Re: DSCP AF class

I can think of one, not so great, reason that you might want to put your
higher priority traffic in AF4 instead of AF1. Backwards compatibility with
IP Prec.

Prec 5 = EF
Prec 4 = AF4
Prec 3 = AF3
Prec 2 = AF2
Prec 1 = AF1
Prec 0 = Best effort

Chris

On Tue, Apr 21, 2009 at 9:27 PM, Ravi Singh <way2ccie_at_googlemail.com> wrote:

> Hi Jay,
>
> As per my understanding, while inherently there is no advantage of a
> packet in one class over a packet in another, the AF PHB states that
> packets with a higher value of the AF class should be given queuing
> preference over packets in a lower value AF class . For example,
> packets in class AF4 should be given better queuing treatment than
> packets in AF1.In other words, when you classify and mark packets and
> assign them the AF PHB, a packet with a PHB of AF4 is not bound to
> receive better queuing treatment than the AF1 packet. It is totally
> upto you on what queuing treatment to give packets in different AF
> classes . If you have a network domain totally under your control, you
> may wish to provide packets in the AF1 class to be given the best
> treatment,whatsoever.
>
> And while I am writing this, Bryan has already responded on how you
> could achieve this configuration. Thanks Bryan.
>
> Regards,
> Ravi
>
>
> On Tue, Apr 21, 2009 at 11:40 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:
> > Hi Ravi,
> > In this case I have the same understanding then 'all AF class have the
> same
> > priority'.
> >
> > Is there anyway to make AF4 with higher priority than AF1?
> >
> > thanks
> > Jay
> >
> > On Tue, Apr 21, 2009 at 11:32 PM, Ravi Singh <way2ccie_at_googlemail.com>
> > wrote:
> >>
> >> 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 preference with
> >> 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
> >> >>> > >
> >> >>> > >
> >> >>>
> >> >>>
> _______________________________________________________________________
> >> >>> > > Subscription information may be found at:
> >> >>> > > http://www.groupstudy.com/list/CCIELab.html
> >> >>>
> >> >>>
> >> >>> Blogs and organic groups at http://www.ccie.net
> >> >>>
> >> >>>
> >> >>>
> _______________________________________________________________________
> >> >>> Subscription information may be found at:
> >> >>> http://www.groupstudy.com/list/CCIELab.html
> >> >
> >> >
> >> > Blogs and organic groups at http://www.ccie.net
> >> >
> >> >
> _______________________________________________________________________
> >> > Subscription information may be found at:
> >> > http://www.groupstudy.com/list/CCIELab.html
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Fri Apr 24 2009 - 17:21:33 ART