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So can any tech expert explain this?

 

 

Thank you.

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Ok, heres a primer on DSCP.

 

One very important thing. Prioritization of values is controlled by QoS policies and can be implemeted however you choose (eg: You can prioritize AF11 packets over EF) This writeup assumes best practices based on DSCP standards

 

DSCP was designed to be more granular and more scalable than IP precedence BUT with backward compatibility.

 

The priority field (or type of service, ToS) was originally 3 bits, giving it the IP prec.values 0-7 (0 being the lowest priority, 7 the highest). DSCP has an 8 bit field, of which 6 bits are used for markings (the 6th bit is always 0). This gives it a larger number of values (both for per hop behavior, or PHB, and drop precedence. More on this later)

 

The last 2 bits are used for ECN or explicit congestion notification. This is a brand new feature (as of Windows Vista) and is purportedly going to revolutionize internetwork traffic flow. More on this later.

 

With the 6 bits allotted to DSCP, the first three (left to right) are used for Major Class, or Per Hop Behavior. These match up with the old IPprec values of 0-7.

 

The second 3 bits identify the drop precedence. Higher = more likely to be dropped. This means that a DSCP marking of AF21 (major class 2, drop precedence of 1) will be preferred over AF22 or AF23. An AF3x will beat any AF1x or AF2x--the major class of 3 is higher than the major class, or PHB, of 2.

 

 

It is important to note the drop precedence is only used on classes 1-4. (here is a table from wikipedia) A marking of 0 indicates 'best effort'

 

Class 1 Class 2 Class 3 Class 4

Low Drop AF11 (DSCP 10) AF21 (DSCP 18) AF31 (DSCP 26) AF41 (DSCP 34)

Med Drop AF12 (DSCP 12) AF22 (DSCP 20) AF32 (DSCP 28) AF42 (DSCP 36)

High Drop AF13 (DSCP 14) AF23 (DSCP 22) AF33 (DSCP 30) AF43 (DSCP 38)

 

The notation DSCP xx is the bit notation. Eg AF12 = 001 100 which is a decimal value of 12. Here's another one. AF43 = 100 110 (38)

 

EF, which it equal to IPprec of 5 is assigned a major class of 5 and a drop pecedence of 3 (this is odd, I don't know why this was done) The decimal value for EF is DSCP 46 or 101 110.

 

Marking is done via a policy-map, let me give you a whole example. I am going to mark FTP, HTTP, RDP and VoIP traffic. Then I will prioritize FTP above HTTP and RDP traffic, and VoIP traffic as a priority queue.

 

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Note: Don't do this during production hours. This enables NBAR and can spike your CPU

 

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Note: You can use "set ip dscp" or "set dscp" interchangably. Also you can use the AF/EF notations or their decimal equivilants

 

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Note: If you want to reserve more than 75% of the interface bandwidth you have to change the max-reserved-bandwidth

 

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I hope that helps you learn. Post any questions or comments.

 

~sly

 

Oh, forgot about ECN. In a nutshell routers will send an ECN slow down message to hosts if their links are becoming saturated. This will "ask" the host to decrease their TCP window size and slow down.

 

The ECN field is the last 2 bits of the ToS byte.

 

00 = not ECN capable

10 or 01 = ECN capable

11 = ECN slow-down message

 

It is enabled on Cisco routers via WRED:

 

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