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how to check / find out the root cause of Link down on SFP Fiber Link

Userlevel 3
How to check / find out the root cause of Link down on SFP Fiber Link

Fiber cable error ? SFP module error ? Why was the Link down?

The link is sharing port. LACP L2 (1:21 and 2:21)
Now the link is up and running.
I didn't see any collisions/congestion/anomalyrxerrors/txerrors on the port

Link State: Active, 1Gbps, full-duplex and AUTO

following is the log

09/25/2015 11:31:22.76 [i] Slot-1: Add port 1:21 to aggregator09/25/2015 11:31:22.72 [i] Slot-1: Port 1:21 link UP at speed 1 Gbps and full-duplex
09/25/2015 11:31:22.15 [i] Slot-1: Remove port 1:21 from aggregator
09/25/2015 11:31:22.14 [i] Slot-1: Port 1:21 is Down, remove from aggregator 1:21
09/25/2015 11:31:22.14 [i] Slot-1: Port 1:21 link down

Appreciate your help.


10 replies

Userlevel 6
Sometimes remote fault or local fault is reported like below.
09/30/2015 09:16:55.14 [i] MSM-A: Port 9:2 link down - Local fault

In your case this is not reported, does the other side report something ?
Are the values in "show port transceiver info detail" within the limits or is it marginal ?
Userlevel 3
Hi OscarK,

transceiver is not support.

sw# show ports 2:24 transceiver information detail
Port : 2:24
Error: DDMI is not supported on this LX transceiver


Slot-1 sw01.1 # debug hal show optic-info slot 1
Port 21
Signal: present
TX Fault: no
SFP/SFP+ Part Number: TRF5735AALB206
SFP/SFP+ Serial Number: C09B08646
SFP/SFP+ Manufacture Date: 090214
Connector: LC
GE Compliance: 1000BASE-LX
Wavelength: 1310
Userlevel 6
Hello Paul,

that is not so good, not much we can see then other that the SFP notifies us the link goes down.

best regards
Userlevel 2
Conf Port 1:21,2:21 debounce timer 150
Userlevel 2
I had exactly the same error with flapping links and it was fixed by TAC with the above command.
Userlevel 3
Please advice any info to find out root cause. thanks.
Userlevel 4
Have you tried looping back the fiber to test the SFP+ modules and the fiber path?
Userlevel 3
Curtis Parish wrote:

Have you tried looping back the fiber to test the SFP+ modules and the fiber path?

Hi Curtis,

thanks for your suggestion. actually i didn't aware that portion. It really help me to troubleshoot the error. since link flap error are mostly from Physical Layer issue Bad Cable, ...

Thanks 😃
Userlevel 7
The command given by Marco will add a delay to help resolve false flapping detection. It shouldn't be used everytime, everywhere. In your case, if it flapped only once, that can be due to the other side, or something in the path (you have LX so I'm assuming there can be other active devices in-between). There's too little information to help find the root cause.

Is it happening a lot / often?
Userlevel 3
Thanks Stephane,

It only happen about 14sec link flap up/down about 4 times within 14sec.
Now the link is up and running. yes i understand that it's not enough info and difficult to find out the root cause. link flap may be because of Physical layer issue.

Thanks for your time.