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Quality of Service
Defining Queues
Cisco Small Business SG200 Series 8-port Smart Switch 167
11
Queue Configuration Recommendations
It is recommended that higher numbered queues be configured with higher
priority, weight, and minimum-bandwidth settings.
The following are recommended scenarios for strict priority (SP) and WRR
queues:
• All eight queues in SP mode (q8 > q7 > q6 > q5 > q4 > q3 > q2 > q1). q8 is
allocated bandwidth as long as there are packets to serve in q8. Then Q7 is
served, followed by Q6, and so forth.
• All 8-queues in WRR mode (q8:q7:q6:q5:q4:q3:q2:q1 = A:B:C:D:E:F:G:H). In
this mode, each queue is allocated its minimum bandwidth according to the
weights configured.
• One queue in SP mode and all other queues in WRR mode (q8 > q7/q6/.../
q1 and q7::q1 = A::G). In this scenario q8 is configured in SP mode and q7
through q1 in WRR mode.
• Four queues in SP mode and four queues in WRR mode (q8 > q7 > q6 >
q5 > q4/q3/q2/q1 and q4/q3/q2/q1 = A:B:C:D): In this scenario q8, q7, q6, and
q5 are configured in strict mode with q4, q3, q2 and q1 in WRR mode.
When there are more ingress ports with traffic destined to different queues on
egress ports, a system might encounter a Head of Line Blocking (HOL) condition.
HOL could result in higher numbered queues getting more bandwidth, although
higher numbered queues are configured with lower bandwidth and weight. It is
always recommended that higher numbered queues with higher weight be
configured in SP mode, so that even in a HOL condition, the desired egress
segregation is achieved.
Configuring Queues
To configure QoS properties:
STEP 1 Click Quality of Service > Queue in the navigation window.
STEP 2 Select from the Interface drop-down menus the Port or the LAG to configure.
STEP 3 Select one of the following modes for each queue on the selected interface:
• Strict Priority—Select to have the scheduler forward traffic strictly based
on the priority levels in the queues. The queue with the highest priority traffic
has access to the egress port until all such traffic is forwarded. Strict priority
mode provides low-latency service to higher priority classes of traffic.