QOS congestion -BE or AFn3 ?

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Jun 12th, 2009
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I am trying to clarify whether the BE class will drop down to nothing before traffic from AF classes starts dropping (highest drop precedence first, of course). This would mean an important but greedy application could kill all general network traffic?

Could someone point me to the relevant documentation where this is explained? I can't seem to find it.

This is really about ISR routers at the WAN edge egress.

Thank you all,


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Laurent Aubert Fri, 06/12/2009 - 06:20
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Hi Davey,

During high congestion, each class of service will have its minimum bandwidth guaranteed. If the default class has no minimum guarantee, it will have the bw left after each other class being served.

If a class wants to use more than its minimum guaranteed during congestion, the excess will be dropped because there is no excess bw available.



Joseph W. Doherty Fri, 06/12/2009 - 08:55
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It depends on your ISR QoS configuration.

On usual ISR WAN interfaces, except for FIFO or WFQ, you'll usually need to define how traffic types share, or not, bandwidth, and what they classify on (which can be DSCP values and/or something else).

Classical WFQ use IP Precedence (1st 3 bits DSCP), to help determine a flow's proportion allowance for bandwidth. I.e., it will indirectly use BE and AFxy without explicit configuration.

Generally, except for PQ, some traffic won't starve other traffic from all bandwidth, although it might leave very little. Single/shared FIFO can also leave very little bandwidth for some traffic flows.

For most QoS, which requires explicit DSCP classification analysis, you can easily have BE push aside AFxy, or even EF, marked traffic. I.e., again, most QoS requires you to define bandwidth allocations/ratios for different traffic types; usually nothing implicit by DSCP markings although there are recommendations on how it should work.

In other words, actual QoS policies (and implementation) would determine BE class vis-à-vis an AF class. Just AF or BE markings don't define how traffic between the two will be treated in all cases.


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