Wednesday, November 12, 2008

MQC - Shape vs Bandwidth Parameter

In this post i examine the difference between the MQC shape and bandwidth parameters. Often when applying a config to answer a lab question i find that i have made the incorrect choice in my selection, using one option, shape or bandwidth, when in fact the other was the required solution.

My confusion was cleared up when i found the following example on the doc cd.

Router(config)# policy-map shape-cbwfq
Router(config-pmap)# class cust1
Router(config-pmap-c)# shape average 384000
Router(config-pmap-c)# bandwidth 256
Router(config-pmap)# class cust2
Router(config-pmap-c)# shape peak 512000
Router(config-pmap-c)# bandwidth 384
Router(config-pmap-c)# configure terminal

In the above scenario both parameters, shape and bandwidth, were used at the same time! In reality this is because each parameter performs a different function.

In summary: bandwidth allocates a minimum bandwidth to the selected traffic flow, whereas shape enables the traffic flow to rise to the assigned throughput before throttling occurs. A subtle but explicitly different function.

Note to myself here, is to read carefully what the question requires. Use bandwidth to reserve throughput and shape to limit throughput.

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