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Chapter 1 Overview

What is the Quality of Service Toolset?

Figure 1-9

Generic Queueing Example

Queueing buffers are finite in capacity and act very much like a funnel for water being poured into a
small opening. However, if water is continually entering the funnel much faster than it exits, then
eventually the funnel will being overflowing from the top. When queueing buffers begin overflowing,
packets may be dropped either as they arrive (tail-drop) or selectively before all buffers are filled.
Selective dropping of packets when the queues are filling is referred to as congestion avoidance.
Congestion avoidance mechanisms work best with TCP-based applications, as selective dropping of
packets causes the TCP windowing mechanisms to 'throttle-back' and adjust the rate of flows to
manageable rates.

Congestion avoidance mechanisms are complementary to queueing algorithms; queueing algorithms
manage the front of a queue, congestion avoidance mechanisms manage the tail of the queue. Therefore,
congestion avoidance mechanisms indirectly affect scheduling.

Scheduling tools include:

Class-Based Weighted-Fair Queueing

Low-Latency Queueing

Weighted-Random Early Detect

Tip

For more information, see Congestion Management in the Cisco IOS Quality of Service Solutions
Configuration Guide, Release 12.2.

Class-Based Weighted-Fair Queueing

Class-Based Weighted-Fair Queueing (CBWFQ) is a hybrid queueing algorithm, combining the ability
to guarantee bandwidth (from Custom Queueing) with the ability to dynamically ensure fairness to other
flows (from Weighted-Fair Queueing). CBWFQ allows for the creation of up to 64 classes of traffic,
each with its own reserved queue. Each queue is serviced in a Weighted-Round-Robin (WRR) fashion.

CBWFQ is a highly efficient algorithm for data applications, but is not able to provide strict-priority
servicing to real-time applications, such as voice or video.

To avoid bandwidth starvation of background applications (such as routing protocols, network services,
and Layer 2 overhead and keepalives), it is recommended that you not provision total bandwidth
guarantees to exceed 75% of the link's capacity (see

Figure 1-11

Note

Distributed CBWFQ (dCBWFQ) is the distributed counterpart of CBWFQ.

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Router

Prioritization can be strict,

bandwidth guarantee by traffic class,

or automatically weighted

Voice

Video

Data