Transmission Priority Scheduling

Low Latency Queuing (LLQ) Overview

On platforms that support low latency queuing (LLQ). LLQ enables delay-sensitive data to have preferential treatment over other traffic. A low-latency queue has the highest priority over any other priority queues, including strict-high queues, as well as a low delay scheduling profile.

For port scheduling of virtual output queues (VOQs), low latency VOQs receive their own dedicated egress queue. High priority VOQs receive a second dedicated egress queue, and low priority VOQs receive a third dedicated egress queue.

Due to the scheduling hierarchy of hierarchical class of service (HCoS), a hierarchical scheduling can use a maximum of two egress queues. Therefore for hierarchical scheduling of VOQs, low latency VOQs and high priority VOQs receive a common dedicated egress queue, and low priority VOQs receive the second dedicated egress queue.

Note:

We recommend the following when configuring low-latency VOQs:

• Use policers to normalize the burstiness of traffic before it reaches a low-latency VOQ.

• Configure a maximum of two low-latency VOQs on a physical or logical interface.

• Classify and schedule traffic (that is, reserve bandwidth) for low-latency VOQs so that there is no congestion for those queues.

Note:

Low-latency queues receive the same buffers as other queues to efficiently use the limited hardware VOQ buffer profiles.

Strict-High Priority Configuration Overview

Depending on the platform can configure one or more queues per interface to have strict-high priority, which works the same as high priority, but provides unlimited transmission bandwidth. As long as the queue with strict-high priority has traffic to send, it receives precedence over all other queues, except queues with high priority. Queues with strict-high and high priority take turns transmitting packets until the strict-high queue is empty, the high priority queues are empty, or the high priority queues run out of bandwidth credit. Only when these conditions are met can lower priority queues send traffic.

On platforms that support multiple strict-high queues per interface, the hardware services the queues in the descending order of queue numbers marked with strict-high priority.

When you configure a queue to have strict-high priority, you do not need to include the transmit-rate statement in the queue configuration at the [edit class-of-service schedulers scheduler-name] hierarchy level because the transmission rate of a strict-high priority queue is not limited by the WRR configuration. If you do configure a transmission rate on a strict-high priority queue, it does not affect the WRR operation. The transmission rate does, however, affect the calculation of the delay buffer and also serves as a placeholder in the output of commands such as the show interface queue command.

strict-high priority queues might starve low priority queues, and under certain circumstances might limit high priority queues. The high priority allows you to protect traffic classes from being starved by traffic in a strict-high queue. For example, a network-control queue might require a small bandwidth allocation (say, 5 percent). You can assign high priority to this queue to prevent it from being underserved.

A queue with strict-high priority supersedes bandwidth guarantees for queues with lower priority; therefore, we recommend that you use the strict-high priority to ensure proper ordering of special traffic, such as voice traffic. You can preserve bandwidth guarantees for queues with lower priority by allocating to the queue with strict-high priority only the amount of bandwidth that it generally requires by applying the rate-limit option to the strict-high queue’s transmission rate. For example, consider the following allocation of transmission bandwidth:

• Q0 BE—20 percent, low priority

• Q1 EF—30 percent, strict-high priority

• Q2 AF—40 percent, low priority

• Q3 NC—10 percent, low priority

This bandwidth allocation assumes that, in general, the EF forwarding class requires only 30 percent of an interface’s transmission bandwidth. However, if short bursts of traffic are received on the EF forwarding class, and the rate-limit option is not applied, 100 percent of the bandwidth is given to the EF forwarding class because of the strict-high setting.