When two QFX-5120-48Y switches both running version 21.2R1.10 are connected via the 25g et interface, the connection is UP.
After upgrading one of the QFX-5120-48Y switches from 21.2R1.10 to 21.4R3-S3.4, the 25g connection is physical down.
Rolling back to 21.2R1.10 could resolve it.
Below table shows the summary:
SW1 SW2 25G UP
21.2R1.10 21.2R1.10 YES
21.2R1.10 21.4R3-S3.4 NO
21.4R3-S3.4 21.4R3-S3.4 YES
root > show chassis hardware detail no-forwarding
Xcvr 1 REV 01 740-071562 Y2201xxxxxx SFP28-25G-BASE-LR
The new Junos release supports FEC. FEC is not supported on 21.2R1.10. It is supported on 21.4R3-S3.4 . If it mismatches, link will not come up.
So there are two options to solve it. You can choose either one.
1. Disable fec on devices running 21.4R3-S3.4 to ensure consistency with opposite device running the old Junos version.
root# set interfaces <interface name> gigether-options fec none
2. Upgrade both switches to FEC supported Junos version.
Customer can check the FEC status via command 'show interface extensive'.
Here are the output samples.
------------------------------
Root> show interfaces et-0/0/46 extensive | match fec
Active defects : LINK, LOCAL-FAULT
Ethernet FEC Mode : FEC74 <<< support and enabled
Ethernet FEC statistics Errors
FEC Corrected Errors 0
FEC Uncorrected Errors 0
FEC Corrected Errors Rate 0
FEC Uncorrected Errors Rate 0
------------------------------
root> show interfaces et-0/0/46 extensive
Active alarms : None
Active defects : None
PCS statistics Seconds
Bit errors 0
Errored blocks 0
Ethernet FEC Mode : NONE <<< support but disabled
Root> run show interfaces et-0/0/46 extensive | match fec
Active defects : LINK, LOCAL-FAULT <<< not support