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QOS LLQ Question

Posted on 2010-08-17
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Last Modified: 2012-05-10
Priority queue is limited to a specific percentage so to not starve other  queues.

If I dedicated 20% of an interface bandwidth to an LLQ strict priority queue does this mean that at all time only %80 of the bandwidth would be available to other traffic? Even if 0% of the llq priority queue was not being used?
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Question by:question01
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decoleur earned 500 total points
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absolutely not.

the strict priority queue is used to send out traffic that has been labeled "priority" ahead of everything else, all things considered equal. when you allocate 20% of your bandwidth to the priority queue, it is only used when there is contention. if you have 15% priority traffic and 60% best effort traffic your interface will only have 75% utilization, with 15% in the priority queue and 60% in the best effort queue. if you have 50% priority traffic only, then 20% will be put into the priority queue and the rest will be treated as best effort with a total of 50% utilization on your interface. if you have 20% priority and 90% best effort, then 20% priority will be passed along and only the available remaining space will be made available to the best effort. if the priority utilization drops to 5% then all the best effort will be sent as long as the priority queue does not grow.

I hope this makes sense and this is all fine and dandy on your network, what you also need to take into consideration is that your carrier may or may not honor your traffic classification based on your SLA with them. Some times they do not allow priority traffic classification unless you have paid additional fees. One carrier calls it the Gold CAR and f you didn't pay for it they rate limited it to 8Kb and dropped all else, so us setting up a priority queue of 512Kb had disastrous results.

hope this helps,

-t
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by:question01
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Thanks decoleur,
good answer.
 

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