Committed Burst Calculation; Excess Bursts - Cisco 10000 Series Configuration Manual

Committed Bursts and Excess Bursts

Committed Burst Calculation

To calculate committed burst, use the following formula:
1.5 seconds is the typical round-trip time.
Note
For example, if the committed information rate is 512000 bps, then using the committed burst formula,
the committed burst is 96000 bytes.
Note When the be value equals 0, we recommend that you set the egress bc value to be greater than or equal
to the ingress bc value plus 1. Otherwise, packet loss can occur. For example:
be = 0
egress bc >= ingress bc + 1

Excess Bursts

The excess burst (be) parameter of the police command implements the second, exceeding (yellow)
token bucket that the router uses to meter traffic. The exceeding token bucket is initially full and the
token count is equal to the excess burst size (EBS). Thereafter, the meter updates the token counts the
number of times per second indicated by the committed information rate (CIR).
The following describes how the meter uses the exceeding token bucket to send packets:
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Excess Burst Calculation
To calculate excess burst, use the following formula:
For example, if you configure a committed burst of 4000 bytes, then using the excess burst formula, the
excess burst is 8000 bytes.
The default excess burst size is 0.
Cisco 10000 Series Router Quality of Service Configuration Guide
6-22
bc = CIR bps * (1 byte) / (8 bits) * 1.5 seconds
bc = 512000 * 1/8 * 1.5
bc = 64000 * 1.5 = 96000
When the first token bucket (the conforming bucket) meets the committed burst size (CBS), the
meter allows the traffic flow to borrow the tokens needed from the exceeding token bucket. The
meter marks the packet yellow and then decrements the exceeding token bucket by the number of
bytes of the packet.
If the exceeding token bucket does not have the required tokens to borrow, the meter marks the
packet red and does not decrement the conforming or the exceeding token bucket. Instead, the meter
performs the exceed-action configured in the police command (for example, the policer drops the
packets).
be = 2 * committed burst
be = 2 * 4000 = 8000
Chapter 6 Policing Traffic
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