Re: Af Clasess

Some times it helps to understand the bits and see the chart. I am sorry if
this is very basic for some and i know that we all know it, but it
*MAY*help some people.

*QOS:*

*8*

*7*

*6*

*5*

*4*

*3*

*2*

*1*

*D*

*T*

*R*

*ECN*

*ECN*

IP Precedence bits

Using IP Precedence we can only get 8 different traffic classification
groups, binary 000  111, this was not enough for current and future
networks, therefore, they expanded the IPP section to include the DTR
(Delay, Throughput and Reliability), and as a result of that, we now have
the capability to classify 64 different types of traffic (2 to the power of
6). They called that DSCP. From the 64 different values, they only named 21
of them and set standards based on the 21 of those values.

The first 8 values within the standard are called Class Selector (CS), these
are backwards compatible with IPP, and they are as follows:

*CS*

*Binary*

*DSCP*

*IPP*

*Name of IPP*

*Default*

*000 000*

*0*

*0*

*Routine*

*1*

*001 000*

*8*

*1*

*Priority*

*2*

*010 000*

*16*

*2*

*Immediate*

*3*

*011 000*

*24*

*3*

*Flash*

*4*

*100 000*

*32*

*4*

*Flash-override*

*5*

*101 000*

*40*

*5*

*Critical*

*6*

*110 000*

*48*

*6*

*Routing/P*

*7*

*111 000*

*56*

*7*

*Signalling*

Note in the above chart the *DTR bits* are not used and you are basically
left with the *IPP bits,* and this is why they are backwards compatible to
IPP.

The remaining 12 standards are called Assured Forwarding and they are as
follows:

They divided the 8 bits as follows:

*8*

*7*

*6*

*5*

*4*

*3*

*2*

*1*

*I*

*P*

*P*

*D*

*T*

*R*

*ECN*

*ECN*

*CLASS*

*CLASS*

*CLASS*

*DP*

*DP*

*DP always zero*

*4*

*2*

*1*

*2*

*1*

*0*

*32*

*16*

*8*

*4*

*2*

*1*

The first line identifies the bits 1  8.

The second line identifies the IPP (IP Precedence) bits (6, 7, and 8), the
DTR bits (3, 4 and 5) and the ECN bits (bits 1 and 2).

The third line reveals the Class and Drop probability sections.

The forth line identifies the decimal value of the corresponding bit in each
section, note how the bits value in the Class section start from 1, whereas,
in the DP section it starts from 0.

The fifth line identifies the DSCP section and the decimal value of the 6
bits.

 These are the classes and their corresponding values, both AF values and
the DSCP values. Within each class the higher DP values are more likely to
be dropped.

Class

*Drop Probability 1*

*Drop Probability 2*

*Drop Probability 3*

* *

*Class 1*

**
**
**
*AF11*

*001 01 0*

*DSCP = 10*

* *

*AF12*

*001 10 0*

*DSCP = 12*

* *

*AF13*

*001 11 0*

*DSCP = 14*

* *

*Class 2*

* *

*AF21*

*010 01 0*

*DSCP = 18*

* *

*AF22*

*010 10 0*

*DSCP = 20*

* *

*AF23*

*010 11 0*

*DSCP = 22*

* *

*Class 3*

* *

*AF31 *

*011 01 0*

*DSCP = 26*

* *

*AF32*

*011 10 0*

*DSCP = 28*

* *

*AF33*

*011 11 0*

*DSCP = 30*

* *

*Class 4*

* *

*AF41*

*100 01 0*

*DSCP = 34*

* *

*AF42*

*100 10 0*

*DSCP = 36*

* *

*AF43*

*100 11 0*

*DSCP = 38*

* *

The 21st standard was assigned to Voice traffic and its called *Expedited
Forwarding* or (*EF*). The Binary value of this is: *101* *110 *or *DSCP
value of 46*

Please ignore the typos, this was done in 10 minutes.
On Thu, Oct 22, 2009 at 5:03 PM, Johnny B CCIE <jbccie_at_gmail.com> wrote:

> AF32 may have a "higher drop probability" however AF21 will get
> dropped first due to lower class. Reference EF = 5 and then AF41 being
> video, and in that order. Also take into account some providers don't
> even take into account AF21 and lower anyway and so this traffic might
> even be treated as best effort or 0.
>
>
>
> On Thu, Oct 22, 2009 at 6:13 PM, Divin Mathew John <divinjohn_at_gmail.com>
> wrote:
> > Which class has a Higher Drop Probability?
> >
> > AF21 or AF32?
> >
> > AF21 = 010 010
> > AF32= 011 100
> >
> >
> > --
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--
Narbik Kocharians
CCSI#30832, CCIE# 12410 (R&S, SP, Security)
www.MicronicsTraining.com
Sr. Technical Instructor
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