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  • Content created by Rene Molenaar (CCIE #41726)


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Forum Replies

  1. Hi Rene, Great explanation of shaping, its quite hard to find this sort of detail in an easy to understand format. One question regarding rate-limiting vs shaping. Although rate-limiting does not buffer/queue packets, is it capable of slowing the rate to the contracted bandwidth? For example, if I have a 1GB fibre to a provider switch but only have say 50mb contracted, can I use rate-limiting to prevent the router transmitting at a 1GB clock rate or is shaping the only method of achieving this?

  2. May i know, at which cases Policer will be usefull ? At which situation we can use both Policer & shaping ?

  3. Please help me out to implement QoS over ADSL link. I want to prioritize my voice traffic and assign 15% of BW.
    Please check the config of my usual WAN link not sure what would be on ADSL.

    class-map match-any Client-Signal-Class
     match precedence 3
     match ip dscp af31
     match ip dscp cs3
    class-map match-any Client-VOIP-Class
     match precedence 5
     match ip dscp ef
    policy-map 50Mb-VOIP-PMAP
     class Client-VOIP-Class
      priority percent 10
     class Client-Signal-Class
      bandwidth percent 5
     class class-default
    policy-map 50Mb-VOIP
     class class-default
      shape average 49000000 504000
       service-policy 50Mb-VOIP-PMAP

    Not sure what would be the shaping reference BW (Here - 49000000) on ADSL link. Your prompt reply highly appreciated.

  4. This example looks correct. You are shaping to 49 Mbit, with a Bc of 504000 you have a Tc of ~11ms which is fine. On Cisco IOS, you have to configure the shaper first and then use a sub-policy with LLQ like you did…10% priority queue for RTP and 5% (CBWFQ) for your signaling traffic.

  5. Hello Prashant,

    Both policing and shaping have a common goal: to “rate-limit” exceeding traffic.

    How they do it is different though. A shaper will “buffer” the traffic while a policer “drops” the traffic.

    A practical example could be an ISP router that is connected to multiple customer routers. Let’s say that the routers are connected with GigabitEthernet links but the customers are only paying for a 100/100 Mbit connection.

    The ISP will then use a policer to drop all incoming traffic that exceeds 100 Mbit.

    On the customer end, you probably don’t want your traffic to get dropped so you can then configure an outgoing shaper that is configured for 100 Mbit. This will not drop your traffic but buffer it, which guarantees that all traffic is sent and the ISP’s policer won’t drop it.

    Hope this helps!


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