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Continuous Increment In 'Drop On No Resources' Counters

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TitleContinuous Increment In 'Drop On No Resources' Counters
Symptoms
Continuous increments in "Dropped On No Resources" counter on a port. Degraded performance of devices connected to this port.
Multiple iterations of "show port-statistics" can help to identify the issue, and to determine its magnitude.
Environment
  • ERS 3500,3600,4500,4800,4900,5500,5600,5900
  • VSP 7024
  • All supported BOSS software versions
Cause

"Dropped On No Resources" counter represents the cumulative number of packets dropped from the egress queues. Egress queue drops result from over-subscription.

Resolution
The issue can be addressed in several ways (or a combination of these)

Increase the amount of shared buffer memory available to a port (available settings: "regular", "large", "maximum"):
1. Type enable
2. Type configure terminal (you will be in global configuration mode for the following commands)
3. Type qos agent buffer maximum
WARNING: Setting the buffer size of "qos agent buffer" to "max" allows the offending port to 'borrow' buffer space from other ports which can result in the other ports dropping packets due to lack of buffer space. Please use with caution.


If possible, reduce the number of queues by decreasing the queue-set value. The more queues are configured, the less buffer resources are available per queue:
1. Type qos agent queue-set <number>
Note: For this change to be effective, the device needs to be rebooted.

If possible, move the traffic type for which the drops are observed, to another queue with more buffer resources and bigger bandwidth allocation. Example:
Unit/Port  Queue       Out Packets          Drop Packets    
                        Out Bytes            Drop Bytes     
---------  -----  --------------------  --------------------
1/1        1      54900345              0                   
                  11424058718           0                   
           2      49134                 0                   
                  7267064               0                   
           3      128                   0                   
                  26348                 0                   
           4      0                     0                   
                  0                     0                   
           5      45                    0                   
                  3060                  0                   
           6      37108769              0                   
                  6651224738            0                   
           7      0                     0                   
                  0                     0                   
           8      5081257632            26022255            
                  2272865743121         36843372643


Drop count is seen for queue 8, which serves QoS unmarked traffic (1p=0). Over a period of time no traffic was hitting queue 4, so the QoS unmarked traffic can be assigned to it.
Note: That queue 4 has much more buffer resources allocated to it

1. Type show qos queue-set 8
Set Queue        General        Bandwidth Bandwidth  Service  Size
ID   ID        Discipline          (%)    Allocation  Order  (Bytes)

___ _____ _____________________ _________ __________ _______ _______
8   1     Priority Queuing      100       Relative   1       32768
8   2     Weighted Round Robin  41        Relative   2       30720
8   3     Weighted Round Robin  19        Relative   2       28672
8   4     Weighted Round Robin  13        Relative   2       26624
8   5     Weighted Round Robin  11        Relative   2       23552
8   6     Weighted Round Robin  8         Relative   2       20480
8   7     Weighted Round Robin  5         Relative   2       16896
8   8     Weighted Round Robin  3         Relative   2       16896

The "best effort" traffic can be assigned to queue 4 with this command:
1. Type qos queue-set-assignment queue-set 8 1p 0 queue 4
Note: To help with congestion you can add more ports to the link on which the drops are seen.
Additional notes

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