ip precedence and dscp

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Nov 19th, 2007
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Hi all, can anyone tell me the differences in the above, and what is the tos value, is that used for precedence where as diffserv uses the dscp value ? whats the difference ?

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bvsnarayana03 Mon, 11/19/2007 - 05:49
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Precedence uses 3 bits of the TOS field in IP header. Thus Ip precedence can classify trafic in to 8 different categories.

Whereas diffserv modifies the IP header to include 6 bit field, thus helping in classifying traffic in to 64 different categories. DSCP has backward compatibility with precedence or intserv.

Both of these are marking tools used for marking packets which are later serviced accordingly.

Pls refer this link for detailed study.


pls rate if helped.

guruprasadr Mon, 11/19/2007 - 06:20
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As i know TOS and DSCP are only Marking Tools.

DSCP has certain chart to follow inorder to mark various kinds of Packet Types.

Refer Cisco QOS Configuration Guide for the same.

Do Rate all Helpful POsts

Best Regards,

Guru Prasad R

carl_townshend Mon, 11/19/2007 - 09:25
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When we talk about RED/WRED, are these different that WFQ ?, Are all these to do with how the queues are serviced ?

allan.thomas Mon, 11/19/2007 - 11:12
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RED/WRED are different from WFQ, essentially RED and WRED are congestion avoidance techniques whereas WFQ is a queuing technique.

RED/WRED simply monitor network traffic in an effort to anticipate and avoid congestion at common network and internetwork bottlenecks before it becomes a problem.

If supported on the hardware, WRED would be recommended instead of RED as it services queues and drops packets based on IP precedence value. The higher the value the less likely it will be dropped.

WFQ simply queues traffic in order to ensure that latency sensitive traffic such voice for example can be forwarded before less critical.




carl_townshend Tue, 11/20/2007 - 05:01
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Hi there

thanks for that, so do you have to configure your router for WRED, or is this default behaviour ?

allan.thomas Tue, 11/20/2007 - 05:26
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As I have mentioned previously, you will firstly need to determine whether your hardware or more specifically your interfaces supports WRED.

You have to specifically enable or configure WRED on the interface, it is not the default. If WRED was enabled by default then this would be indicated on the 'show interface' or 'show queueing' commands.

Please review the following link, which explains in more detail about WRED and also has configuration examples if interested:-




Pls rate this post and others if you find the information provided helpful.

carl_townshend Wed, 11/21/2007 - 03:59
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Hi there

so can you tell me how the queues work ? what kind of things do we assign to the queues ? and then what do we do with the queues?

also is RED/WRED part of QOS

can you put me a step by step info on what to do when configuring qos, ie 1, decide what traffic, 2 classify etc?

this would be very helpful



bvsnarayana03 Wed, 11/21/2007 - 05:28
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The links provided here & in the other post by you for a question on QoS, are god enough for understanding most of the QoS concepts.

Also, QoS guide from Cisco press by Wendell Odom is a Bible for this topic.All topics have been covered very extensively. I'm currently spending most of my time on this book for last few days.

This book can help meet your quench for deeper understanding of these topics.

Have a great reading.


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