QoS question

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May 14th, 2008
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Hi everyone


A QoS scenario which has been troubling me for a while:


1. fair-queue enabled on interface

2. traffic-shape matching ACL - 2048kbps


Will packets with a higher precedence matching the ACL get preference in being transmitted out of the interface? If not, can one achieve this without using class-maps and service-policy?


Thanks

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bmcginn Wed, 05/14/2008 - 15:12
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Hi there,


It is definitely true that the precedence of a packet impacts how the WFQ scheduler works. WFQ gives more bandwidth to flows with higher IP Precendence values. With FQ, each flow gets an equal amount of bandwidth. eg 128kbps bandwidth may give 1.28kbps to each of 100 flows. WFQ gives preference to higher-precedence flows. eg, with 10 flows on a 128kbps link and 5 of those flows are precendence 0 and the other 5 have precedence 1, WFQ may give the precedence 1 flows twice as much bandwidth as the others. eg 8.5kbps to each IPP 0 flows and 17kbps to each IPP 1 flow. If I remember correctly, WFQ provides a share roughly on the ratio of each flow's precedence + 1. so IPP 1 compared to 0 would be 2, IPP2 compared to 0 would be 3 etc


Brad

bvsnarayana03 Wed, 05/14/2008 - 22:03
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Packets with higher precedence definitely get a better treatment than the normal pkts. However with WFQ, its based on the algorithm & scheduler to conreol the traffic treatment.


If you want to be in control of way the traffic has to treated then class-maps & service-policy are good options on L3. For switches, the number of queues are already defined & we cant do much about it however how the scheduler treats each queue can be defined by us.

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