Re: DSCP AF class

Jay,

If your LLQ is already being used, you can still give other classes a
guaranteed portion of the bandwidth. Classifying based on CS or AF values
doesn't really make a difference. It's how you treat those markings that
matters.

Bryan Bartik
CCIE #23707, CCNP
Sr. Support Engineer - IPexpert, Inc.
URL: http://www.IPexpert.com

On Tue, Apr 21, 2009 at 8:40 PM, Jay Pal <jay.b.pal_at_gmail.com> wrote:

> Hi Ravi, Gaurav, Bryan and WorkerBee,
> thanks for the inputs/explaination.
> This is where it gets confusing to me. The Qos book (mentioned earlier in
> the post by Ravi) that 'there is no inherent advantage to being in class 4
> versus class 1'. And in another post Ravi mentions that
> '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.'
> So I am confused as to why AF4 has better queue treatment than AF1 in a
> DSCP
> aware network. I can understand that if the network is not DSCP aware then
> it would give better treatment for AF4 than AF1 (e.g. when you just have
> WFQ on the interface). I have read few materials on the web and an RFC but
> it still not quite clear.
>
> Bryan, Just to put another requirement on this scenario, lets just say that
> you already have LLQ with voice running on EF. You could probably use CS 4
> and CS1 to achieve this but i am trying to see if there is any other way of
> doing it by using AF41 and AF11.
>
> thanks
> Jay
>
> On Wed, Apr 22, 2009 at 10:01 AM, Ravi Singh <way2ccie_at_googlemail.com
> >wrote:
>
> > Exactly, That's why I said if you have a network domain totally under
> > your control you may fiddle with the markings. Forgot to add that
> > obviously such non-standard markings will not be of much use when sent
> > outside the domain to other DSCP compliant networks. Thanks for adding
> > this up WorkerBee.
> >
> > Regards,
> > Ravi
> >
> > On Wed, Apr 22, 2009 at 2:42 AM, WorkerBee <ciscobee_at_gmail.com> wrote:
> > > Ravi/Jay,
> > >
> > > AF classification is meant to standardize QoS service offering. It is
> > > well understood that EF class is known to every engineer that it meant
> > > LLQ/Priority while Best Effort is DSCP 0 marking.
> > >
> > > Of course you can choose to reverse the logic and implement the other
> > > way round but you won't be integrating very well with other DSCP
> > > compliant networks.
> > >
> > > That's why, you don't have the luxury of control the entire domain
> > > but following the standards help to improve the QoS treatment of
> > > properly marked DSCP packets.
> > >
> > > Engineering is fun. :)
> > >
> > > On Wed, Apr 22, 2009 at 9:27 AM, 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
> > >>>> >>> > >
> > >>>> >>> > >
> > >>>> >>>
> > >>>> >>>
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