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6.
Advanced Settings
6.1.
Bandwidth Control
6.1.1.
QoS
6.1.1.1.
Introduction
Each egress port can support up to 8 transmit queues. Each egress transmit queue contains a list
specifying the packet transmission order. Every incoming frame is forwarded to one of the 8
egress transmit queues of the assigned egress port, based on its priority. The egress port transmits
packets from each of the 8 transmit queues according to a configurable scheduling algorithm,
which can be a combination of Strict Priority (SP) and/or Weighted Round Robin (WRR).
Typically, networks operate on a best-effort delivery basis, which means that all traffic has equal
priority and an equal chance of being delivered in a timely manner. When congestion occurs, all
traffic has an equal chance of being dropped.
When you configure the QoS feature, you can select specific network traffic, prioritize it
according to its relative importance, and use congestion-management and congestion-avoidance
techniques to give preferential treatment. Implementing QoS in your network makes network
performance more predictable and bandwidth utilization more effective.
The Switch supports 802.1p priority queuing. The Switch has 8 priority queues. These priority
queues are numbered from 7 (Class 7) — 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:
Priority : 0 1 2 3 4 5 6 7
Queue
: 2 0 1 3 4 5 6 7
Priority scheduling is implemented by the priority queues stated above. The Switch will empty
the four hardware priority queues in order, beginning with the highest priority queue, 7, to the
lowest priority queue, 0. Each hardware queue will transmit all of the packets in its buffer before
permitting the next lower priority to transmit its packets. When the lowest hardware priority
queue has finished transmitting all of its packets, the highest hardware priority queue will begin
transmitting any packets it may have received.
QoS Enhancement
You can configure the Switch to prioritize traffic even if the incoming packets are not marked
with IEEE 802.1p priority tags or change the existing priority tags based on the criteria you select.
The Switch allows you to choose one of the following methods for assigning priority to incoming
packets on the Switch:
802.1p Tag Priority
- Assign priority to packets based on the packet’s 802.1p tagged
priority.
Port Based QoS
- Assign priority to packets based on the incoming port on the
Switch.
