Congestion Management Technique Comparison - HP MSR4080 Configuration Manual

Emergency queue—Enqueues emergent packets. The emergency queue is a FIFO queue without
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bandwidth restriction.
Low Latency Queuing (LLQ)—An EF queue. Because packets are fairly treated in CBQ,
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delay-sensitive flows like video and voice packets might not be transmitted timely. To solve this
problem, LLQ combines PQ and CBQ to preferentially transmit delay-sensitive flows like voice
packets. When defining traffic classes for LLQ, you can configure a class of packets to be
preferentially transmitted. The packets of all priority classes are assigned to the same priority queue.
Bandwidth restriction on each class of packets is checked before the packets are enqueued. During
the dequeuing operation, packets in the priority queue are transmitted first. To reduce the delay of
the other queues except the priority queue, LLQ assigns the maximum available bandwidth to each
priority class. The bandwidth value polices traffic during congestion. When no congestion is
present, a priority class can use more than the bandwidth assigned to it. During congestion, the
packets of each priority class exceeding the assigned bandwidth are discarded.
Bandwidth queuing (BQ)—An AF queue. The BQ provides guaranteed bandwidth for AF traffic,
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and schedules the AF classes proportionally. The system supports up to 64 AF queues.
Default queue—A WFQ queue. The default queue transmits the BE traffic by using the remaining
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interface bandwidth.
The system matches packets with classification rules in the following order:
Match packets with priority classes and then the other classes.
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• Match packets with priority classes in the configuration order.
Match packets with other classes in the configuration order.
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Match packets with classification rules in a class in the configuration order.
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Congestion management technique comparison

Congestion management techniques offer different QoS capabilities to meet different application
requirements, as explained in
Table 3 Congestion management technique comparison
Type
FIFO
Table
Number of queues
1
3.
Advantages
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No need to configure, easy
to use.
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Easy to operate, low delay.
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Disadvantages
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All packets are treated equally.
The available bandwidth, delay
and drop probability are
determined by the arrival order of
packets.
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No restriction on traffic from
connectionless protocols
(protocols without any flow control
mechanism, UDP, for example),
resulting in bandwidth loss for
traffic of connection-oriented
protocols (TCP, for example).
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No delay guarantee for
time-sensitive real-time
applications, such as VoIP.