Committed Burst Calculation; Excess Bursts; Excess Burst Calculation - Cisco ASR 9000 Series Configuration Manual
Aggregation services router modular quality of service
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Configuring Modular QoS Congestion Management
The default committed burst size is the greater of 2 milliseconds of bytes at the police rate or the network
maximum transmission unit (MTU).
Committed Burst Calculation
To calculate committed burst, use the following formula:
bc = CIR bps * (1 byte) / (8 bits) * 1.5 seconds
Note
1.5 seconds is the typical round-trip time.
For example, if the committed information rate is 512000 bps, then using the committed burst formula, the
committed burst is 96000 bytes.
bc = 512000 * 1/8 * 1.5
bc = 64000 * 1.5 = 96000
When the be value equals 0, we recommend that you set the egress bc value to be greater than or equal to
Note
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:
• 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).
Excess Burst Calculation
To calculate excess burst, use the following formula:
be = 2 * committed burst
For example, if you configure a committed burst of 4000 bytes, then using the excess burst formula, the excess
burst is 8000 bytes.
be = 2 * 4000 = 8000
The default excess burst size is 0.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide, Release 6.1.x
Regulation of Traffic with the Policing Mechanism
61
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