings. If you need a cable run longer then you have to use single mode.

2 - Buy single mode transceivers LRM and single mode patch cables. The single mode transceivers and cables are more expensive.

https://en.wikipedia.org/wiki/Single-mode_optical_fiber

fibre optic cables

fibre cables have different terminations. You need to select the right cable which can connect to your deivce and the the other end to fiber patch panel so you need to check both. Some times the cables are mixed up in the patch so you need to pop the cables out and swap them around

ST (straight tip)
SC (Subscriber connector)
LC (Local connector)
MT-RJ 

LR = long range / single mode

SR = short range / multimode

Have seen 10g LRM SFPs work on both multimode/singlemode cable.

You need to see how it is terminated at each patch point.
You need to have the correct SFP in your network device so you can plug the fibre in.
You also need to select the right cables SC-LC, ST-ST etc.

Look a bit into the SFP you should see a light on one side. This is the transmit side and needs to be the same all the way. You won't be able to see the light with single mode (its too narrow for the human eye). Need to use device db signals

-> red light transmit
<- no light receive

There are OM values that relate to cable distance will update more on that later

10Gig-Base-SR (closer to 0 is better)
Typical working transmit range: -1 to -7
Typical working transmit range: -1 to -9.9

When we see a Rx power around -14 dBm or lower there is typically some sort of fault in the cable plant (bad splice, dirty connector, poorly seated jumper etc.) that's causing excessive signal loss.

If either Tx or Rx is in the -30 dBm or lower range that's usually indicative of there being no actual signal received and the transceiver is reporting the "noise floor" of the receiver stage. (https://en.wikipedia.org/wiki/Noise_floor)

https://www.cisco.com/c/en/us/products/interfaces-modules/transceiver-modules/index.html

CLI commands:

Router:

sh hw-module subslot 0/0 transceiver 2 status

sh hw-module subslot 0/0 transceiver 2 idprom

Switch:

sh int g1/0/1 status

sh int gig1/0/1 capabilities

sh int gig1/0/1 transceiver