QOS question

[Arun]

I understand that the Expedited Forwarding (EF) model is used to provide resources to latency sensitive real-time, interactive traffic.The EF model uses one marking -- DSCP 46.

so the binary value is 101 11 0. Here bits 2 and 1 specify the drop probability (11 - high);  so that means high probablity of dropping the packet.instead of this, cisco should have used 101 01 0 (dscp value 42 and drop propabilty here is low ) to acheive the best results Right?.. Any thoughts?..

If EF does NOT follow the drop preference rules of the assured forwarding model,then how the IOS handle this.. what is the meaning of those two drop preference bits here.

[cadet alain · Accepted solution]

Hi,

EF doesn't follow the drop precedence rule and these bits have no special meaning for IOS.It's not Cisco who decided the value of the EF and AFxx DSCP code points. I think these values for these bits was chosen from the tos bits meaning:

http://tools.ietf.org/html/rfc1349

Regards.

Alain

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[Arun]

Hi Alain,

Thanks for answering this question..

So that means as soon as the ios sees 1 0  1 in the left most bits, the drop precedence bits have no special meaning right?

Is my understanding  correct..

Thanks,

Arun

[Joseph W. Doherty]

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So that means as soon as the ios sees 1 0  1 in the left most bits, the drop precedence bits have no special meaning right?
Is my understanding  correct..

Yes and no.  First you need to understand QoS RFCs are recommendations.  I.e. ToS interpretation/treatment is up to you.  Second, by default, most Cisco devices generally ignore ToS.  I.e. you need to configure the device to treat ToS as you desire, although some Cisco devices do have some default behaviors with "activation" of QoS features.

Additionally, as Alain noted, Expedited Forwarding (http://tools.ietf.org/html/rfc2598, http://tools.ietf.org/html/rfc3246) isn't part of the RFC for Assured Forwarding (http://tools.ietf.org/html/rfc2597, http://tools.ietf.org/html/rfc3260), i.e. drop precedence wasn't defined for it.

PS:

You might also want to review http://tools.ietf.org/html/rfc4594 to see how it all works together.