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correspond to match criteria, including ACLs, protocols, and input interfaces. Traffic that
matches the class criteria belongs to that specific class. Each class has a defined queue that
corresponds to an output interface.
After traffic has been matched and belongs to a specific class, you can modify its charac-
teristics, such as assigning bandwidth, maximum queue limit, and weight. During periods
of congestion, the bandwidth assigned to the class is the guaranteed bandwidth that is de-
livered to the class.
One of CBWFQ's key advantages is its modular nature, which makes it extremely flexible
for mo st s it uat ion s. It is often refer red to as MQC or Modular QoS CLI, which is the
framework for building QoS policies. Many classes can be defined to separate your net-
work traffic as needed in the MQC.
Low-Latency Queuing
Low-latency queuing (LLQ) adds a strict priority queue (PQ) to CBWFQ. The strict PQ al-
lows delay-sensitive traffic such as voice to be sent first, before other queues are serviced.
That gives voice preferential treatment over the other traffic types. Unlike priority queu-
ing, LLQ provides for a maximum threshold on the PQ to prevent lower priority traffic
from being starved by the PQ.
Without LLQ, CBWFQ would not have a priority queue for real-time traffic. The addi-
tional classification of other traffic classes is done using the same CBWFQ techniques.
LLQ is the standard QoS method for many VoIP networks.
Tr a f f i c S h a p i n g a n d Po l i c i n g
Traffic shaping and polic ing are mechanis m s that in s pec t t raffic and take an ac t ion bas ed
on the traffic's characteristics, such as DSCP or IP precedence bits set in the IP header.
Traffic shaping slow s dow n the rate at which packets are s ent out an inter face (e g re s s) by
matching certain criteria. Traffic shaping uses a token bucket technique to release the
packets into the output queue at a preconfigured rate. Traffic shaping helps eliminate po-
tential bottlenecks by throttling back the traffic rate at the source. In enterprise environ-
ments, traffic shaping is used to smooth the flow of traffic going out the provider. This is
desirable for several reasons. In provider networks, it prevents the provider from dropping
traffic that exceeds the contracted rate.
Policing tags or drops traffic depending on the match criteria. Generally, policing is used
to set the limit of incoming traffic coming into an interface (ingress) and uses a “leaky
bucket mechanism.” Policing is also referred to as committed access rate or CAR. One ex-
ample of using policing is to give preferential treatment to critical application traffic by el-
evating to a higher class and reducing best-effort traffic to a lower-priority class.
When you contrast traffic shaping with policing, remember that traffic shaping buffers
packets while policing can be configured to drop packets. In addition, policing propagates
bursts, but traffic shaping does not.
 
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