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Notable 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. This is one awesome posts about QoS policing and shaping .U don't find like this many out there.And I should thank U for this.
    BTW, I have few questions reg this,

    1.In this example that you have demonstrated on what basis the Tc (Time Interval) value was chosen. i,e 62.5 in this case ? (62.5 + 62.5 =125).Is there any logic behind it?

    Expecting the earliest reply..

    Thanks in Advance,

  3. Good question. A shaper will "buffer" your exceeding traffic while the policer will "drop" all exceeding traffic.

    Anything that is time-sensitive like VoIP or realtime video should not be shaped because it adds delay. You could shape FTP traffic for example since it's not delay-sensitive and could consume most of your bandwidth.

    An ISP might use policing to enforce your contract, anything that exceeds what you pay for will be dropped by them.


  4. Hi Rene.Thanks for your Perfect article. Lets say traffic is passing from R1 to R2 and I configured shaping (avarage shaping 1mbps).

    show policy-map interface 
      Service-policy output: TEST
        Class-map: class-default (match-any)  
          13774 packets, 1089848 bytes
          30 second offered rate 0000 bps, drop rate 0000 bps
          Match: any 
          queue limit 64 packets
          (queue depth/total drops/no-buffer drops) 0/0/0
          (pkts output/bytes output) 358/43444
          shape (average) cir 1000000, bc 4000, be 4000
          target shape rate 1000000

    This is a CSR1000V and default Tc value is 4 ms

    it means we have 250 section. From each section during the 4 ms , 4000 token(bit) can pass. On condition that more bits pass through the R1 druring the 4 ms , (not 4000 bit lets say 6000 bit) they wil drop. Can you explain me please, when the fragmentation will occur in this case?

  5. Hi Kamil,

    Glad to hear you like it! Fragmentation is typically only done when the IP Packet doesn't fit in the MTU of the data link layer, it doesn't have anything to do with shaping. When you shape and it doesn't fit...the packet will be dropped, no fragmentation will occur.


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