10.16 Oversubscription with Legacy CAR and WFQ

Hi Experts,

 

In the solution of 10.16, the configuration of CAR is done as follows:

 

interface fastethernet0/1

rate-limit out access-group 111 64000 3200 3200 conform set-prec-transmit 1 exceed continue
 rate-limit out access-group 111 128000 3200 3200 conform set-prec-transmit 0 exceed drop
 
 rate-limit out access-group 116 64000 3200 3200 conform set-prec-transmit 1 exceed continue
 rate-limit out access-group 116 128000 3200 3200 conform set-prec-transmit 0 exceed drop

 

As per my understanding, for an averaging interval of 200ms, the configuration should be as follows:

 


interface fastethernet0/1

rate-limit out access-group 111 64000 1600 1600 conform set-prec-transmit 1 exceed continue  
rate-limit out access-group 111 128000 3200 3200 conform set-prec-transmit 0 exceed drop
 
 rate-limit out access-group 116 64000 1600 1600 conform set-prec-transmit 1 exceed continue 
 rate-limit out access-group 116 128000 3200 3200 conform set-prec-transmit 0 exceed drop

 

Why in the solution the value of 3200 is used for Bc instead of 1600 for the rate-limit statements (in Bold letters above)? Am I missing anything?

 

Thanks

 

Comments

  • Can somebody please throw some light here?

     

    Thanks

  • I have the same query as you and think this should be 200ms per policer as I see no reason to set it to 400ms for the smaller, guaranteed, 64k flow.

    Setting the value the same for both flows breaks the question's requirement.  If any of the quesiton's requirements were set up on their own the values would be as you mentioned above.

    I hope I'm not missing anything here?

  • "The burst size is calculated based on a maximum transmit rate of 128Kbps and averaging interval of 200ms, where Bc = 128000/8 *200/1000 = 3200 bytes. Note that CAR takes the burst value in bytes, not bits, so 128000bps is divided by 8 to convert to bytes, and 200ms is divided by 1000 to convert to seconds."

    that text quated from SG answer section,and Cisco as follwoing says Bc is derived from configured rate "not from maximum rate"  which is here "64000 bit" so i got confused here..

    Note that IOS doesn't allow Bc values higer than 2000



    Cisco: http://www.cisco.com/en/US/docs/ios/12_2/qos/command/reference/qrfcmd8.html#wp1037428

    Policing Traffic with CAR



    CAR embodies a rate-limiting feature for policing traffic. When policing
    traffic with CAR, Cisco recommends the following values for the normal
    and extended burst parameters:


    normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds


    extended burst = 2 * normal burst

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