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QoS AFxx drop probability

Unanswered Question
May 22nd, 2012
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Dears

Would like your assistance please in understanding QoS AFxx concept


I know that we have 4 Assured Forwarding and each class has 3 drop probability

For example If we have AF11 , AF12 and AF13 then AF13 should have higher drop probablity than AF11.

My question is how do router know that AF13 has higher drop probablity than AF11 ?

I believe this can only be done by wRED but what confuses me why it is not written in documents that wRED is a must for this case !

Am I missing something ???

Appreciate your assistance

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Joseph W. Doherty Tue, 05/22/2012 - 16:43
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My question is how do router know that AF13 has higher drop probablity than AF11 ?

Am I missing something ???

How does a router "know"?  It's doesn't unless you configure it to "know".  I.e. you're not missing something.


Don't confuse a RFC recommendation with device features.

Sherif Atef Ahm... Wed, 05/23/2012 - 14:07
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Thanks Joseph for your reply


So in Cisco there is no internal mechanism that can differentiate between AFx1 and AFx3

Also is there any other way that allow me to differentiate between AFx1 and AFx3 rather than wRED ?


Many Thanks

Joseph W. Doherty Wed, 05/23/2012 - 17:01
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The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.


Liability Disclaimer


In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.


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Unsure what you would consider "internal".  You've already mentioned WRED, which some Cisco devices QoS features can utilize any DSCP marking for a different drop treatment (within the same queue).  Some Cisco devices (switches) support multiple (settable) drop limits, WTD, for DSCP marked packets or WRED.


BTW, to be a little more precise, on Cisco routers that I've used, generally, by default, all interfaces only have a single FIFO queue with a single default tail drop except for "slow" serial interfaces which default to WFQ.  Different drop treatments, require activation of interface WRED or WRED within a CBWFQ class.  Once activated defaults will drop DSCP marked packets differently.


On Cisco QoS capable switches, their default tends to be no QoS, another instance of a single FIFO queue with tail drop.  On those, activation of QoS often also defaults to different drop treatments if using DSCP markings.

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