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We are facing one confusion with regards to burst size value against rate limit or policer in Juniper MX. Need to understand two things :-

(1) Is the burst size limit defined in firewall policer dependent or calculated based on physical interface speed and independent of bandwidth limit (Mbps) defined under a policer for a user?

For example , suppose we have 1G interface and 2 x customers are terminated on this interface . One customers need 2 Mbps while other need 10 Mbps. Since interface is 1G , the burst size against both of these customers would be same i.e. 625,000 bytes while bandwidth limit would be 2 Mbps and 10 Mbps respectively if burst-size-limit is dependent on physical interface and independent of bandwidth limit value? Please correct if am wrong.

(2) How to calculate the burst size for 10G interface in Juniper?

This is taken from JNCIA Study Guide (Old Book) , Chapter No.10.

Suppose you have a Gigabit Ethernet interface in your router. The interface can receive 1000 megabits per second. The burst-size-limit is entered in bytes per second, so we translate 1000Mbps into 125MBps. That further translates into 125,000KBps and finally 125,000,000Bps. In a single millisecond, the interface receives 125,000 bytes, so a total of 625,000 bytes are received in 5 milliseconds. The policer statement becomes burst-size-limit 625000.

Based on above reference , if the burst size of 1G interface is 625 ,000 bytes then burst size of 10G interface would be 625 , 000 x 10 = 6250 , 000. Is this correct? If not , please guide as per the Juniper best practices.

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The burst-size is most effective when the policer rate-limit is low enough that a single TCP flow can reach the rate-limit and consume all the burst tokens before experiencing any packet loss.

As the number of TCP (or other application protocol) flows increases, or the used bandwidth commonly approaches the rate-limit, the actual effect of the configured burst-size is reduced.

You should configure burst-size based on the policer's permitted bandwidth, not based on the interface speed. If you configure a 2Mb/s policer with a 625KB burst-size, TCP transmit rate will fluctuate above the policer more and the result will be more congestion & retransmits.

As policer speeds scale up, this matters less. At low speeds like 2Mb/s it is noticeable and you can easily observe the effect in a lab test.

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  • There is also challenge in selecting the burst interval if the burst size is based on policer's bandwidth. How to cope with this. – NABEEL NASIR Jan 30 at 16:55

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