Description

This article explains the reason that the counters retrieved from MIB OID is different from the value reported in 'show interfaces extensive' or 'show interfaces queue'.


Symptoms

On EX series devices, the SNMP MIB OID 'jnxCosIfqTailDropPkts' or 'jnxCosQstatTailDropPkts' is used to get the tail drop value from interfaces.  The 'show interfaces extensive' and 'show interfaces queue' commands also report the tail drop value.

Below is an example showing the commands to retrieve the tail drop counters via SNMP and via the 'show interfaces' command.  The values are expected to be the same. 

{master:0}
root> show snmp mib walk jnxCosQstatTailDropPkts.546    
jnxCosQstatTailDropPkts.546.0 = 712949
jnxCosQstatTailDropPkts.546.1 = 0
jnxCosQstatTailDropPkts.546.2 = 0
jnxCosQstatTailDropPkts.546.3 = 0
jnxCosQstatTailDropPkts.546.4 = 0
jnxCosQstatTailDropPkts.546.5 = 0
jnxCosQstatTailDropPkts.546.6 = 0
jnxCosQstatTailDropPkts.546.7 = 0


{master:0}
root> show interfaces queue ge-0/0/21 | no-more 
Physical interface: ge-0/0/21, Enabled, Physical link is Up
  Interface index: 132, SNMP ifIndex: 546
Forwarding classes: 16 supported, 4 in use
Egress queues: 8 supported, 4 in use
Queue: 0, Forwarding classes: best-effort
  Queued:
  Transmitted:
    Packets              :               8774793
    Bytes                :             561586776
    Tail-dropped packets :                712949
Queue: 1, Forwarding classes: assured-forwarding
  Queued:
  Transmitted:
    Packets              :                     0
    Bytes                :                     0
    Tail-dropped packets :                     0
Queue: 5, Forwarding classes: expedited-forwarding
  Queued:
  Transmitted:
    Packets              :                     0
    Bytes                :                     0
    Tail-dropped packets :                     0
Queue: 7, Forwarding classes: network-control
  Queued:
  Transmitted:
    Packets              :                   399
    Bytes                :                 33734
    Tail-dropped packets :                     0
root>

However, the EX device may encounter a situation where these two values are different.

Solution

The mismatch is caused when the "clear interfaces statistics" command is run.

For example: 

root> clear interfaces statistics ge-0/0/21                          

{master:0}
root> show snmp mib walk jnxCosQstatTailDropPkts.546    
jnxCosQstatTailDropPkts.546.0 = 712949
jnxCosQstatTailDropPkts.546.1 = 0
jnxCosQstatTailDropPkts.546.2 = 0
jnxCosQstatTailDropPkts.546.3 = 0
jnxCosQstatTailDropPkts.546.4 = 0
jnxCosQstatTailDropPkts.546.5 = 0
jnxCosQstatTailDropPkts.546.6 = 0
jnxCosQstatTailDropPkts.546.7 = 0


{master:0}
root> {master:0}
root> show interfaces queue ge-0/0/21 | no-more    
Physical interface: ge-0/0/21, Enabled, Physical link is Up
  Interface index: 132, SNMP ifIndex: 546
Forwarding classes: 16 supported, 4 in use
Egress queues: 8 supported, 4 in use
Queue: 0, Forwarding classes: best-effort
  Queued:
  Transmitted:
    Packets              :                     0
    Bytes                :                     0
    Tail-dropped packets :                     0
Queue: 1, Forwarding classes: assured-forwarding
  Queued:
  Transmitted:
    Packets              :                     0
    Bytes                :                     0
    Tail-dropped packets :                     0
Queue: 5, Forwarding classes: expedited-forwarding
  Queued:
  Transmitted:
    Packets              :                     0
    Bytes                :                     0
    Tail-dropped packets :                     0
Queue: 7, Forwarding classes: network-control
  Queued:
  Transmitted:
    Packets              :                     2
    Bytes                :                   136
    Tail-dropped packets :                     0
When more tail dropped packets are injected, the output values are still not the same: 
{master:0}
root> show snmp mib walk jnxCosQstatTailDropPkts.546    
jnxCosQstatTailDropPkts.546.0 = 3840995
jnxCosQstatTailDropPkts.546.1 = 0
jnxCosQstatTailDropPkts.546.2 = 0
jnxCosQstatTailDropPkts.546.3 = 0
jnxCosQstatTailDropPkts.546.4 = 0
jnxCosQstatTailDropPkts.546.5 = 0
jnxCosQstatTailDropPkts.546.6 = 0
jnxCosQstatTailDropPkts.546.7 = 0

{master:0}
root> show interfaces queue ge-0/0/21 | no-more         
Physical interface: ge-0/0/21, Enabled, Physical link is Up
  Interface index: 132, SNMP ifIndex: 546
Forwarding classes: 16 supported, 4 in use
Egress queues: 8 supported, 4 in use
Queue: 0, Forwarding classes: best-effort
  Queued:
  Transmitted:
    Packets              :              21635339
    Bytes                :            1384661696
    Tail-dropped packets :               3128046
Queue: 1, Forwarding classes: assured-forwarding
  Queued:
  Transmitted:
    Packets              :                     0
    Bytes                :                     0
    Tail-dropped packets :                     0
Queue: 5, Forwarding classes: expedited-forwarding
  Queued:
  Transmitted:
    Packets              :                     0
    Bytes                :                     0
    Tail-dropped packets :                     0
Queue: 7, Forwarding classes: network-control
  Queued:
  Transmitted:
    Packets              :                    89
    Bytes                :                  7537
    Tail-dropped packets :                     0


root>
In the output above, the tail drop counters retrieved from SNMP MIB OID, 3840995, is different from the 'show interfaces' value of 3128046.
712949 + 3128046 = 3840995.  This shows us that the value retrieved from SNMP MIB is an accumulative value.

This is expected behavior if the 'clear interfaces statistics' command has been executed sometime before through the CLI (Command Line Interface) on the switch.  See PR747155 .

Juniper MIB counters are real-time. The number of packets shown in the counter statistics of Snmpwalk ('jnxCosIfqTailDropPkts' or 'jnxCosQstatTailDropPkts') is the number of packets (dropped) since system start up. These stats are stored in the kernel and the values are NOT refreshed or reset after a particular time interval. It persists even when there is an interface flap or when interface statistics are cleared or when snmp/mib2d restarts. In case of a SNMP or mib2d restart, snmpd reads the counters from the kernel again. The only way to reset the statistics is by rebooting the device.

This behavior is as same for all SNMP MIB OIDs under the 'jnxCosIfqStatsTable' tree, i.e the "Transmitted bytes" OID 'jnxCosIfqTxedBytes'.