Network Traffic Shape Rate Calculator
The guaranteed bandwidth you wish to allocate.
The time slice used for the token bucket refill. Standard Cisco default is often 125ms (1/8th of a second).
Calculation Results
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Understanding Traffic Shaping Rates
Traffic shaping is a bandwidth management technique used on computer networks to delay some or all datagrams to bring them into compliance with a desired traffic profile. This calculator helps network engineers determine the correct Committed Burst Size ($B_c$) based on the desired Committed Information Rate (CIR) and the Time Interval ($T_c$).
The Token Bucket Algorithm
Most network equipment, including Cisco routers, uses a token bucket algorithm to shape traffic. The relationship between the variables is defined by the following fundamental physics:
$CIR (bps) = \frac{B_c (bits)}{T_c (seconds)}$
Where:
- CIR (Committed Information Rate): The average rate of traffic (speed) you want to allow over time.
- $B_c$ (Committed Burst): The maximum amount of data (in bits) that can be sent during a specific time interval ($T_c$).
- $T_c$ (Time Interval): The frequency at which the token bucket is refilled.
How to Use This Calculator
- Enter CIR: Input your target bandwidth (e.g., 10 Mbps).
- Define $T_c$: Enter the time interval in milliseconds.
- Lower $T_c$ (e.g., 4ms) results in smoother traffic flow for real-time applications like VoIP.
- Higher $T_c$ (e.g., 125ms) allows for larger immediate bursts of data but may introduce higher jitter.
- Calculate: The tool will output the necessary Burst Size in both bits and bytes, required for configuration commands.
Common Interval Values
While the formula allows for any interval, standard network equipment often defaults to specific values:
- 125 ms: The standard default for many Cisco IOS versions (8 intervals per second).
- 4 ms: Often recommended for Voice over IP (VoIP) traffic to minimize delay serialization (250 intervals per second).
Why Calculate Burst Size?
If you configure a shaping rate without specifying the burst size, the router assumes a default. However, if the default $B_c$ is too small for high-speed links, your effective throughput might be lower than the configured CIR because the bucket cannot hold enough tokens to sustain the rate. Conversely, if $B_c$ is too large, you may cause bufferbloat downstream.