[j-nsp] Burst Reset question on Juniper M5

sthaug at nethelp.no sthaug at nethelp.no
Fri May 16 08:31:02 EDT 2008

> Michael's basically right.  If you imagine that you have a bucket that
> is sized for 10xMTU of the link (e.g. 15000 Bytes) and the link is a
> 1Mbps link, it would only take 15*8/1000 seconds (120ms) to totally
> empty a full burst buffer.  If you have a transmit-rate of 1kbps on
> your 1Mbps link, then you'd take 120s to empty your buffer completely
> if you received no more packets during that 120s!

The biggest problem here, IMHO, is the rather sparse JunOS documentation
about how to configure the burst size. E.g. the follow 9.1 page:


It says, "To determine the value for the burst-size limit, the preferred
method is to multiply the bandwidth of the interface on which you are
applying the filter by the amount of time you allow a burst of traffic
at that bandwidth to occur; for example, 5 milliseconds."

This does not at all relate to the *practical* requirement that we have
a customer connected to a high speed port (e.g. FE or GE) and wish to
limit the customer to a fraction of the link capacity (e.g. 5 Mbps on a
FE port).

On Cisco boxes we typically have a rule of thumb which says to configure
a burst size of 1 to 1.5 seconds of traffic. *Useful* Juniper doc would
include something similar. I'm afraid the sentence "If you do not know
the interface bandwidth, you can multiply the maximum transmission unit
(MTU) of the traffic on the interface by 10 to obtain a value" does not
count as particularly useful documentation...

Steinar Haug, Nethelp consulting, sthaug at nethelp.no

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