Shaping Tools | Network Quality of Service

Shapers operate in a manner similar to policers in that they invoke token bucket algorithms to meter traffic. However, the principle difference between a policer and a shaper is that where a policer re-marks or drops traffic as a policy action, a shaper merely delays traffic. Even though TelePresence is a real-time application, you might encounter scenarios, such as subline rate access scenarios, which require shaping policies to be included within a comprehensive TelePresence QoS policy. Figure 1 illustrates generic traffic shaping.

 
Figure 1: Generic traffic shaping effect on traffic flow

Shapers are particularly useful when traffic must conform to a specific rate of traffic to meet a service-level agreement (SLA) or to guarantee that traffic offered to a service provider is within a contracted rate. Additionally, shapers have long been associated with Non-Broadcast Multiple-Access (NBMA) WAN topologies, such as ATM and Frame Relay, where potential speed-mismatches exist. However, shapers are becoming increasingly popular on newer Layer 3 WAN access circuits, such as Ethernet-based handoffs, to conform to subline access rates.

Shapers, like policers, use token bucket algorithms. On most platforms, by default, IOS shapers set the Bc to equal CIR/8, which yields an interval (Tc) of 125 ms. Although this interval value might be adequate for data applications, it introduces unnecessary interpacket delays for real-time networks. Consider the example shown in Figure 2.

 
Figure 2: Shaping behavior with default Bc values

A shaper uses the CIR, Bc, and Be to smooth a traffic stream to a specified rate. It achieves the given CIR by taking the line speed of the interface and dividing it into equal length timeslots (Intervals, or Tc) and then sends a smaller portion (Bc) of the traffic during each timeslot. The timeslot size is governed by the Bc parameter (Tc = Bc / CIR). In Figure 2, a line rate of 128 kbps is shaped to 64 kbps. The Bc, by default, is one eighth of the CIR (or eight kbps). Each second is divided into eight timeslots of 125 ms each, and the shaped rate of 64,000 bps is divided into eight bursts of 8000 bps each (Bc). Each burst will take 62.5 ms to transmit (at a 128 kbps line rate). So the shaper transmits information for the first 62.5ms of each 125 ms timeslot and is silent for the remaining 62.5 ms of the timeslot. Over the span of a second, this achieves the average rate of CIR.

Additionally, you can use the Be value to determine the peak Rate of sending, which is calculated as follows:

Peak Rate = CIR (1 + Be / Bc)

Peak-rate shaping allows the router to burst higher than average-rate shaping. However, when peak-rate shaping is enabled, any traffic exceeding the CIR can be dropped if the network becomes congested.

A router interface might be configured to shape using average rates, through the shape average policy-action command, or to a peak rate, through the shape peak policy-action command. The configuration for a basic Class-Based Shaping policy to shape to an average rate of 5 Mbps is as follows:

Router(config)# policy-map CB-SHAPING-5MBPS
Router(config-pmap)# class class-default
Router(config-pmap-c)# shape average 5000000

Because shaping involves buffering, various queuing techniques might be activated when the shaping buffer has been filled to capacity.