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C H A P T E R 8
130
Quality of Service (QoS)
•
Maximum bandwidth – The maximum percentage of total link
bandwidth that may be transmitted by a hardware queue on a
physical port. The default value on all maximum bandwidth
parameters is 100%.
•
Priority – The level of priority assigned to a hardware queue on a
physical port. The switch has 8 different available priority
settings. By default, each of the default QoS profiles is assigned a
unique priority. Prioritization is used under the following
circumstances:
-
When two or more hardware queues on the same physical
port are contending for transmission on the same physical
port, only after their respective bandwidth management
parameters have been satisfied. If two hardware queues on
the same physical port have the same priority, a round-robin
algorithm is used for transmission.
-
When configured to do so, the priority of a QoS profile
determines the 802.1p bits used in the priority field of a
transmitted packet.
-
The priority of a QoS profile determines the DiffServ code
point value used in an IP packet when the packet is
transmitted.
•
Buffer – This parameter reserves buffer memory for use
exclusively by a QoS profile across all affected ports. The default
value for buffer settings is 0%. The summed value of all QoS
profile buffer parameters should not exceed 100%. Reserving
buffer memory for a QoS profile affects the dynamic buffer space
available to other QoS profiles. You should not modify the buffer
parameter unless specific situations and application behavior
indicates this need.
A QoS profile does not alter the behavior of the switch until it is
assigned to a traffic grouping. Recall that QoS profiles are linked to
hardware queues. There are multiple hardware queues per physical
port. By default, a QoS profile links to the identical hardware queue
across all the physical ports of the switch.
The settings for the default QoS profiles are summarized in
Table 31.
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