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