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DES-3500 Series Layer 2 Stackable Fast Ethernet Managed Switch User Manual
83
Understanding QoS
The Switch has four priority queues. These priority queues are labeled as 3, the high queue to 0, the lowest queue. The eight
priority tags, specified in IEEE 802.1p are mapped to the Switch's priority tags as follows:
• Priority 0 is assigned to the Switch's Q1 queue.
• Priority 1 is assigned to the Switch's Q0 queue.
• Priority 2 is assigned to the Switch's Q0 queue.
• Priority 3 is assigned to the Switch's Q1 queue.
• Priority 4 is assigned to the Switch's Q2 queue.
• Priority 5 is assigned to the Switch's Q2 queue.
• Priority 6 is assigned to the Switch's Q3 queue.
• Priority 7 is assigned to the Switch's Q3 queue.
For strict priority-based scheduling, any packets residing in the higher priority queues are transmitted first. Multiple strict priority
queues empty based on their priority tags. Only when these queues 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 DES-3500 Series switches has four priority queues (and four Classes of Service) for each port on the Switch.
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