The primary reasons to use traffic shaping are to control access to available bandwidth, to ensure that traffic conforms to specific policies, and to regulate the flow of traffic in order to avoid congestion. Some example reasons for using traffic shaping follow:

Control access to bandwidth when policy dictates that the average rate of a given interface should not exceed a certain rate.

Configure traffic shaping on an interface if you have a network with differing access rates. Suppose that one end of the link in a Frame Relay network runs at 256 kbps and the other end of the link runs at 128 kbps. Sending packets at 256 kbps could cause failure of the applications using the link.

A similar, more complicated case would be a link-layer network giving indications of congestion with differing access rates on different attached data terminal equipment (DTE) devices. The network may be able to deliver more transit speed to a given DTE device at a specific time than at another time.

If you offer a subrate service, traffic shaping enables you to use the router to partition your T1 or T3 links into smaller channels.

Traffic shaping prevents packet loss. Its use is especially important in Frame Relay networks because the switch cannot determine which packets take precedence or which packets should be dropped when congestion occurs.

Specifying peak rate shaping allows you to make better use of available bandwidth by allowing more data than the CIR to be sent if the bandwidth is available.

Standard traffic shaping (the shape average command) will keep the average rate of the traffic to the limit you specify, which should be the committed rate of your circuit.

Peak shaping will also do traffic shaping (using queues, etc), guaranteeing a minimum rate, but will allow traffic to levels to burst above your configured (committed) rate.

From the documentation:

Traffic shaping limits the rate of transmission of data. In addition to using a specifically configured transmission rate, you can use Generic Traffic Shaping (GTS) to specify a derived transmission rate based on the level of congestion.

You can specify two types of traffic shaping; average rate shaping and peak rate shaping. Average rate shaping limits the transmission rate to the CIR. Using the CIR ensures that the average amount of traffic being sent conforms to the rate expected by the network.

Peak rate shaping configures the router to send more traffic than the CIR. To determine the peak rate, the router uses the following formula:

peak rate = CIR(1 + Be / Bc)

where:

•Be is the Excess Burst size.

•Bc is the Committed Burst size.

Peak rate shaping allows the router to burst higher than average rate shaping. However, using peak rate shaping, the traffic sent above the CIR (the delta) could be dropped if the network becomes congested.

If your network has additional bandwidth available (over the provisioned CIR) and the application or class can tolerate occasional packet loss, that extra bandwidth can be exploited through the use of peak rate shaping. However, there may be occasional packet drops when network congestion occurs. If the traffic being sent to the network must strictly conform to the configured network provisioned CIR, then you should use average traffic shaping.

Examples

The following example sets the uses average rate shaping to ensure a bandwidth of 256 kbps:

shape average 256000

The following example uses peak rate shaping to ensure a bandwidth of 300 kbps but allow throughput up to 512 kbps if enough bandwidth is available on the interface:

bandwidth 300

shape peak 512000