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Shaping behaviour


I've a question related to IOS shaping tools (e.g. CB-shaping and FRTS)...

Consider for instance a default FRTS configuration for a serial interface (frame-relay traffic-shaping configured at interface level):

R1#sh traffic-shape

Interface   Se0/0

       Access Target    Byte   Sustain   Excess    Interval  Increment Adapt

VC     List   Rate      Limit  bits/int  bits/int  (ms)      (bytes)   Active

106           56000     875    7000      0         125       875       -

105           56000     875    7000      0         125       875       -

here Token bucket's size is 7000 bit (7000/8 = 875 Byte) and it is refilled with 7000 bit every Tc = 125 msec interval

Now, I did a simple test with 1500 Byte ping packets train going trough that 'shaped' interface: even if each packet's size is greater than Token bucket's size you can see (entires) packets are forwarded to the destination without any fragmentation

Which is the shaper behaviour in this 'specific' condition ?


Everyone's tags (3)
Hall of Fame Expert

Re: Shaping behaviour

Hello Carlo,

>> packets are forwarded to the destination without any fragmentation

A whole packet is sent or a whole packet is delayed.

To send the packet some bytes are borrowed from next Tc interval. After sending the packet at Tc time interval i, the router waits to cumulate enough tokens to send the next packet so next packet  is sent at TC time interval i+2.

In average the router is shaping. In a specific Tc interval are sent 1500 bytes or 0 bytes.

Fragmentation is not part of shaping action.

Hope to help


Re: Shaping behaviour

Thanks Giuseppe,

so here the Token bucket 'metaphor' has not to be intended in a strict sense (in other words Token bucket's depth is not fixed at Bc bit). It seems just like a 'deficit' behaviour....

Does it make sense ?

Hall of Fame Expert

Shaping behaviour

Hello Carlo,

token bucket depth has to reach the packet size for the packet to be transmitted.

In this case it takes more then one Tc to cumulate enough credit to transmit one packet.

The Bc is not the size of the bucket, but it is the filling rate of the bucket it receives 875 credits every Tc time interval.

Hope to help


Shaping behaviour


reading for example and I understand that Token bucket's size (or depth) is  equal to (Bc + Be) bits while Bc worth of bits are replenished every  Tc...

Maybe there is some different detail in the IOS shaper implementation.....

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