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Cisco Bug: CSCvu04431 - P1 queues without a policer can accumulate large deficit tokens leading to slow queue drains

Last Modified

May 18, 2020

Products (1)

  • Cisco ASR 9000 Series Aggregation Services Routers

Known Affected Releases

7.1.2.BASE

Description (partial)

Symptom:
On ASR9k Ethernet line cards, a priority 1 queue can accumulate very large token deficit if traffic
being enqueued to this queue is not rate limited by a policer. This can lead to a long time to drain
other queues as the deficit is overcome through periodic token refresh.

This can become visible as stuck queues or very slow in place modification at scale when there
is a low rate parent shaper.

Conditions:
The problem is specific to a queue marked as "priority level 1" through a user configured policy.

If such a queue has no policer action associated with the same class-map, then the queue can
accumulate a very large token deficit as P1 traffic is allowed to be transmitted on deficit tokens
to avoid critical traffic drops during micro bursts.

The problem happens when the parent reference rate is very low (such as a low rate parent,
grand-parent or port shaper) and the incoming rate for the queue is high
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Bug Details Include

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