Hi.

Looks like normal behavior:

https://supportportal.juniper.net/s/article/PTX-FEC-corrected-errors-increasing-on-link-between-QSFP-100GBASE-SR4-740-058734-and-QSFP-100G-SR4-T2-740-061405?language=en_US

"An incrementing FEC Corrected Errors counter is normal for a link that
is running FEC. It just indicates that the errored bits have been
corrected by FEC. "

"Therefore, the incrementing FEC Corrected Errors counter might only be
indicating an interoperability issue between the optics from ......"

---
Fredrik Holmqvist
I2B & BBTA tjänster AB
08-590 90 000

On 2024-04-17 21:36, Aaron Gould wrote:
> We recently added MPC10E-15C-MRATE cards to our MX960's to upgrade our
> core to 400g.  During initial testing of the 400g interface
> (400GBASE-FR4), I see constant FEC errors.  FEC is new to me.  Anyone
> know why this is occurring?  Shown below, is an interface with no
> traffic, but seeing constant FEC errors.  This is (2) MX960's cabled
> directly, no dwdm or anything between them... just a fiber patch
> cable.
>
> {master}
> me@mx960> clear interfaces statistics et-7/1/4
>
> {master}
> me@mx960> show interfaces et-7/1/4 | grep rror | refresh 2
> ---(refreshed at 2024-04-17 14:18:53 CDT)---
>   Link-level type: Ethernet, MTU: 1514, MRU: 1522, Speed: 400Gbps,
> BPDU Error: None, Loop Detect PDU Error: None, Loopback: Disabled,
> Source filtering: Disabled,
>     Bit errors                             0
>     Errored blocks                         0
>   Ethernet FEC statistics              Errors
>     FEC Corrected Errors                    0
>     FEC Uncorrected Errors                  0
>     FEC Corrected Errors Rate               0
>     FEC Uncorrected Errors Rate             0
> ---(refreshed at 2024-04-17 14:18:55 CDT)---
>   Link-level type: Ethernet, MTU: 1514, MRU: 1522, Speed: 400Gbps,
> BPDU Error: None, Loop Detect PDU Error: None, Loopback: Disabled,
> Source filtering: Disabled,
>     Bit errors                             0
>     Errored blocks                         0
>   Ethernet FEC statistics              Errors
>     FEC Corrected Errors                 4302
>     FEC Uncorrected Errors                  0
>     FEC Corrected Errors Rate               8
>     FEC Uncorrected Errors Rate             0
> ---(refreshed at 2024-04-17 14:18:57 CDT)---
>   Link-level type: Ethernet, MTU: 1514, MRU: 1522, Speed: 400Gbps,
> BPDU Error: None, Loop Detect PDU Error: None, Loopback: Disabled,
> Source filtering: Disabled,
>     Bit errors                             0
>     Errored blocks                         0
>   Ethernet FEC statistics              Errors
>     FEC Corrected Errors                 8796
>     FEC Uncorrected Errors                  0
>     FEC Corrected Errors Rate             146
>     FEC Uncorrected Errors Rate             0
> ---(refreshed at 2024-04-17 14:18:59 CDT)---
>   Link-level type: Ethernet, MTU: 1514, MRU: 1522, Speed: 400Gbps,
> BPDU Error: None, Loop Detect PDU Error: None, Loopback: Disabled,
> Source filtering: Disabled,
>     Bit errors                             0
>     Errored blocks                         0
>   Ethernet FEC statistics              Errors
>     FEC Corrected Errors                15582
>     FEC Uncorrected Errors                  0
>     FEC Corrected Errors Rate             111
>     FEC Uncorrected Errors Rate             0
> ---(refreshed at 2024-04-17 14:19:01 CDT)---
>   Link-level type: Ethernet, MTU: 1514, MRU: 1522, Speed: 400Gbps,
> BPDU Error: None, Loop Detect PDU Error: None, Loopback: Disabled,
> Source filtering: Disabled,
>     Bit errors                             0
>     Errored blocks                         0
>   Ethernet FEC statistics              Errors
>     FEC Corrected Errors                20342
>     FEC Uncorrected Errors                  0
>     FEC Corrected Errors Rate             256
>     FEC Uncorrected Errors Rate             0
>
> {master}
> me@mx960> show interfaces et-7/1/4 | grep "put rate"
>   Input rate     : 0 bps (0 pps)
>   Output rate    : 0 bps (0 pps)
>
> {master}
> me@mx960> show interfaces et-7/1/4
> Physical interface: et-7/1/4, Enabled, Physical link is Up
>   Interface index: 226, SNMP ifIndex: 800
>   Link-level type: Ethernet, MTU: 1514, MRU: 1522, Speed: 400Gbps,
> BPDU Error: None, Loop Detect PDU Error: None, Loopback: Disabled,
> Source filtering: Disabled,
>   Flow control: Enabled
>   Pad to minimum frame size: Disabled
>   Device flags   : Present Running
>   Interface flags: SNMP-Traps Internal: 0x4000
>   Link flags     : None
>   CoS queues     : 8 supported, 8 maximum usable queues
>   Schedulers     : 0
>   Last flapped   : 2024-04-17 13:55:28 CDT (00:36:19 ago)
>   Input rate     : 0 bps (0 pps)
>   Output rate    : 0 bps (0 pps)
>   Active alarms  : None
>   Active defects : None
>   PCS statistics                      Seconds
>     Bit errors                             0
>     Errored blocks                         0
>   Ethernet FEC Mode  :                 FEC119
>   Ethernet FEC statistics              Errors
>     FEC Corrected Errors               801787
>     FEC Uncorrected Errors                  0
>     FEC Corrected Errors Rate            2054
>     FEC Uncorrected Errors Rate             0
>   Link Degrade :
>     Link Monitoring                   :  Disable
>   Interface transmit statistics: Disabled
>
>   Logical interface et-7/1/4.0 (Index 420) (SNMP ifIndex 815)
>     Flags: Up SNMP-Traps 0x4004000 Encapsulation: ENET2
>     Input packets : 1
>     Output packets: 1
>     Protocol inet, MTU: 1500
>     Max nh cache: 75000, New hold nh limit: 75000, Curr nh cnt: 1,
> Curr new hold cnt: 0, NH drop cnt: 0
>       Flags: Sendbcast-pkt-to-re
>       Addresses, Flags: Is-Preferred Is-Primary
>         Destination: 10.10.10.76/30, Local: 10.10.10.77, Broadcast:
> 10.10.10.79
>
> --
> -Aaron