QoS – Quick Post on Low Latency Queuing
A friend was looking for some input on low latency queuing yesterday. I thought the exchange we had could be useful for others so I decided to write a quick post.
The query was where the rule about the priority queue being limited to 33% came from. The follow up question is how you handle dual priority queues.
This is one of those rules that are known as a best practice and doesn’t really get challenged. The rule is based on Cisco internal testing within technical marketing. Their testing showed that data applications suffered when the LLQ was assigned a to large portion of the available bandwidth. The background to this rule is that you have a converged network running voice, video and data. It is possibly to break this rule if you are delivering a pure voice or pure video transport where the other traffic in place is not business critical. Other applications are likely to suffer if the LLQ gets too big and if everything is priority then essentially nothing is priority. I have seen implementations using around 50-55% LLQ for VoIP circuits which is a reasonable amount.
How should dual LLQs be deployed? The rule still applies. Continue reading