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DES-3550 Fast Ethernet Layer 2 Switch
Figure 6- 42. Mapping QoS on the Switch
The picture above shows the default priority setting for the Switch. Class-3 has the highest priority of the four priority
queues on the Switch. In order to implement QoS, the user is required to instruct the Switch to examine the header of a
packet to see if it has the proper identifying tag tagged. Then the user may forward these tagged packets to designated
queues on the Switch where they will be emptied, based on priority.
For example, lets say a user wishes to have a video conference between two remotely set computers. The administrator can
add priority tags to the video packets being sent out, utilizing the Access Profile commands. Then, on the receiving end,
the administrator instructs the Switch to examine packets for this tag, acquires the tagged packets and maps them to a class
queue on the Switch. Then in turn, the administrator will set a priority for this queue so that will be emptied before any
other packet is forwarded. This results in the end user receiving all packets sent as quickly as possible, thus prioritizing the
queue and allowing for an uninterrupted stream of packets, which optimizes the use of bandwidth available for the video
conference.
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. •
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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.
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