Calculate Committed Information Rate (CIR), Normal Burst (Bc), and Excess Burst (Be) for Cisco IOS QoS configurations.
Kbps
Mbps
Gbps
Standard (1.5 seconds) – Recommended
Tight/Strict (0.25 seconds)
Extended (2.0 seconds) – For High Latency
Determines the size of the token bucket (Tc).
Policing Parameters
Bit Rate (bps)0
Normal Burst (Bc)0 bytes
Excess Burst (Be)0 bytes
Cisco Command Preview
Legacy CAR (Committed Access Rate):
rate-limit input …
MQC (Modular QoS CLI):
police …
How to Calculate Cisco Rate Limits
Configuring traffic policing on Cisco routers and switches requires understanding the relationship between the Committed Information Rate (CIR), the Committed Burst (Bc), and the Excess Burst (Be). Unlike simple speed tests, router configurations require specific byte values for the token buckets.
The Formula:
Normal Burst (Bc) = (CIR in bps * Burst Time in seconds) / 8 The division by 8 converts bits to bytes.
Understanding the Parameters
CIR (Committed Information Rate): This is the guaranteed bandwidth you wish to allocate, usually expressed in bits per second (bps).
Bc (Normal Burst): This represents the size of the "token bucket." It is the maximum number of bytes that the router can transmit in a single interval without exceeding the rate limit. Cisco generally recommends a burst size based on 1.5 seconds of traffic for general data to account for TCP windowing mechanisms.
Be (Excess Burst): This is the additional allowance for traffic spikes. In many standard configurations, this is set to double the normal burst (Be = 2 * Bc) or equal to the normal burst depending on how strictly you need to drop packets.
Common Calculation Standards
While you can mathematically set the burst to any value, setting it too low (e.g., < 8000 bytes) on high-speed links will cause TCP starvation and poor throughput even if the link isn't saturated. This calculator uses the standard Cisco recommendation:
Bc = (Rate * 1.5) / 8
This formula ensures there is enough "depth" in the bucket to allow a full 1.5 seconds worth of data to pass during momentary spikes, ensuring smooth TCP acknowledgement flows.