Configuring Qos - 3Com 7700 Configuration Manual

150 C 7: Q S/ACL O
HAPTER O

Configuring QoS

PERATION
Traffic Mirroring
The traffic mirroring function copies the specified data packets to the monitoring
port for network diagnosis and troubleshooting.
Traffic Counting
With flow-based traffic counting, you can request a traffic count to count and
analyze the packets.
RED
When the congestion reaches a certain degree, the Switch 7700 selects some
frames to drop, using the RED algorithm. The RED alogrithm can alleviate the
excessive congestion. In addition, the global TCP synchronization caused by the
Tail-Drop algorithm can be avoided.
In the RED algorithm, every queue has a pair of high and low limits. This algorithm
also regulates that:
If the queue length is smaller than the low limit, no packet is discarded.
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If the queue length is greater than the high limit, all the packets that arrive
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after the limit is reached are discarded.
If the queue length is between the high and low limits, the packets are
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discarded randomly as they arrive. Every new packet is given a random number.
This random number is compared with the discarding probability for the
current queue. Any packet whose random number is greater than the
probability is discarded. The longer the queue, the higher the discarding
probability . However, there is a maximum discarding probability.
Through randomly discarding packets, RED avoids global TCP synchronism. When
some packets of a TCP connection are discarded and the transmission speed is
lowered, other TCP connections can still keep the higher transmission speed. In
this way, there are always some TCP connections with higher transmission speeds,
that make a better use of the line bandwidth.
QoS configuration includes:
Traffic Policing
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Configure Priority Marking
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Configure Queue Scheduling
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Configure the RED Operation
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Configure Bandwidth Assurance
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Configure Traffic Statistics
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Before configuring these QoS tasks, you must define the corresponding ACL. The
packet filtering function is realized by activating the ACL.
Traffic Policing
Traffic policing is the flow-based rate limit. It takes corresponding actions to deal
with the flows at excessive speed, such as discarding or lowering the priority.