QoS Bc and Packet size

Unanswered Question
Aug 14th, 2009
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Hi Experts,

If we have a link with 128 Kbps speed, for a particular class we shape it

to CIR=64 Kbps, Tc= 125 msec -- Bc= 64000x125/1000= 8000 bit = 1000 byte.

Given that, at the Bust period for this class:

1- What will happen to a Packet from this class with 1500 byte length ??

the Bc is only 1000 byte !!!. this packet requires more 500 bytes bits over the Bc !

2- What will happen to the third Packet of three consequent packets from

that class with 400 byte per each packet. The summation of the first two

packets is 800 bytes still can be transmitted within the Bc (1000 byte), but the third packets requires 200 bytes over the Bc !

In those cases, Packets will be queued or transmitted, or fragmented ?

Thanks in advance


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Joseph W. Doherty Fri, 08/14/2009 - 08:48
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#1 I recall (somewhere?) Cisco documenting that Bc should always be large enough to pass the largest packet, otherwise the shaper wouldn't work correctly (or some such). However, although I haven't tested it, I suspect a shaper just might delay such a packet for multiple Tcs until it logically accumulates enought Bc.

#2 The 3rd packet should be held until the beginning of the next Tc. I.e. the 1st two packets would be transmitted back-to-back at line rate (128 Kbps) at the beginning of the first Tc, the 3rd packet also transmitted at line rate 125 ms after the first packet began its transmission.


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