Specifications

10-3
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
Chapter 10 Traffic Shaping
About Traffic Shaping
Traffic Shaping and Rate of Transfer
Traffic shaping limits the rate of transmission of data. You can limit the data
transfer to one of the following:
• A specific configured rate
• A derived rate based on the level of congestion
The rate of transfer depends on three components that constitute the token bucket:
burst size, mean rate, and measurement (time) interval. The mean rate is equal to
the burst size divided by the interval.
When traffic shaping is enabled, the bit rate of the interface does not exceed the
mean rate over any integral multiple of the interval. During every interval, the
burst size is usually the maximum number of bits that can be sent. Within the
interval, however, the bit rate may be faster than the mean rate at any given time.
One additional variable applies to traffic shaping: Excess Burst Size (called the
Be size). The Be Size corresponds to the number of noncommitted bits—those
outside the committed information rate (CIR)—that are still accepted by the
Frame Relay switch but are marked as discard eligible (DE).
The Be size allows more than the burst size to be sent during a time interval in
certain situations. The switch allows the packets belonging to the Excess Burst to
go through but it will mark them by setting the DE bit. Whether the packets are
sent depends on how the switch is configured.
When the Be size equals 0, the interface sends no more than the burst size every
interval, achieving an average rate no higher than the mean rate. However, when
the Be size is greater than 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. Whenever
the number of bits sent during an interval is less than the burst size, the remaining
number of bits can be sent in a later interval.