IP Precedence and DSCP Values

Hello Berthol

This lesson describes the components of the IP header, and specifically the ToS field, that is used to implement QoS. The values found within the ToS field are interpreted in specific ways using the logic described with the PHB, AF, EF, and so on.

Now at the end of the lesson, Rene explains what I think you are asking. He says that all of these values found in the ToS field will actually do nothing unless a router is configured to act upon those values. How it acts upon them, and how it interprets them are based on the network vendor of the device. Typically, they will conform to the industry standard. For example, for voice traffic, IP Precedence value 5 or DSCP EF will normally be used for voice traffic while IP precedence value 3 or DSCP CS3 or AF31 is used for call signaling. Different vendors may apply different behavior, but even so, you can configure the QoS to respond differently if you like.

I hope this has been helpful!

Laz

ok thank you for the intervention even as I am not really satisfied; my real issue was understanding how useful this is for the CCNP .

Hello Berthol

No need to apologize, that’s why we’re here… to answer questions!! :slight_smile:

When such questions arise, always go back to the official list of exam topics published by Cisco. For the CCNP 350-401 ENCOR Exam the only topic that includes anything about QoS or precedence or DSCP or any related topic is 1.6:

  • 1.6 Describe concepts of wired and wireless QoS
  • 1.6.a QoS components
  • 1.6.b QoS policy

For the CCNP 300-410 ENARSI Exam, there is no specific topic that covers QoS concepts.

However, Cisco’s exam blueprints are always accompanied by a disclaimer that indicates that related topics may be included in the exam that may not be explicitly listed in the topics list.

The truth is that it is not very likely that the content in this specific lesson will appear on the exam. However, because it is fundamental to understanding QoS, it’s a good idea to have at least a working knowledge of it.

I hope this has been helpful!

Laz

Thanks for the clarification.

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Hi Mr Rene AF31 = 0110101 then AF came from where as 001=3 and 001=1
i want to regarding AF

Hello Sulthan

AF31 is equal to 011 010. However, the value of 31 in the name AF31 does not come from the value of the bits, but from the class and drop probability. To understand this further, here’s the table once again, as in the lesson, but this time I’ve added row numbers:

image

  • AF11 is in class 1 and row 1 (low), therefore, it is AF11
  • AF12 is in class 1 and row 2 (medium), therefore, it is AF12
  • AF31 is in class 3 and row 1 (low), therefore, it is AF31
  • AF23 is in class 2 and row 3 (high), therefore, it is AF23

So the numbers in the AF notation denote class and drop probability.

I hope this has been helpful!

Laz