The Network Consultants Handbook, published by Cisco Press, helps value-added resellers (VARs) analyze and enhance their clients' basic network topology or set up a computer network from the beginning. In this excerpt from Chapter 15, author Matthew Castelli discusses best practices for managing a client's network traffic through frame relay traffic shaping.
Frame Relay Traffic Shaping
Traffic shaping supports the controlling of the traffic going out of an interface. This control matches the flow of traffic to the speed of the remote destination (or target) interface and ensures that the traffic conforms to policies contracted for the interface. Traffic adhering to a particular profile can be shaped to meet downstream requirements, eliminating bottlenecks in topologies with data-rate mismatches.
The primary reasons for using traffic shaping are to control access to available bandwidth, to ensure that traffic conforms to the policies established for the available bandwidth, and to regulate the flow of traffic to avoid congestion. Congestion can occur when the sent traffic exceeds the access speed of its destination (target) interface across a VC.
Following are some examples of when to use traffic shaping:
- To control access to bandwidth when policy dictates that the rate of a given interface should not, on the average, exceed a certain rate, even though the access rate exceeds the speed.
- To 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 that are using the link.
NOTE Regarding a similar, more complicated case, a link-layer network giving indications of congestion that has differing access rates on different attached DTE; the network might be able to deliver more transit speed to a given DTE device at one time than another. (This scenario warrants that the token bucket be derived, and then its rate maintained.)
- To partition the T1 or T3 links into smaller channels in a subrate service scenario. Traffic shaping prevents packet loss. The use of traffic shaping is especially important in Frame Relay networks because the switch cannot determine which frames take precedence and therefore which frames should be dropped when congestion occurs. It is important for real-time traffic, such as VoFR, that latency be bounded, thereby bounding the amount of traffic and traffic loss in the data link network at any given time by keeping the data in the router that is making the guarantees. Retaining the data in the router allows the router to prioritize traffic according to the guarantees that the router is making.
Traffic shaping limits the rate of transmission of data, limiting the data transfer to one of the following:
- A specific configured rate
- A derived rate based on the level of congestion
The transfer rate depends on three components that constitute the token bucket: burst size, mean rate, measurement (time) interval.
The mean rate is equal to the burst size divided by the interval, as demonstrated by the following equation:
Mean rate = Burst Size (B
) / Time Interval (T
When traffic shaping is enabled, a maximum burst size can be sent during every time interval. However, within the interval, the bit rate might be faster than the mean rate at any given time.
BE size is an additional variable that applies to traffic shaping. The excess burst size corresponds to the number of noncommitted bits—those bits outside the CIR—that are still accepted by the Frame Relay switch but marked as DE.
The BE size allows more than the burst size to be sent during a time interval. The switch will allow the frames that belong to the excess burst to go through, but it will mark them by setting the DE bit. The switch configuration determines whether the frames are sent.
When BE size equals 0 (BE = 0) the interface sends no more than the burst size every interval, realizing an average rate no higher than the mean rate. When BE size is greater than 0 (BE > 0) the interface can send as many as BC + BE bits in a burst, if the maximum amount was not sent in a previous time period. When less than the burst size is sent during an interval, the remaining number of bits, up to the BE size, can be used to send more than the burst size in a later interval.