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QoS and DLSw+ 533
Custom Queuing
Custom queuing was discussed in Chapter 9, but it applies to DLSw+ as well. Custom queuing
is particularly important for time-sensitive protocols such as SNA, which requires predictable
response times. As mentioned, priority queuing can be unfair. Custom queuing can be
considered fair, because you can define up to 16 queues. The queues are serviced in a round-
robin fashion, so even the lower queues get serviced. An added benefit is that a byte weight can
be applied to each queue so that many of the bytes get through before another queue is served.
NOTE
Earlier IOS versions supports only up to ten custom queues.
Weighted Fair Queuing
This type of QoS can be stated as "WFQ favors low-volume conversations." An analogy would
be an amusement park in which large groups of people are waiting to get on a ride that has only
a few seats available. The park attendants search for one or two people to fill the seats,
neglecting the people who have been waiting in line the longest. Low-volume traffic streams,
which comprise the majority of network traffic, get preferential treatment. This ensures that
user applications receive satisfactory response. Batch file transfers (FTP, for example) would
share the remaining bandwidth.
With standard WFQ, packets are classified by flow. Packets with the same IP address,
destination address, or source or destination TCP or UDP port belong to the same flow. WFQ
allocates an equal share of the bandwidth to each flow. Flow-based WFQ is also called fair
queuing because all flows are equally weighted.
How is it configured?
WFQ requires no configuration and is on by default.
Class-Based Weighted Fair Queuing
Class-Based Weighted Fair Queuing (CBWFQ) uses the 3 bits in the IP Precedence field to
define what traffic should be favored even further. Traffic must first be classified as important
using extended access lists, which define which traffic gets the IP Precedence bits set.
IP Precedence
The IP Precedence field has been around since IP was developed, but now new schemes are
finally utilizing this field.
IP Precedence lets administrators establish service classes using the three precedence bits in the
Type of Service field in the IPv4 header. The attraction of this is that administrators can use the
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