On SRX4600 devices, 100G interfaces may fail to come up when using QSFP-100G-SR4 or similar transceivers. This issue is typically encountered when connecting to other devices (such as QFX, MX, or third-party platforms) using 100G SR4 optics. The problem can stem from transceiver compatibility, fiber polarity, or interface configuration mismatches

# All four transceivers are identified correctly as 100G-SR4 modules.

> show chassis hardware detail no-forwarding

FPC 1            REV 17   7xx-xxxxxx     BCDGxxxx         SRX4600 MPC
  PIC 0                   BUILTIN      BUILTIN           MRATE-4xQSFPP-XGE-XLGE-CGE
    Xcvr 0       REV 01   7xx-xxxxxx   Gxxxxxxxxxx      QSFP-100GBASE-SR4
    Xcvr 1       REV 01   7xx-xxxxxx   Gxxxxxxxxxx      QSFP-100GBASE-SR4
    Xcvr 2       REV 01   7xx-xxxxxx   Gxxxxxxxxxx      QSFP-100GBASE-SR4
    Xcvr 3       REV 01   7xx-xxxxxx   Gxxxxxxxxxx      QSFP-100GBASE-SR4

# The link is physically down, and speed is incorrectly identified as 40Gbps, despite having a 100G optic.
# FEC is disabled (FEC Mode: NONE), while the peer device (QFX) is configured for FEC91.
# No errors recorded, indicating a link negotiation mismatch, not transmission degradation.

> Physical interface: et-1/0/0, Enabled, Physical link is Down. <<<<<<<<<
  Interface index: 151, SNMP ifIndex: 517, Generation: 188
  Link-level type: Ethernet, MTU: 1518, MRU: 1526, Speed: 40Gbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Disabled
  Device flags   : Present Running Down
  Interface Specific flags: Internal: 0x100000
  Interface flags: Hardware-Down SNMP-Traps Internal: 0x4000
Active alarms  : LINK
  Active defects : LINK, LOCAL-FAULT   <<<<<<<<<
  PCS statistics                      Seconds
    Bit errors                             0
    Errored blocks                         0
  Ethernet FEC Mode  :                   NONE.  <<<<<<<<<<<< 
    FEC Codeword size                       0
    FEC Codeword rate                   0.000
  Ethernet FEC statistics              Errors
    FEC Corrected Errors                    0.  <<<<<<<<<
    FEC Uncorrected Errors                  0
    FEC Corrected Errors Rate               0
    FEC Uncorrected Errors Rate             0

# QFX configuration : QFX side is operational and has no reported FEC errors.

 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

# No physical or optical issues are present

Verify the FEC mode configuration on both the SRX and the QFX (or other peer device). Ensure that both ends are configured with matching FEC settings. If a mismatch is found, update the port configuration on both sides to use a consistent Ethernet FEC mode—either none or FEC91, depending on the transceiver type and link requirements.

> show interfaces et-1/0/0 extensive

Physical interface: et-1/0/0, Enabled, Physical link is Up

Link-level type: Ethernet, MTU: 1518, MRU: 1526, Speed: 100Gbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, Loopback: Disabled, Source filtering: Disabled,

Active alarms : None

Active defects : None

Ethernet FEC Mode : FEC91

On the qfx5120:

 Ethernet FEC Mode :         FEC91

FEC91 is the standard for 100G Ethernet, providing better error correction, while NONE means no error correction, which is suitable only for very short, clean links.