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malik1
Posts: 164
Registered: ‎11-14-2009
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Significance of Using Low/High With DSCP Rewrite Rule

Hello Community

 

Hope every one is doing fine.

 

1-Can some one please shed a light on why do we use the low/high keyword when defining a DSCP Rewrite/Classfication

 

rule.As to me the last 3 least significant bits of the 6 bit DSCP code point refer to the low/high thing then why is it neccassary

 

to use the high/low keyword.i.e forwarding-class ef loss priority low code point 101100

 

2-In case of Exp Classifier/Rewrite rule how do we express low or high drop precedence as there are only three bits.

 

Thanks & Regards

Malik

 

 

Contributor
wtucker
Posts: 16
Registered: ‎03-05-2010
0

Re: Significance of Using Low/High With DSCP Rewrite Rule

The bits don't mean anything until you assign meaning to them - but encoding loss priority in those bits is common practice for some forwarding classes (particularly AF4, AF3, etc.)

 

Packets with a higher loss priority are more likely to be dropped by WRED (if you make use of it). You always have the option of classifying everything with the same loss priority.

 

There is no way to encode loss priority in the MPLS EXP or 802.1p priority bits. If you need to pass that information along it will need to be in the DSCP bits (which of course only works for IP traffic). I believe your ieee-802.1 and exp rewrite rules need to have entries for each priority you use under a forwarding class, e.g.:

 

forwarding-class af1 {
    loss-priority low code-point af1;
    loss-priority high code-point af1;
}

 

Hope this helps.

 

:w

 

Contributor
malik1
Posts: 164
Registered: ‎11-14-2009
0

Re: Significance of Using Low/High With DSCP Rewrite Rule

ThanksTucker

 

Can you please tell , if defining low or high changes some bit settings in the DSCP field or packets,i mean if any flag is set,

 

Also if one such flag is set then y do we again define the classifier again at ingress with low and high code points.

 

E.g. Ingress---->R1(Rewrite AF11 PLP low)--->(Classifier AF11 PLP low) R2

 

Why do we again define low with Af11 at R2 as it is already conveyed by R1.

 

2-I think the last 3 least significant bits of DSCP code points for assured forwarding class convey the low or high meaning , am i right ?

 

3-If i iddentify some traffic as part of EF forwarding class at R1 & i am using the default DSCP rewrite then to which code point will this traffic map to at egress of R1 as we know that default DSCP table has both low & high code points defined for EF class & one type of traffic can not be mapped to 2 code points at the same time i.e low & high.

 

4-Why is it so that both high & low code points for EF class in Default DSCP table are same for high and low.

 

5-Why a re-write rule is called a rewrite when nothing is written in TOS byte then why do we call it rewrite.

 

   This only seems true when the traffic is already being marked by server or switch.other wise it should be just write rule.

 

    Or does it mean that if we use MF classifier and set PLP it means that something is written in TOS byte by defining PLP.

 

I will be really thank ful to you for your answers.

 

Thanks & Regards

Malik

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