[MX] Different BPS values displayed in interface and queue statistics

Summary:

Sometimes, users may notice that the Bits Per Second (BPS) values in the show interfaces extensive output and the show interfaces queue output are different. This can cause confusion when troubleshooting interface bandwidth or Class of Service (CoS) related issues.

This article provides an explanation for the BPS values to be different in these command outputs, and includes the formulae to use to calculate different traffic rates.

Symptoms:

When checking BPS values on egress interfaces by using the show interfaces extensive and show interfaces queue commands, significant differences in the values are observed in the two outputs:

user@host> show interfaces xe-5/0/1 extensive 

Physical interface: xe-5/0/1, Enabled, Physical link is Up
 .....
  Traffic statistics:
   Input  bytes  :        1143907959514           5800027488 bps
   Output bytes  :        1143916433329           5800027720 bps <<< 5.8Gbps
   Input  packets:          11987204562             12500059 pps
   Output packets:          11987261126             12500059 pps
 ......


user@host> show interfaces queue xe-5/0/1 

Physical interface: xe-5/0/1, Enabled, Physical link is Up
 .....
Queue: 0, Forwarding classes: BE
  Queued:
    Packets              :           12429718082              12499952 pps
    Bytes                :         1691620731707            9999961984 bps <<< 10Gbps
  Transmitted:
    Packets              :           12429718082              12499952 pps
    Bytes                :         1691620731707            9999961984 bps <<< 10Gbps
....

Notice that the Packet Per Second (PPS) values are nearly the same in the above two outputs but the BPS values are very different (5.8 Gbps Vs. 10 Gbps).

Solution:

This is expected behavior due to the following:

The show interfaces queue command displays Layer 1 BPS values.
The show interfaces extensive command displays Layer 2 or Layer 3 BPS values (the interface can be Layer 2 or Layer 3).

On Ethernet interfaces, different traffic rates can be calculated as below:

L3 rate (bps) = pps * 8 bytes * layer3 packet size
L2 rate (bps) = pps * 8 bytes * ethernet frame size
L1 rate (bps) = pps * 8 bytes * (ethernet frame size + preamble + inter-frame gap)

It is recommended that users be aware of the type of traffic rate displayed in each command and use the above formulas to convert traffic rates in different layers.

Knowledge Base