Let’s look at examples of setting up policers to limit speed on Juniper routers, and also talk about how to calculate the burst-size-limit value.

Burst-size-limit calculation for juniper

To configure the speed limit, the first thing you need to do is calculate the burst-size value. There are different calculation options. I suggest using the simplest way to calculate the burst-size from the interface speed with a maximum burst time of 5 ms. Actually this method is recommended by juniper.

We recommend that you use a value of 5 ms (0.005 seconds) as the starting point for the allowable amount of time for a burst of traffic

Formula for calculating burst size limit:

burst-size limit in bytes = interface media rate x allowable time for bursty traffic / 8 bits

For convenience, we write the formula in abbreviated form:

burst-size-limit = (INTERFACE_RATE * 0.005)/8 

Depending on the interface speed, we get the following values:

  • Fast Ethernet - 62,500 bytes
  • Gigabit Ethernet - 625,000 bytes
  • 10-Gigabit Ethernet - 6,250,000 bytes
  • 25-Gigabit Ethernet - 15,625,000 bytes
  • 40-Gigabit Ethernet - 25,000,000 bytes
  • 100-Gigabit Ethernet - 62,500,000 bytes

So we got several options for the burst-size-limit value depending on the speed of the physical interface:

  • burst-size-limit 62.5k - for Fast Ethernet
  • burst-size-limit 625k - for Gigabit Ethernet
  • burst-size-limit 6250k - for 10-Gigabit Ethernet
  • burst-size-limit 15625k - for 25-Gigabit Ethernet
  • burst-size-limit 25m - for 40-Gigabit Ethernet
  • burst-size-limit 62m - for 100-Gigabit Ethernet