Bandwidth & Police command in the same class

[lekhraj01]

Hi Folks,

i am little bit confused in HQOS configuration where CBWFQ have bandwidth command configured as well have policing. 

For example:

class ce_af4

     bandwidth 170
police  64000 12000 24000  conform-action set-dscp-transmit af41 exceed-action set-dscp-transmit af41 violate-action set-dscp-transmit af42

!

With this configurations, will the AF4 class packets get atleast 170Kbps bandwidth during congestion ? But 64000bps is CIR for this class which means 64000 bits will be sent per second with Bc=12000.

For your information, above AF4 queue is the child queue of shaping queue of 3Mbps.

Need a good explanation please ?

Thanks.

[Joseph W. Doherty · Accepted solution]

The class's bandwidth statement sets a minimum guarantee. This class should always obtain at least 170 Kbps.

The police statement sets a cap. In this case, your policer is only being used to "color" overrate traffic, actual transmission speed won't be limited. However, ToS marking will reflect transmission rate going through the policer. (Traffic will be marked with an AF41 unless it exceeds 64 Kbps during a 24000 burst, then it will be marked with an AF42.)

[lekhraj01]

Thanks Joseph for the explanation. However, I have few more queries as listed below.

What would be the data rate per second then ? As i understand policer comes before the queueing, scheduler & as per policer command: BC = 12000 bytes & Be = 24000bytes. As i understand, Bc is the amount of data could be sent per Tc. Here, Tc is coming as 96000 bps (Bc)/ 64000bps (CIR) = 1.5sec

Now, i apply this logic then we can send committed 96000bits in one Tc (1.5sec). If it is true then how can 64000bps be achieved as CIR :(, its bit confusing. If we say 170kbps bandwidth could be minimum bandwidth allocated for this class then it is really confusing with policer Bc & Tc.

if i assume, above shouldn't work this way then please advise what could be the data rate per second with Bc, Be & How do we configure this CIR, Bc & Be values for 170Kbps ? I mean the best practises for this configurations.

Please advise.

Thanks.

[lekhraj01]

Just to add one more thing, shaping values for the the classes like AF4 are given below:

class class-default
   shape average 3552000  14208 0 
  service-policy Out_MPLS

what is the role of Bc values in shaping queue here when every class have Bc values in policer statement ?

[Joseph W. Doherty]

In either a shaper or policer, the purpose of Bc and/or Be is to set measurement periods.

When used together within the same policy, shapers and/or policers can be configured such that one effectively precludes the other from its configured function. From what you've posted, so far, that's not the case with your policy.

Your parent shaper is setting an overall transmission cap, likely to insure you don't go over rate for some provider setting, and the policer, again, just "colors" some of your traffic based on its transmission rate.

[Joseph W. Doherty]

Actual transmission rate is always what's supported by the interface/medium.

What the Bc and/or Be do is they define a Tc, which is a time interval that the transmission volume is measured against. Say your Tc was 10 ms, and your logical rate was 10 Mbps, then the total bit rate allowance would be 100 Kb during the 10 ms period.

How to set Bc and/or Be depends how you want to control/measure your traffic. Smaller values are closer to what simulated rate behavior would be during a short time period, but with bursty traffic, you may simulate a long term rate lower than the simulated rate behavior would be.

One way to think of Bc and/or Be, image a 100 Mbps ingress link and and 10 Mbp egress link. The Bc and/or Be sort of set the max queue length supported on the egress.

[Joseph W. Doherty]

With both shapers and policers, physical transmission rate of a frame/packet is always what the interface/media supports. Both, though, can impact average transmission rate over time.

A shaper queues frames/packets to keep the average transmission rate as configured.

A policer, if configured for drops, drops over rate frames/packets to keep the average transmission rate as configured.

Smaller Bc/Be tends to more closely simulate an average transmission period, for the configured rate, during short time periods.

[lekhraj01]

Thanks Joseph for the explanation.....Cheers..!!

[Saran]

Hi Joseph ,

Can you also confirm once the police rate of 64kbps is reached and also Bc and Be is exceeded i.e. we hit violate action and now packets are re-marked as AF42.

Now AF42 marked packets will check bandwidth availability in class AF4 or will take bandwidth from any available anywhere in the pipe.

Rgds
Saran

[Joseph W. Doherty]

Yes, to your remark question.

Insufficient information to answer your second question.