You are asked to implement CoS on an EX Series switch. You attempt to configure the priority for
the voice and data queue schedulers to medium-high and medium-low priority, respectively.
However, you notice that the only parameters available for the priority is strict high and low.
Why are strict high and low the only available parameters for configuration?
A.
The loss priority for the queues must first be set to medium-low and medium-high, respectively.
B.
The switch only supports the strict high and low queue priorities.
C.
The shared buffer feature must be configured prior to configuring scheduler priority.
D.
The scheduler must be applied to an interface prior to configuring scheduler priority.
http://www.juniper.net/documentation/en_US/junos12.1/topics/concept/cos-qfx-series-schedulers-understanding.html
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Scheduling Priority
Scheduling priority determines the order in which an output interface transmits traffic from the queues, thus ensuring that queues containing important traffic receive better access to the outgoing interface. The priority setting in the scheduler determines the priority for the queue.
Two levels of scheduling priority are supported:
Low—Low-priority queues transmit traffic based on the weighted round robin (WRR) algorithm. The scheduler first determines if an individual queue is within its defined bandwidth profile. The scheduler then regularly reevaluates whether each individual queue is within its defined bandwidth profile and compares the amount of data the queue transmits to the amount of bandwidth the scheduler allocates to the queue. When the transmitted amount is less than the allocated amount, the queue is considered to be in profile. A queue is out of profile when its transmitted amount is larger than its allocated amount. Out of profile queue data is transmitted only if bandwidth is available. Otherwise, it is buffered if buffer space is available. If no buffer space is available, the traffic may be dropped.
Strict-high—You can configure only one queue as strict-high priority. The other 11 queues are low priority.
The strict-high priority queue receives preferential treatment over the low-priority queues. The strict-high priority queue receives all of its configured bandwidth before low-priority queues are serviced. Low-priority queues do not transmit traffic until the strict-high priority queue is empty. Carefully consider how much bandwidth you want to allocate to the strict-high priority queue to avoid starving the low-priority queues.
If you configure a strict-high priority queue, you must observe the following rules:
You must create a separate forwarding class set (priority group) for the strict-high priority queue.
Only one forwarding class set can contain strict-high priority queues.
Strict-high priority queues cannot belong to the same forwarding class set as queues that are not strict-high priority.
A strict-high priority queue cannot belong to a multidestination forwarding class set.
You cannot configure a minimum guaranteed bandwidth for a strict-high priority queue. (You cannot configure a transmit rate for a strict-high priority queue scheduler, and you cannot configure a guaranteed rate for a forwarding class set that has a strict-high priority queue.)
Junos OS performs priority queueing using the following steps:
Services the strict-high priority queue before any other queues are served
Services the minimum bandwidth (transmit rate) of low-priority queues until the minimum is met or the queues are empty
Services all other low-priority queues and needs that exceed the minimum bandwidth
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