How switch detects fiber link is not connected in other side?

[Ali Norouzi]

I want to know how a Cisco switch detects fiber link is not connected.

I know how UDLD works and tested it between two switches. Then I disabled UDLD globally and under the port but the port was still down while RX side of fiber was OK.

interface Ethernet1/20
  no lldp transmit
  no lldp receive
  no cdp enable
  speed 10000
  duplex full
  no negotiate auto
  udld disable

sh int et 1/20
Ethernet1/20 is down (Link not connected)
admin state is up, Dedicated Interface
Hardware: 100/1000/10000 Ethernet, address: 1c6a.7a46.16fb (bia 1c6a.7a46.16fb)
MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, medium is broadcast
Port mode is access
full-duplex, 10 Gb/s, media type is 10G
Beacon is turned off
Auto-Negotiation is turned off FEC mode is Auto
Input flow-control is off, output flow-control is off
Auto-mdix is turned off
Rate mode is dedicated
Switchport monitor is off
EtherType is 0x8100

!

SFP Detail Diagnostics Information (internal calibration)
----------------------------------------------------------------------------
Current Alarms Warnings
Measurement High Low High Low
----------------------------------------------------------------------------
Temperature 40.31 C 80.00 C -10.00 C 70.00 C 0.00 C
Voltage 3.28 V 3.63 V 2.97 V 3.46 V 3.13 V
Current 6.34 mA 15.00 mA 2.00 mA 12.00 mA 3.00 mA
Tx Power -2.04 dBm 1.99 dBm -9.50 dBm 0.00 dBm -7.52 dBm
Rx Power -1.97 dBm 1.99 dBm -14.20 dBm 0.00 dBm -12.14 dBm

I want to know now that auto negotiation and UDLD is off and switch rx is OK why the interface is still down? How switch detects it?

[marce1000]

 - Lets summarize/collapse your question to a fiber link connectivity issue , some advices : use compatible sfp's for both devices involved , for that look at : https://tmgmatrix.cisco.com/ . Usual advices and things to test are , 1) set interface  Ethernet 1/20  to default configuration (default int Eth1/20) , 2) disable auto negotation , 3) flip the fiber pair at the sfp-connection , 4) make a local fiber loop on the  switch and check if that can work.

 M.

[balaji.bandi]

what is other side device, what is the output of other side ? show interface x/x

make sure below things checked :

1. Make sure both the side same SFP Module in your case 10GB SFP

2. what kind of patch cables used here, single mode and MM

3. how far each device located ?

as other post suggested, if this going via different comms rooms, twisting the fibre lead help you.

UDLD - nothing to do with, if the port go error disabled, it will show you reason.

More can be find here what is UDLD :

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10591-77.html

[MHM Cisco World]

may be the other side SW run UDLD and detect failure and down the interface which make your SW interface  down also.