Specifications

DGS-3024 Layer 2 Switch CLI Reference Manual
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QOS COMMANDS
The DGS-3024 Switch supports 802.1p priority queuing. The Switch has 4 priority classes of service. These priority classes of
service are numbered from 3 (Class 3) — the highest priority class of service — to 0 (Class 0) — the lowest priority class of
service. The eight priority queues specified in IEEE 802.1p (p0 to p7) are mapped to the Switch’s priority classes of service is as
follows:
• Priority 0 is assigned to the Switch’s Q1 class.
• Priority 1 is assigned to the Switch’s Q0 class.
• Priority 2 is assigned to the Switch’s Q0 class.
• Priority 3 is assigned to the Switch’s Q1 class.
• Priority 4 is assigned to the Switch’s Q2 class.
• Priority 5 is assigned to the Switch’s Q2 class.
• Priority 6 is assigned to the Switch’s Q3 class.
• Priority 7 is assigned to the Switch’s Q3 class.
Priority scheduling is implemented using two types of methods, strict priority and round-robin priority. If no changes are made
to the QoS priority scheduling settings, the method used is strict priority.
For strict priority-based scheduling, packets residing in the higher priority classes of service are transmitted first. Only when
these classes of service are empty, are packets of lower classes of service allowed to be transmitted. Higher priority packets
always receive preference regardless of the amount of lower priority packets in the buffer, and regardless of the time elapsed
since any lower priority packets have been transmitted. By default, the Switch is configured to empty the buffer using strict
priority.
NOTICE: The default QoS scheduling arrangement is a strict priority
schedule. To customize scheduling to set up round-robin queue clearing,
the MAX. Latency and MAX. Packets values need to be changed using
the config scheduling command. See config scheduling below.
To implement round-robin (weighted) priority, the Switch’s four priority classes of service should be configured to reduce the
buffer in a round-robin fashion - beginning with the highest priority class of service, and proceeding to the lowest priority class
of service before returning to the highest priority classes of service.
The weighted-priority based scheduling alleviates the main disadvantage of strict priority-based scheduling − in that the lower
priority class of service gets starved of bandwidth − by providing a minimum bandwidth to all classes of service for
transmission. This is accomplished by configuring the maximum number of packets allowed to be transmitted from a given
priority class of service, and the maximum amount of time a given priority class of service will have to wait before being
allowed to transmit its accumulated packets. This establishes a Class of Service (CoS) for each of the Switch’s four hardware
priority classes of service.
The possible range for maximum packets is: 0 to 15 packets.
The QoS commands in the Command Line Interface (CLI) are listed (along with the appropriate parameters) in the following
table.
Command Parameters
config scheduling <class_id 0-3> max_packet <value 0-15>
show scheduling
config 802.1p user_priority <priority 0-7> <class_id 0-3>
show 802.1p user_priority
config 802.1p default_priority [<portlist> | all] <priority 0-7>
show 802.1p default_priority {<portlist>}
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