Re: DSCP AF class

Hi Jay,

 Below is a paste of some relevant lines from the Cisco QoS Exam Cert
Guide which will make this clear

"An individual PHB describes what happens in a single hop, most
typically a router. In the case of AF,
each PHB contains two separate QoS function, typically performed by
two different QoS tools. The
first function is queuing. Each router classifies the packets into
four different classes, and packets
from each class are placed in a separate queue.

The AF PHB defines Congestion Avoidance as the second behavior that
comprises the AF PHB.

Unlike the CS PHB, AF does not follow the bigger-is- better
logic for the AF DSCPs. First, AF11, AF12, and so on are names for
DSCP values, not the binary
of decimal equivalent. Given the names, at least you can
think of the first digit after the AF to be the queuing
classification  for example, all AF4x code
points are in the same class for queuing. No specific queuing
parameters are implied for any of these
classes, so there is no inherent advantage to being in class 4 versus class 1.

Similarly, the second numeric digit in the AF DSCP names imply the
drop preferencewith
3 meaning highest likelihood of being dropped, and 1 meaning the least
likelihood. In other words,
inside a single class, an AFx3 DSCP would mean that these packets
would be dropped more quickly
(more aggressively) than AFx2, which would be dropped more
aggressively than AFx1 packets. In
the actual DSCP names, a bigger number for the second numeric digit
actually implies a less desirable
QoS behavior.

HTH,
Ravi

On Tue, Apr 21, 2009 at 2:23 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:
> ok. I am clear on the drop probability. what about the first numerical
> value. Does AF4 have preference over AF1?
>
> On Tue, Apr 21, 2009 at 9:19 PM, GAURAV MADAN <gauravmadan1177_at_gmail.com>wrote:
>
>> correct !
>>
>> first integer is "class selector" and second is " drop precedence"
>>
>> for drop prec : 1,2,3 .... 1 means low 2 means medium and 3 means high
>>
>> HTH
>> Gaurav Madan
>>
>> On Tue, Apr 21, 2009 at 6:43 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:
>>
>>> Actually, drop preference is the second numeric value in a particular
>>> class
>>> like AF11 (low drop), AF12 (medium drop) and AF13 (high drop).
>>>
>>> I was talking more on AF class (AF1, AF2, AF3 and AF4).
>>>
>>> On Tue, Apr 21, 2009 at 9:07 PM, Divin Mathew John <divinjohn_at_gmail.com
>>> >wrote:
>>>
>>> > AF1, AF2 etc... 1,2 is the drop preference. like AF1 will be dropped
>>> > less than AF2 and all
>>> > Thanking You
>>> >
>>> > Yours Sincerely
>>> >
>>> > Divin Mathew John
>>> > divinjohn_at_gmail.com
>>> > divin_at_dide3d.com
>>> > http://www.dide3d.com
>>> > +91 9945430983
>>> > +91 9846697191
>>> > +974 5008916
>>> > PGP PUBLIC KEY BLOCK @ http://www.dide3d.com/divin_Public_PGP_key.txt
>>> > Sent from Bangalore, KA, India
>>> >
>>> >
>>> > On Tue, Apr 21, 2009 at 6:31 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:
>>> > > Hi all,
>>> > >
>>> > > Just to make sure my understanding is correct on DSCP AF class. AF1,
>>> AF2,
>>> > > AF3 and AF4 have the same priority when using DSCP and there is no
>>> > > advantage of using AF4 to AF1 or AF2.
>>> > >
>>> > > If this is correct is there a way to make one of the AF class with
>>> more
>>> > > priority than other?
>>> > >
>>> > > thanks
>>> > > Jay
>>> > >
>>> > >
>>> > > Blogs and organic groups at http://www.ccie.net
>>> > >
>>> > >
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>
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Received on Tue Apr 21 2009 - 16:32:20 ART