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On a Cisco router, when setting up a policy-map to shape traffic and drop packets based on DSCP (such as the example below), which class of packet will be dropped first: DSCP 0 or AF13? DSCP AF13 is described as most likely to be dropped, yet DSCP 0 is described as "best effort", which one will win?

policy-map mypolicy
 class class1
  shape average 128000
  random-detect dscp-based  <-- which will be dropped first, DSCP 0 or AF13?
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up vote 1 down vote accepted

Here's some good documentation from Cisco about DSCP:

There may very well be something I'm missing, so hopefully someone else can verify this (not a CCIE, yet), but the important bit to know is how these DSCP values map to the binary values, and what AF (Assured Forwarding) is.

DSCP 0 (decimal value 0) would be 000000 in binary. For AF, you can see that document for more details, but AF13 would be 001110, which is 14 in decimal.

However, it does not go directly off of the decimal value. If you do not specify any other configurations there, it will use the defaults. You can find these in this table:

So, DSCP 0 (default in that table) would be more likely to be dropped in that the threshold where it could be dropped begins at 20% vs 24% for AF13.

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So if you change the default thresholds, you can change the drop rate of any of the DSCP to anything you want. Fascinating. Excellent link! It looks like the 20% and 24% are not percents but the minimum packets in the queue before packets will start randomly dropping, with 20 and 24 packets respectively. – Brain2000 Feb 1 '12 at 3:46

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