Understanding Qos - D-Link xStack DGS-3400 Series User Manual

xStack DGS-3400 Series Layer 2 Gigabit Ethernet Managed Switch

Understanding QoS

The xStack DGS-3400 Series supports 802.1p priority queuing. The Switch has 8 priority queues. These priority queues are
numbered from 6 (Class 6) — the highest priority queue — to 0 (Class 0) — the lowest priority queue. The eight priority tags
specified in IEEE 802.1p (p0 to p7) are mapped to the Switch's priority queues as follows:
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Priority 0 is assigned to the Switch's Q2 queue.
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Priority 1 is assigned to the Switch's Q0 queue.
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Priority 2 is assigned to the Switch's Q1 queue.
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Priority 3 is assigned to the Switch's Q3 queue.
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Priority 4 is assigned to the Switch's Q4 queue.
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Priority 5 is assigned to the Switch's Q5 queue.
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Priority 6 is assigned to the Switch's Q6 queue.
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Priority 7 is assigned to the Switch's Q6 queue.
For strict priority-based scheduling, any packets residing in the higher priority classes of service are transmitted first. Multiple
strict priority classes of service are emptied based on their priority tags. Only when these classes are empty, are packets of lower
priority transmitted.
For weighted round-robin queuing, the number of packets sent from each priority queue depends upon the assigned weight. For a
configuration of 8 CoS queues, A~H with their respective weight value: 8~1, the packets are sent in the following sequence: A1,
B1, C1, D1, E1, F1, G1, H1, A2, B2, C2, D2, E2, F2, G2, A3, B3, C3, D3, E3, F3, A4, B4, C4, D4, E4, A5, B5, C5, D5, A6, B6,
C6, A7, B7, A8, A1, B1, C1, D1, E1, F1, G1, H1.
For weighted round-robin queuing, if each CoS queue has the same weight value, then each CoS queue has an equal opportunity
to send packets just like round-robin queuing.
For weighted round-robin queuing, if the weight for a CoS is set to 0, then it will continue processing the packets from this CoS
until there are no more packets for this CoS. The other CoS queues that have been given a nonzero value, and depending upon the
weight, will follow a common weighted round-robin scheme.
Remember that the xStack DGS-3400 switch series has 7 configurable priority queues (and seven Classes of Service) for each port
on the Switch.
NOTICE: The Switch contains eight classes of service for each port on the Switch. One of
these classes is reserved for internal use on the Switch and is therefore not configurable. All
references in the following section regarding classes of service will refer to only the seven
classes of service that may be used and configured by the administrator.
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