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xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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802.1p User Priority
The DES-3800 Series allows the assignment of a user priority to each of the 802.1p priorities. Click QoS > 802.1p User Priority,
to view the screen shown below.
Figure 9- 6. QoS Class of Traffic window
Once you have assigned a priority to the port groups on the Switch, you can then assign this Class to each of the 8 levels of 802.1p
priorities. Click Apply to set your changes.
WRED Settings
WRED or Weighted Random Early Discard is another implementation for QoS that will help the overall throughput for your QoS
queues. Based on the egrees queue of the QoS function set on the Switch, this method will analyze these packets and their QoS
queue to determine if there will be an overflow of packets entering the QoS queues and consequentially, minimize the packet flow
into these queues by dropping random packets. WRED employs two methods of avoiding congestion within the QoS queue.
1. Every QoS queue has a minimum and a maximum level for acceptance of packets. Once the maximum threshold has
been reached for this queue, the switch will begin discarding all ingress packets, this minimizing the allotted bandwidth
for QoS. When below the minimum threshold, the switch will accept all ingress packets.
2. When the ingress packets are somewhere between the maximum and minimum queue, the Switch will use a slope
probability function to determine a random method of dropping packets based on the fill percentage of the QoS queue. If
queues are closer to being full, the Switch will increase the discarding of random packets to even out the flow to the
queues and avoid overflows to higher priority queues.
Figure 9- 7. WRED Settings window