On QFX5110 / QFX5120 and EX4100 / EX4400 series switch, you may observe the issue that the logical interface output stats with bytes format decreases sometimes.
This is a limitation which can be applied to QFX5110 / QFX5120 / EX4100 / EX4400 series switch. This KB describes this behavior in detail.

The issue could be found by SNMP when collecting interface output stats with bytes format byifHCOutOctets or ifOutOctets and there is no user traffic, but only control packets such as BGP and OSPF are passing through interfaces.
Here is the sample of outputs, ifHCOutOctets and Output bytes are decreasing 65030 bytes on xe-0/0/3.
--------------------
Thu Jan 16 18:24:42 UTC 2025
ifHCOutOctets.552 = 477491966113

Thu Jan 16 18:25:42 UTC 2025
ifHCOutOctets.552 = 477491900672   <<<========================================= Here

Thu Jan 16 18:26:43 UTC 2025
ifHCOutOctets.552 = 477491902475
--------------------

The same behavior is seen on the stats by "show interface XXX" command.

This is a chip limitation and here are the details:

1)
The PFE cannot calculate the byte count of egress control packets for logical interfaces.
As a result, the byte counter for control packets on logical interfaces does not increase on the PFE side.

2)
The values shown in CLI outputs or retrieved via SNMP are taken from the kernel, which maintains its own counters.

3)
To ensure accuracy, the kernel periodically synchronizes its counters with the PFE.
When this synchronization occurs, the kernel’s counter values may also decrease.
 
4)
There is no issue with IFD (physical interfaces), and packet statistics themselves are also unaffected.
 
5)
The same behavior must be occurred when transit traffic is going out with control packets, but it's difficult to notice because the stats values keep increasing.

2025-05-22 : Updated affected product
2025-04-18 : Article Created