[EX/QFX] PFE Discard statistics counter does not incremement on the device.

Summary:

This article explains the behavior of PFE discard statistics counters on the following devices:

QFX3500
QFX5100
QFX5110
QFX5120
QFX5200
QFX5210
EX4600
EX4650

Symptoms:

PFE Discard Statistics Counter shows 0 on QFX3500, QFX5100, EX4600, QFX5110, QFX5200, QFX5210, QFX5120, EX4650 devices.

user@qfx5100> show pfe statistics traffic <snip>

Packet Forwarding Engine hardware discard statistics: Timeout : 0 Truncated key : 0 Bits to test : 0 Data error : 0 TCP header length error : 0 Stack underflow : 0 Stack overflow : 0 Normal discard : 0 Extended discard : 0 Invalid interface : 0 Info cell drops : 0 Fabric drops : 0 Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics: Input Checksum : 0 Output MTU : 0

Cause:

This is due to a product limitation.

Solution:

This is an expected behavior and is a product limitation on QFX3500, QFX5100, EX4600, QFX5110, QFX5200, QFX5210, QFX5120, EX4650 devices. Since these devices do not support these counter values, there is no increment seen. This does not mean that there is an issue with the device. This is a default expected behavior.

In other Junos products like MX and PTX, these discards are incremented and when there is a discard seen for a specific category. This differentiation of discard counters on MX as seen on QFX solely depends on the underlying hardware and cannot be manually modified.

Example output from MX960 device:

user@host> show pfe statistics traffic 
<snip>
Packet Forwarding Engine hardware discard statistics:
    Timeout                    :                    0
  *****LINE OMITTED*****
    Bits to test               :                    0
    Data error                 :                    0
    Stack underflow            :                    0
    Stack overflow             :                    0
    Normal discard             :             81704366
    Extended discard           :                    0
    Invalid interface          :                    0
    Info cell drops            :                    0
    Fabric drops               :                  227
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
    Input Checksum             :                    0
    Output MTU                 :                    0

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