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620 Chapter 16: Voice Techniques
Class-Based Weighted Fair Queuing
Class-Based Weighted Fair Queuing (CBWFQ) is the preferred queuing mechanism for voice.
With CBWFQ, priority queues get established for voice, so voice queues get served before
other data queues. CBWFQ is recommended over WFQ because WFQ is too fair to the other
processes and allows other traffic to flow ahead of voice. CBWFQ combines the best features
of priority, custom, and weighted fair queuing.
Here are the steps to implement CBWFQ:
Step 1
Define traffic classes to specify the classification policy.
Step 2
Apply policies to classes.
Step 3
Attach policies to interfaces.
CBWFQ allows the exact amount of bandwidth to be allocated for a specific class of traffic.
Taking into account available bandwidth on the interface, 64 classes can be configured.
CBWFQ allows the use of access control lists and protocols or input interface names to define
how traffic will be classified, thereby providing coarser granularity. Packets satisfying the
match criteria meet the traffic class. These packets are assigned characteristics such as
bandwidth, weight, and maximum packet limit. The policies are then assigned to an interface.
Figure 16-9 shows an example of CBWFQ.
NOTE
A number of QoS features are scattered over various IOS releases. It is important to audit the
existing hardware platforms to determine whether the hardware supports the features that
are needed.
Table 16-4
IP Precedence Bit Values
Bit Value
Priority
0
Normal traffic
1
Priority
2
Immediate
3
Flash
4
Flash override
5
Critical
6
Internet
7
Network
87200333.book Page 620 Wednesday, August 22, 2001 1:41 PM