service policy : police vs shape

Discussion in 'Cisco' started by Tassos, Dec 11, 2003.

  1. Tassos

    Tassos Guest

    From Modular QoS CLI documentation, when creating a Service Policy:

    Router(config-pmap-c)# police bps burst-normal burst-max conform-action action
    exceed-action action violate-action action

    Router(config-pmap-c)# shape {average | peak} mean-rate [burst-size [excess-burst-size]]

    What is the difference in router cpu load of the above 2 commands?
    Which one should be prefered?

    Is the command
    police 10000000 312500 conform-action transmit exceed-action drop
    "equal" to
    shape peak 10000000 312500
    if we just want to limit the bw of an interface to 10mbps?

    PS: From what i have read, "shape" delays packets instead of dropping them, but what
    exactly does this mean?
    For how long does it delay the packets?
    Does it have any impact on router's cpu load?
    Tassos, Dec 11, 2003
    #1
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  2. "Tassos" <> wrote in message
    news:1071122705.135786@athprx02...
    > PS: From what i have read, "shape" delays packets instead of dropping

    them, but what
    > exactly does this mean?


    It means they stay in queue until a "credit" is available for them to get
    out. It is similar to leaky token bucket queue. Packets go only when
    credits are available, and credits are generated periodically at your
    desired traffic rate. So the data is "shaped" to the desired output speed.

    > For how long does it delay the packets?


    Depends on your shaping parameters. BUSRT_SIZE is sent every period. You
    can set the burst size. You can get 128kbps by sending 128kb every second,
    64k every half of a second, 32k every quarter of a second, or 16k every
    eighth of a second. That is burst size and period. Each "shapes" the
    traffic differently. IIRC, the default burst perio is 1/8 second if you
    just set a bandwidth and no burst period.

    > Does it have any impact on router's cpu load?


    Not as much as you'd think :cool:
    Phillip Remaker, Dec 11, 2003
    #2
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