Management and Configuration Guide K/KA/KB.15.15

Table 15 Per-port outbound priority queues
Outbound priority queue for a given port802.1p Priority settings in tagged VLAN packets
1
11 (low)
22 (low)
30 (normal)
43 (normal)
54 (medium)
65 (medium)
76 (high)
87 (high)
1
The switch processes outbound traffic from an untagged port at the "0" (normal) priority level.
You can use GMB to reserve a specific percentage of each port's available outbound bandwidth
for each of the eight priority queues. This means that regardless of the amount of high-priority
outbound traffic on a port, you can ensure that there will always be bandwidth reserved for
lower-priority traffic.
Since the switch services outbound traffic according to priority (highest to lowest), the highest-priority
outbound traffic on a given port automatically receives the first priority in servicing. Thus, in most
applications, it is necessary only to specify the minimum bandwidth you want to allocate to the
lower priority queues. In this case, the high-priority traffic automatically receives all unassigned
bandwidth without starving the lower-priority queues.
Conversely, configuring a bandwidth minimum on only the high-priority outbound queue of a port
(and not providing a bandwidth minimum for the lower-priority queues) is not recommended,
because it may "starve" the lower-priority queues.
NOTE: For a given port, when the demand on one or more outbound queues exceeds the minimum
bandwidth configured for those queues, the switch apportions unallocated bandwidth to these
queues on a priority basis. As a result, specifying a minimum bandwidth for a high-priority queue
but not specifying a minimum for lower-priority queues can starve the lower-priority queues during
periods of high demand on the high priority queue. For example, if a port configured to allocate
a minimum bandwidth of 80% for outbound high-priority traffic experiences a demand above this
minimum, this burst starves lower-priority queues that do not have a minimum configured. Normally,
this will not altogether halt lower priority traffic on the network, but will likely cause delays in the
delivery of the lower-priority traffic.
The sum of the GMB settings for all outbound queues on a given port cannot exceed 100%.
Impacts of QoS queue configuration on GMB operation
The section on “Configuring Guaranteed Minimum Bandwidth (GMB) for outbound traffic” (page 177)
assumes the ports on the switch offer eight prioritized, outbound traffic queues. This may not always
be the case, however, because the switch supports aQoS queue configuration feature that allows
you to reduce the number of outbound queues from eight (the default) to four queues, or two.
Changing the number of queues affects the GMB commands (interface bandwidth-min and
show bandwidth output) such that they operate only on the number of queues currently
configured. If the queues are reconfigured, the guaranteed minimum bandwidth per queue is
automatically re-allocated according to the following percentages:
Guaranteed minimum bandwidth (GMB) 191