Some 100G interfaces between QFX5210-64C and QFX5110 using QSFP-100G-SR4-T2 optic does not come up on certain ports.

Example:

Port 49 on QFX5210 and port 40 on QFX5100 does not come up, but if we move the port location on the 5210 to another port (for example port 50, the link comes up).  

QFX5210-64C ----et-0/0/49-----------------et-0/0/40----QFX5110  <-- Link is down
QFX5210-64C ----et-0/0/50-----------------et-0/0/40----QFX5110  <-- Link is up

This may be related to FEC errors on the QFX5210-64C side. Check if the link has any FEC errors. 

root@QFX5210> show interfaces et-0/0/40 
Physical interface: et-0/0/40, Enabled, Physical link is Down
  Interface index: 744, SNMP ifIndex: 515
  Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Speed: 100Gbps, BPDU Error: None, Loop Detect PDU Error: None,
  Ethernet-Switching Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Disabled,
  Media type: Fiber
  Device flags   : Present Running Down
  Interface flags: Hardware-Down SNMP-Traps Internal: 0x4000
  Link flags     : None
  CoS queues     : 10 supported, 10 maximum usable queues
  Current address: f4:bf:a8:06:0b:47, Hardware address: f4:bf:a8:06:0b:47
  Last flapped   : 2021-06-07 08:23:52 BOT (07:18:58 ago)
  Input rate     : 0 bps (0 pps)
  Output rate    : 0 bps (0 pps)
  Active alarms  : LINK
  Active defects : LINK, LOCAL-FAULT
 Pre-emphasis(reg 236/237)
                       Register-value                 0
 Ctle(reg 234/235)
                    Register-value                 0
 Differential-amplitude(reg 238/239)
                        Register-value                 0
  PCS statistics                      Seconds
    Bit errors                             0
    Errored blocks                         0
  Ethernet FEC Mode  :                  FEC91
  Ethernet FEC statistics              Errors
    FEC Corrected Errors            258874756
    FEC Uncorrected Errors           27714745
    FEC Corrected Errors Rate               0
    FEC Uncorrected Errors Rate             0
  PRBS Statistics : Disabled
  Interface transmit statistics: Disabled

From the "show interfaces extensive" output, both ports have Ethernet FEC Mode set to FEC91

​{master:0}
root@QFX5210-64c> show interfaces et-0/0/40 extensive | match "FEC"   
  Active defects : None
  Ethernet FEC Mode  :                   FEC91 
  Ethernet FEC statistics              Errors
    FEC Corrected Errors            258874756
    FEC Uncorrected Errors           27714745   
    FEC Corrected Errors Rate               0
    FEC Uncorrected Errors Rate             0


{master:0}
root@QFX5110-48S> show interfaces et-0/0/49 | match FEC 
  Active defects : None
  Ethernet FEC Mode  :                   FEC91 
  Ethernet FEC statistics              Errors
    FEC Corrected Errors                    0
    FEC Uncorrected Errors                  0
    FEC Corrected Errors Rate               0
    FEC Uncorrected Errors Rate             0

​Change the port configuration on both sides and set the Ethernet FEC Mode to none. 

Refer to the technical documentation on FEC Edit Interfaces .

{master:0}[edit]
root@QFX5110-48S# set interfaces et-0/0/49 gigether-options fec none    

{master:0}[edit]
root@QFX5110-48S# commit and-quit 
configuration check succeeds
commit complete
Exiting configuration mode

{master:0}[edit]
root@QFX5210-64C# set interfaces et-0/0/40 gigether-options fec none 

{master:0}[edit]
root@QFX5210-64C# commit and-quit 
configuration check succeeds
commit complete
Exiting configuration mode

{master:0}
root@QFX5210-64C> show interfaces terse et-0/0/40 
Interface               Admin Link Proto    Local                 Remote
et-0/0/40               up    up

{master:0}
root@QFX5110-48S> show interfaces terse et-0/0/49    
Interface               Admin Link Proto    Local                 Remote
et-0/0/49               up    up

• FEC (gigether)