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Juniper MX Series by Harry Reynolds, Douglas Richard Hanks Jr.

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Trio Scheduling and Queuing

The scheduling stage determines when a given queue is serviced, in which order, and how much traffic can be drained at each servicing. In the Trio architecture, schedulers and queues are no longer closely linked, in that you can now have schedulers and shapers at all four levels of the H-CoS hierarchy, with queues only found at level 4.

When you configure a scheduler, you can define parameters for each of up to eight queues. These parameters include the scheduling priority, maximum queue depth/temporal delay, transmit rate, a peak rate (shaping), and how (or if) excess bandwidth is shared. At the queue level, you can also link to one or more WRED profiles; only queue-level schedulers support WRED profile linking. With H-CoS you can also define scheduling and shaping parameters at other levels of the hierarchy, though the specific parameters supported can vary by the node’s position in the hierarchy.

Scheduling Discipline

MX routers with Trio-based MPCs use a form of Priority Queue Deficit Weighted Round Robin (PQ-DWRR) scheduling with five levels of strict priority. PQ-DWRR extends the basic deficit round robin (DWRR) mechanism by adding support for one or more priority queues that exhibit minimal delay. The deficit part of the algorithm’s name stems from the allowance of a small amount of negative credit in an attempt to keep queues empty. The resultant negative balance from one servicing interval is carried over to the next quantum’s credit allocation, ...

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