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DES-3010F/DES-3010FL/DES-3010G/DES-3016/DES-3018/DES-3026 Fast Ethernet Switch Manual
111
Understanding CoS
The DES-3000 series switch supports 802.1p priority queuing. The DES-3000 series 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 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 CoS priority scheduling settings the method used is strict priority.
For strict priority-based scheduling, packets residing in the highest priority class of service are transmitted
first. Once a strict scheduling is implemented for CoS, the highest class will work in strict mode and the
other classes will remain in a weight fair scheduling mode. 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 CoS scheduling arrangement is a strict priority schedule which means
the switch will consider the highest class of service to have strict scheduling only, while the
other queues empty in a round-robin method. See the config scheduling_mechanism
command in this section for more information regarding this subject.
To use implement round-robin (weighted) priority, the Switch’s four priority classes of service can 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 lower priority class of service get 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 weight value range is: 1 to 55 packets.
In networking environments that use alternative priority protocols, the Switch’s CoS can be mapped to
accommodate DSCP priority and Type of Service (ToS) priority. CoS can also be mapped to specified
destination MAC addresses or ports on the Switch.