Notable Replies

  1. Hi Taslim,

    I will, just added it to my list.


  2. 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.

  3. 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!


  4. Hello Jose

    Yes indeed this can be the case. You must coordinate with the ISP so that your shaping policy on the edge of your network doesn’t send more traffic than the ISP is configured to handle especially if the ISP implements a more strict policy than your edge router. Otherwise, as you describe in your post, traffic can and will be dropped by the ISP.

    I hope this has been helpful!


  5. Hello Jose

    There’s no single answer to your question. You are correct in that the Bc that you will use will depend on the traffic that you want to shape. As mentioned in the lesson, Bc depends upon the Tc and upon the actual Committed Information Rate or the required shaped speed of the link, like so:

    Bc = Tc * CIR

    And Tc will depend on the kind of traffic that will be shaped. If you’re shaping voice, Tc should be around 10 ms. So Bc = 10 ms * CIR. Now if your CIR is very large like 5Mbps, then Bc will be 50 kilobits, but if your CIR is relatively small like 128kbps, then Bc = 10 ms * 128kbps = 1280 bits (remember to convert your units when calculating these values).

    If you’re going to make your Tc larger because you’re only forwarding non-time-sensitive traffic, then Bc changes accordingly based on the CIR as well.

    So really, you have to calculate each situation and determine which value is best.

    I hope this has been helpful!


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