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[MX] How the minimum burst-size value for shaping or guaranteed rates are determined



Article ID: KB32591 KB Last Updated: 08 May 2018Version: 1.0

The impact of traffic burst can be managed by configuring a burst-size value with the shaping or guaranteed rate commands as shown bellow:

set class-of-service traffic-control-profiles <name> shaping-rate burst-size <value>
set class-of-service traffic-control-profiles <name> guaranteed-rate burst-size <value>

The smaller burst size is subject to a minimum value that enables the system to achieve the configured rates. However, in some cases on MX-Series routers with MPC cards installed, even the minimum burst-size causes neighboring devices' buffers to be overwhelmed. How is the minimum burst-size value determined? Is it possible to change it?



The MX-Series MPC uses a token bucket algorithm for shaping to enforce a limit on average transmit rate of traffic on an interface. The size of the token bucket cannot be less than the number of tokens it receives per rate update. This is called the Token Bucket Update Period and determines the minimum burst-size allowed for a given shaping-rate. With a smaller number of queues, the token bucket update interval reduces and lowers the minimum burst-size values at different nodes, hence improving jitter. The configuration setting is:

‘set chassis fpc <fpc number> max-queues

The exact minimum burst-size value depends on the MPC type installed in the box. In a hierarchical class of service (HCoS) scenario, different nodes of the scheduling hierarchy have different minimum burst-size values. Please contact JTAC to obtain exact values by specifying the MPC type and the amount of queues required to be supported.      

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