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[QFX] Shared buffer configs to mitigate multicast related tail drops

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Article ID: KB36658 KB Last Updated: 26 Apr 2021Version: 1.0
Summary:

 This article suggests that shared buffer configurations can be used to mitigate the issue of tail drops in an environment with traffic bursts.

Symptoms:

Occasionally, queue drops are seen on interfaces even if interface utilization is less than 100%.

The following is a drop counter comparison with about a 5-minute interval between them.

user@QFX5200> show interfaces ae1 extensive | find "Egress queues"                    

  Egress queues: 10 supported, 9 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0                                0          21181578237                    0
    2                                0                    0                    0
    3                                0                    0                    0
    4                                0                    0                    0
    5                                0                    0                    0
    6                                0              1520936                    0
    7                                0                    0                    0
    8                                0         255054573446                 7774
    9                                0         448103107069                    0

 

user@QFX5200> show interfaces ae1 extensive | find "Egress queues"   

  Egress queues: 10 supported, 9 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0                                0          21343010517                    0
    2                                0                    0                    0
    3                                0                    0                    0
    4                                0                    0                    0
    5                                0                    0                    0
    6                                0              1534997                    0
    7                                0                    0                    0
    8                                0         257902117369                 8565
    9                                0         451503846648                    0

 

user@QFX5200> show interfaces ae1 extensive | find "Egress queues"   

  Egress queues: 10 supported, 9 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0                                0          21430328732                    0
    2                                0                    0                    0
    3                                0                    0                    0
    4                                0                    0                    0
    5                                0                    0                    0
    6                                0              1541829                    0
    7                                0                    0                    0
    8                                0         259457625088                 9379
    9                                0         453342838977                    0

In the above example, the interface utilization is well below 100% but queue drops are still seen. This is for the multicast queue. 

Cause:

In certain situations, especially with cable providers, multicast traffic has a lot of bursts. The queue drops are a symptom of these traffic bursts, which can have a negative impact on customer experience.

Solution:

A shared buffer configuration can be effectively used to mitigate this situation. However, the configuration should be tweaked according to user needs based on the pattern of traffic. This is usually done on trial-and-error basis as traffic pattern varies from site to site. Below is a sample configuration for a shared buffer:

set class-of-service shared-buffer egress percent 100
set class-of-service shared-buffer egress buffer-partition lossless percent 5
set class-of-service shared-buffer egress buffer-partition lossy percent 50
set class-of-service shared-buffer egress buffer-partition multicast percent 45

Output after applying the shared buffer configuration

The following is a drop counter comparison with about a 5-minute interval between them after applying the shared buffer configurations:

user@QFX5200> show interfaces ae1 extensive | find "Egress queues"                     
  Egress queues: 10 supported, 9 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0                                0          12270467126                    0
    2                                0                    0                    0
    3                                0                    0                    0
    4                                0                    0                    0
    5                                0                    0                    0
    6                                0              2411825                    0
    7                                0                    0                    0
    8                                0         144143462335                 3565
    9                                0         337012016258                    0
 
 
user@QFX5200> show interfaces ae1 extensive | find "Egress queues"    
  Egress queues: 10 supported, 9 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0                                0          12431899406                    0
    2                                0                    0                    0
    3                                0                    0                    0
    4                                0                    0                    0
    5                                0                    0                    0
    6                                0              2422714                    0
    7                                0                    0                    0
    8                                0         146991006258                 3968
    9                                0         340412755837                   0
 
 
user@QFX5200> show interfaces ae1 extensive | find "Egress queues"    
  Egress queues: 10 supported, 9 in use
  Queue counters:       Queued packets  Transmitted packets      Dropped packets
    0                                0          12519217621                    0
    2                                0                    0                    0
    3                                0                    0                    0
    4                                0                    0                    0
    5                                0                    0                    0
    6                                0              2429546                    0
    7                                0                    0                    0
    8                                0         148546513977                 4418
    9                                0         342251748166                    0
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