Specifications

Chapter 4: Quality of Service (QoS)
Extreme Networks EAS 100-24t Switch Software Manual
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The previous picture 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, if a user wishes to have a videoconference 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 result for the end user is 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 from 0 to 3, with 3 being the
highest priority and 0 the lowest priority 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 four QoS queues, A~D with their respective weight value:
4~1, the packets are sent in the following sequence: A1, B1, C1, D1, A2, B2, C2, A3, B3, A4.
For weighted round robin queuing, if each QoS queue has the same weight value, then each QoS queue
has an equal opportunity to send packets just like round robin queuing.
The Switch has four priority queues (and eight Classes of Service) for each port.