QFX5120-48Y supports 25GBASE-SR and it mandates support for FEC108 as per IEEE802.3 Standard.
QFX5120-48Y has default FEC configuration of FEC74 on 25GBASE-SR and this sometimes may cause a lot of FEC errors and problem in establishing link if peer device is running on FEC108.
root@switch_1> show chassis hardware detail no-forwarding
Hardware inventory:
Item Version Part number Serial number Description
Chassis XH3722060085 JNP48Y8C-CHAS
Pseudo CB 0
Routing Engine 0 BUILTIN BUILTIN RE-QFX5120-48Y-8C
ada0 9316 MB QEMU QM00001 Virtio Block Disk
ada1 4096 MB QEMU QM00002 Virtio Block Disk
ada2 512 MB QEMU QM00003 Virtio Block Disk
ada3 1024 MB QEMU QM00004 Virtio Block Disk
usb0 (addr 0.1) UHCI root HUB 0 Intel uhub0
usb1 (addr 1.1) XHCI root HUB 0 0x8086 uhub1
FPC 0 REV 15 650-083242 XH3722060085 JNP48Y8C-CHAS
CPU BUILTIN BUILTIN FPC CPU
PIC 0 BUILTIN BUILTIN 48x25G-8x100G
Xcvr 0 REV 01 740-068639 1A1M31A616133 SFP28-25G-BASE-SR
Xcvr 1 REV 01 740-068639 1A1M31A61602S SFP28-25G-BASE-SR
Xcvr 2 REV 01 740-068639 1A1M31A61617E SFP28-25G-BASE-SR
Xcvr 3 REV 01 740-068639 1A1M31A6153YS SFP28-25G-BASE-SR
Xcvr 4 REV 01 740-068639 1A1M31A6160KC SFP28-25G-BASE-SR
Xcvr 5 REV 01 740-068639 1A1M31A61610T SFP28-25G-BASE-SR
Xcvr 6 REV 01 740-068639 1A1M31A6160WM SFP28-25G-BASE-SR
Xcvr 7 REV 01 740-068639 1A1M31A6160R2 SFP28-25G-BASE-SR
Xcvr 8 REV 01 740-068639 1A1M31A61601P SFP28-25G-BASE-SR
Xcvr 9 REV 01 740-068639 1A1M31A61617D SFP28-25G-BASE-SR
Xcvr 10 REV 01 740-068639 1A1M31A616196 SFP28-25G-BASE-SR
Xcvr 11 REV 01 740-068639 1A1M31A6160VY SFP28-25G-BASE-SR
Xcvr 12 REV 01 740-068639 1A1M31A61601L SFP28-25G-BASE-SR
Xcvr 13 REV 01 740-068639 1A1M31A6160UX SFP28-25G-BASE-SR
Xcvr 14 REV 01 740-068639 1A1M31A61619B SFP28-25G-BASE-SR
Xcvr 15 REV 01 740-068639 1A1M31A61611S SFP28-25G-BASE-SR
Xcvr 16 REV 01 740-068639 1A1M31A6161G6 SFP28-25G-BASE-SR
Xcvr 17 REV 01 740-068639 1A1M31A61601B SFP28-25G-BASE-SR <===== 25GBASE-SR
<snip>
Physical interface: et-0/0/17, Enabled, Physical link is Up
Interface index: 673, SNMP ifIndex: 616, Generation: 164
Description: fc1vcfwqa14c4
Link-level type: Ethernet, MTU: 9216, LAN-PHY mode, Speed: 25Gbps, 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
Interface flags: SNMP-Traps Internal: 0x4000
Link flags : None
CoS queues : 10 supported, 10 maximum usable queues
Hold-times : Up 0 ms, Down 0 ms
Current address: ec:7c:5c:af:3c:15, Hardware address: ec:7c:5c:af:3c:15
Last flapped : 2023-03-28 11:17:48 KST (1d 21:43 ago)
Statistics last cleared: Never
Traffic statistics:
Input bytes : 4071847272 416 bps
Output bytes : 1153293017475 22400 bps
Input packets: 10206348 0 pps
Output packets: 2739214114 34 pps
IPv6 transit statistics:
Input bytes : 0
Output bytes : 0
Input packets: 0
Output packets: 0
Input errors:
Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Policed discards: 0, L3 incompletes: 0, L2 channel errors: 0, L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0
Output errors:
Carrier transitions: 25, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0
Egress queues: 10 supported, 5 in use
Queue counters: Queued packets Transmitted packets Dropped packets
0 1955955 1955955 0
3 0 0 0
4 0 0 0
7 8127282 8127282 0
8 2697686217 2697686217 0
Queue number: Mapped forwarding classes
0 best-effort
3 fcoe
4 no-loss
7 network-control
8 mcast
Active alarms : None
Active defects : None
PCS statistics Seconds
Bit errors 0
Errored blocks 0
Ethernet FEC Mode : FEC74 <===== FEC74 mode is being run currently
Ethernet FEC statistics Errors
FEC Corrected Errors 675219 <===== FEC Errors
FEC Uncorrected Errors 370708 <=====
FEC Corrected Errors Rate 0
FEC Uncorrected Errors Rate 0
<snip>{master:0}[edit]root@switch_1# set interfaces et-0/0/17 gigether-options fec ?Possible completions: fec108 IEEE 802.3by Clause 108, Reed-Solomon FEC (RS-FEC) on 25g links fec74 FEC74 enabled fec91 IEEE 802.3bj Clause 91, Reed-Solomon FEC (RS-FEC) none FEC disabled