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I read from this website to understand DSCP and it's corresponding IP precedence value.
For example DSCP value is '000 000' which Drop Probability is N/A(never happen) and it's Equivalent IP Precedence Value is '000'. But in IP Precedence table '000' is least important.
My question is if any packet with DSCP value has very less 'packet drop probability' but why it's less important?
The IPv4 DSCP (IPv6 Traffic Flow) packet header field can be used to mark packets for later treatment in network congestion. There are recommendations and guidelines for interpreting these values a PHBs (Per-Hop Behaviors). It is a best practice to follow the guidelines, but there is no requirement to do so.
By default, network devices do not have QoS enabled, and most packets simply are simply marked BE (Best Effort). Even if packets are marked, network devices do not by default treat the packets any differently. You can configure your network devices to treat the different DSCP markings as you want them to be treated. This is meaningless unless there is congestion.
You should follow the guidelines, and you should configure a consistent set of QoS policies on all your network devices. Failure to have all your network devices configured to treat packet markings the same can create chaos when there is congestion.
The DSCP markings are ignored on the public Internet because everyone has a different interpretation of QoS and what should be prioritized over what when there is congestion.
Edit for your edit:
What you are really asking about is the AF (Assured Forwarding) interpretation of DSCP. With the AF interpretation, you have two digits of three bits each. The first digit is the priority (1 to 4) with a higher number being a higher priority. The second digit (1 to 3) is the drop probability with a higher number being a higher drop probability. There are also two special AF values: BE (Best Effort = 00) and EF (Expedited Forwarding = 46) that do not fit in the drop probability of the regular AF values.
There are also other standard ways to interpret the six bits, including CS and Precedence. Cisco has a chart that may help you understand.
The idea is to have some guidelines, recommendations, and best practices about how to interpret the bits, but you can configure the devices to interpret them and assign your own meanings and behaviors to any value. It is not a good idea to buck the guidelines, but there is really nothing to stop you from doing that.
