Comparison of QoS Mechanisms
QoS Scales to Supports High- Designed to
Mechanism Large Networks Speed Interfaces Support Voice
RSVP No Yes Yes
CBWFQ Yes Yes No
LFI Yes Use on Links < 768k Yes
cRTP Yes Designed for Slow Links Yes
LLQ Yes Yes Yes
Several of these technologies, such as RSVP and LLQ, are currently being
used for voice applications (mostly), and you will find that these more advanced
mechanisms are often used in conjunction with each other, rather than independently.
Such mechanisms, although powerful and useful in their own right, gain
power and functionality when used alongside other mechanisms. LLQ, for
example, is simply Class-Based Weighted Fair Queuing with the addition of a
Priority Queue. For this reason, implementing LLQ in your AVVID environment
will necessitate the use of CBWFQ even if you simply dump all your nonvoice
traffic into the single default class (class-default).
Some of the benefits of more advanced queuing mechanisms include
increased granular control of traffic behavior, and the ability to be far more specific
when classifying and queuing, or dropping, traffic. However, this presents a
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potential problem.There is a trade-off between granular control and flexibility of
use. LLQ, for example, is a very specific mechanism with a very specific purpose,
but it is not well suited for many things other than that particular function.This
is a contributing factor to the reasoning behind using multiple QoS mechanisms
to develop a strong overall Quality of Service strategy for your AVVID environment.
It is particularly important in this chapter to pay attention to recommendations
about where the deployment of these mechanisms is appropriate.
This chapter outlines some of the newer and, therefore, more AVVID-related
QoS mechanisms currently available in Cisco IOS.While some of these mechanisms
are just beginning to be widely deployed on production networks, QoS is
constantly being improved, expanded, and modified, so these technologies will
undoubtedly continue to receive a significant amount of development effort in
the near future. Of course, entirely new QoS mechanisms will continue to
emerge as Cisco’s AVVID strategy pushes forward into a growing number of customer
networks as demand increases for even more advanced QoS controls.Table
8.1 provides an overall comparison between some of the different QoS mechanisms
we will discuss in this chapter, including: Resource Reservation Protocol
(RSVP), Class-Based Weighted Fair Queuing (CBWFQ), Link Fragmentation
and Interleaving (LFI), compressed Real-Time Transport Protocol (cRTP), and
Low Latency Queuing (LLQ).