difference between "bw percent" and police in policy

Answered Question
Mar 16th, 2010

Say I want to limit the http traffic on an interface to 25% of my T1 circuit.  If I use a policy and assign "bandwidth percent 25" to a particular class then in theory I should limit http traffic to 384k.  What about if instead of "bandwidth percent 25" I use something like "police 384000...".  What is the difference?  Right now I am using the "bandwidth percent 25 method and it doesn't seem  to be working.  See output of show policy below.  It says 384k but offered rate is 971k and mt T1 is slammed.  What am I doing wrong?  Should I try the police method?

Thanks,

Diego

    Class-map: http_Outbound (match-any)
      3913962 packets, 4079657739 bytes
      5 minute offered rate 971000 bps, drop rate 0 bps
      Match: access-group 150
        3913962 packets, 4079657739 bytes
        5 minute rate 971000 bps
      Queueing
        Output Queue: Conversation 266
        Bandwidth 25 (%)
        Bandwidth 384 (kbps)Max Threshold 64 (packets)
        (pkts matched/bytes matched) 3913962/4079657739
        (depth/total drops/no-buffer drops) 16/7786/0

Correct Answer by Jon Marshall about 6 years 11 months ago

Diego

Apologies for delay in getting back. Couple of modifcations -

policy-map T1_Shape

shape average 1544000

service-policy QoS

You only need the above in your parent policy. The shape average figure should match the CIR of the circuit.

policy-map QoS

class voice

priority percent 7

class RDP

bandwidth percent 20

class Data

bandwidth percent 15

class SMTP_Outbound

bandwidth percent 15

class class-default

bandwidth percent 40

You can only reserve 75% of the bandwidth in your classes with the bandwidth statements as 25% is reserved. You can change this but not recommended. Just remove class-default as this will then be allocated from bandwidth left over.

Jon

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Jon Marshall Tue, 03/16/2010 - 12:31

Diego

When you use the bandwidth command you are promising a minimum bandwidth for that class of traffic during periods of congestion. If there is available bandwidth from other traffic classes then it can be used if needed. That is why you see more traffic than your 384k.

When you use the police command you are setting a maximum bandwidth for that class of traffic beyond which a certain action is taken ie. drop or remark the packet.

If you want to limit the total amount of bandwidth for that class regardless of the available bandwidth on the link then use policing or better yet use shaping which still imposes a limit on the amount of useable bandwidth for that class but is more efficient than policing.

Jon

DIEGO ALONSO Tue, 03/16/2010 - 12:39

Can I mix and match policing, shaping and percent bandwidth all  in one policy?  BTW,  what is the name of the QoS method that "percent bandwidth" falls under?  I'd like to talk intelligently next time I have an issue with one of these commands.  ;) Thanks, Diego

Jon Marshall Tue, 03/16/2010 - 12:58

diego@ats-corp.com

Can I mix and match policing, shaping and percent bandwidth all  in one policy?  BTW,  what is the name of the QoS method that "percent bandwidth" falls under?  I'd like to talk intelligently next time I have an issue with one of these commands.  ;) Thanks, Diego

Diego

Yes you can use policing/shaping with bandwidth commands in your policy map or as an example, a common use is to shape the overall bandwidth of the interface and then within that limit apply bandwidth commands to traffic classes.

Not all interfaces/devices support this type of mixing though and Catalyst switches in general do not support shaping at all.

The QOS method most in use today is called CBWFQ (Class Based Weighted Fair Queueing) and that is what you will be using.

Jon

DIEGO ALONSO Tue, 03/16/2010 - 13:32

I am working with a T1 on an 1841 router.  Would you happen to have a link to an example of what you just mentioned?  Seems like the way to go but I wouldn't wan't to start from scratch with that type of config on a production router.  Might take a few tries to get it right.  It would be greatlt appreciated and thank you for all your help so far. Rgds, Diego

DIEGO ALONSO Tue, 03/16/2010 - 16:24

Jon,

The parent/child policy you suggested seem scary at first but after a little bit of reading I felt like I might be able to pull it off.  Here is what  I came up with:

policy-map T1_Shape

class class-default

shape peak 1544000

shape average 1200000

service-policy QoS

!

policy-map QoS

class voice

priority percent 7

class RDP

bandwidth percent 20

class Data

bandwidth percent 15

class SMTP_Outbound

bandwidth percent 15

class class-default

bandwidth percent 40

max-reserved bandwidth 97

Couple of assumptions I used to come up with this are:

1) I want to shape ALL traffic on the T1 so I used 1544000 which is the well known T1 max and came up with the 1200000 from some MRTG graphs of the circuit.

2) I want to guarantee each of the five classes above the bandwith shown but allow any of them to use more if it is available.

What do you think?  Will it work?

Thanks,

Diego

Correct Answer
Jon Marshall Thu, 03/18/2010 - 15:13

Diego

Apologies for delay in getting back. Couple of modifcations -

policy-map T1_Shape

shape average 1544000

service-policy QoS

You only need the above in your parent policy. The shape average figure should match the CIR of the circuit.

policy-map QoS

class voice

priority percent 7

class RDP

bandwidth percent 20

class Data

bandwidth percent 15

class SMTP_Outbound

bandwidth percent 15

class class-default

bandwidth percent 40

You can only reserve 75% of the bandwidth in your classes with the bandwidth statements as 25% is reserved. You can change this but not recommended. Just remove class-default as this will then be allocated from bandwidth left over.

Jon

DIEGO ALONSO Fri, 03/19/2010 - 11:09

Jon, please, no apologies needed.  I just thought that maybe the email that alerts you to new posts didn't get to you and I wanted to touch the thread to get another out just in case. 

Last question, I understand what you say about the class-default bandwidth percent.  However I want to give a little more than 25% to this class.  Is it better then to use "max-reserved bandwidth 60" and then remove the class-default line from the map?

Thanks,

Diego

DIEGO ALONSO Thu, 03/18/2010 - 14:05

Jon,

Any chance you can take a quick look at the config snippet I posted and give me some feedback?

I would greatly appreciate it.

Thanks,

Diego

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