BC and BE in policing question

[jon_panes24]

hi guys,

What exactly does the burst bytes do in policing traffic. I notice that if you dont specify the burst bytes, the bc and be of the policy map is auto configured. Is the auto configured bc and be just fine to be used or do I really need to change it manually? Could someone give an easy to understand explanation what is bc and be.

Lets say I want to limit icmp to the router to only 8000 bps, how can i efficiently use the option of using the bc and be on this scenario.

thanks and regards.

[Vasilii Mikhailovskii · Accepted solution]

Hello.

Bc is an amount of traffic that would be "confirmed" per time interval, called Tc.

Tc = Bc / CIR

Be is an amount of traffic that would be "exceeded" per time interval (applicable for 3-colors policers).

Everything else would violate your policer.

So, if you say police 8000 bps with Bc = 8000 bits, it means that during single second (Tc = Bc/CIR = 1 second) you could send no more than 8000 bits of traffic.

If you say police 8000 with Bc=2000 bits, it means that during the period of 250ms (Tc = 2000/8000 = 250ms), you could send no more than 2000 bits of traffic == 8000 bps.

So, the lower Bc, the smoother policing you do (cutting spikes).

[Joseph W. Doherty · Accepted solution]

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Posting

Perhaps to understand Bc/Be, you need to appreciate that policers (or shapers) are measuring a defined transmission rate on a link that always runs at full line rate.  I.e. what does it really mean when we specify a CIR of 3 Mbps on a 10 Mbps link?  Again, remember, transmission rate for the foregoing will always be at 10 Mbps.

Given a CIR, we need to define a measurement period (the Tc).  Perhaps we want a Tc of 1 second, which means we'll allow up to 3 Mb (the Bc) during 1 second (the Tc).  Or, perhaps we want a Tc of 100 ms, which means we'll allow up to 300 Kb (the Bc) during Tc.

(Most/all?) Cisco policers (and shapers) don't let you directly define the Tc, instead you define the Bc (in bytes, not bits).  Using the formula Vasilii provided, you can use CIR and Bc to configure the Tc.

Be provides for additional traffic during a time interval.  In theory, this supports bursting, but Cisco's documentation on its usage is contradictory, and because of that, I don't want to delve into an explanation of its usage.