Medianet Reference Guide - Cisco

Cisco QoS Toolset
Chapter 4
Medianet QoS Design Considerations
Figure 4-15
A Dual-Rate Policer as a Three-Color Marker
Violating Traffic (> PIR)
Offered Traffic
PIR
CIR
Offered Traffic
Y
Y
Y
PIR
CIR
Exceeding Traffic (>CIR < PIR)
Conforming Traffic (< CIR)
Time
Time
A RFC 2698 “Two Rate Three Color Marker” can:
• mark conforming traffic to one value (such as AF31)
• remark exceeding traffic to another value (such as AF32)
• remark violating traffic to yet another value (such as AF33)
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Shaping Tools
Shapers operate in a manner similar to policers, in that they meter traffic rates. However, the principle
difference between a policer and a shaper is that where a policer remarks or drops traffic as a policy
action, a shaper merely delays traffic. Figure 4-16 illustrates generic traffic shaping.
Figure 4-16
Traffic Shaping
Line
Rate
without Traffic Shaping
with Traffic Shaping
CIR
Traffic shaping limits the transmit rate of traffic
to a value (CIR) lower than the interface’s line rate
by temporarily buffering packets exceeding the CIR
226612
Shapers are particularly useful when traffic must conform to a specific rate of traffic in order to meet a
service level agreement (SLA) or to guarantee that traffic offered to a service provider is within a
contracted rate. Traditionally, shapers have been associated with Non-Broadcast Multiple-Access
(NBMA) Layer 2 WAN topologies, like ATM and Frame-Relay, where potential speed-mismatches exist.
However, shapers are becoming increasingly necessary on Layer 3 WAN access circuits, such as
Ethernet-based handoffs, in order to conform to sub-line access-rates.
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Medianet Reference Guide
OL-22201-01