DSCP Madness!

To help me fully grasp the concept of DSCP I have written a few notes below. I was struggling with the drop values and also the structure of the DSCP bit setup.

DSCP – AF Classes

Key point for understanding the Major and Minor drop classes…

  • In the Major Classes, a higher number means /drop value means you are LESS likely to be dropped.
  • In the Minor Classes, a higher number/drop value means you are MORE likely to be dropped.

So for example..

2 DSCP packets come into the router with DSCP markings as follows:

  • AF11
  • AF12

As they are the same Major Class, we look to the Minor Class values… HIGHER number is a HIGHER drop value, therefore AF12 is dropped.

Another example…

  • AF21
  • AF33

Who is dropped? Well the Major Classes here are different, we have AF2 and AF3 Major Classes, therefore AF2 is a LOWER number than AF3 which means a HIGHER drop value, therefore AF21 is dropped over AF33.

Confusing yes… the best way to remember is that the Major and Minor drop classes are the opposite of each other. Just remember Major or Minor and you can’t fail! 😉

DSCP/ECN Bits

226608

If we look at the image above, we have 8 bits in total for DSCP and ECN. ECN occupying the 1st 2 bits on the right hand side and DSCP occupying the other 6 bits.

These 6 DSCP bits are split in half. 3 bits for the Major Class, 3 bits for the Minor Class. However with the Minor Class the last bit is always zero. Therefore we are only interested overall in the 5 bits (right to left)

Therefore a DSCP setup is as follows:

4 2 1 —————————— 2 1

MAJOR—————————MINOR

AF21 = 0 0 1 0 1 0

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