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Before mapping out our per-hat behaviors, we need to look at our DSCP bits.

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So our bits are going to be split into.

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We've got those first three bits that were originally set aside for IP

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precedence, and

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then we have the other three bits that were added in for DSCP, and remember, D

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SCP gets

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all of these.

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And yet IP precedence, from a backward compatibility perspective, is only going

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to look at the first

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set of three.

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They have to be able to coexist, even though IP precedence is pretty well dead
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They have to be able to coexist, even though IP precedence is pretty well dead

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at this

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point.

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If, for example, we have 101, 110, which, by the way, that would actually map

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out to

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DSCP of 46, which is expedited forwarding, as we've said.

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So we run the math there.

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Either way, IP precedence, a device that receives it and can only speak IP

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precedence, would

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view this as a five.

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So kind of interesting how this turns out, how we actually have two different
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So kind of interesting how this turns out, how we actually have two different

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numbers

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here.

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But interestingly, we might notice that because this is, we'll do our binary

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math here, the

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ones, the twos, the fours, this is the eights column here, or the eights number

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.

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IP precedence is essentially just looking at this value as a number 0 through 7

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, and from

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a DSCP perspective, it's going to multiply out by 8.

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For example, IP precedence says that we have five here, well, five times eight
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For example, IP precedence says that we have five here, well, five times eight

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is 40, and

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then we can see the extra six here, 110 is six, and that's how we get to 46.

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That math is going to be useful to us here in a moment when we talk about the

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class selector.

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So we're now diving into our PHBs.

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And the first PHB group that we're going to talk about is class selector.

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We're going to talk about three different groups, class selector, assured

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forwarding,

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and expedited forwarding.
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and expedited forwarding.

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We mentioned the second two in the previous video, but we're going to start

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with class

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selector.

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Class selector defines eight different DSCP values, and it's meant to provide

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backward

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compatibility with IP precedence.

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The idea here is that I can still use DSCP PHBs, specifically these PHBs, and

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yet it's

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something that IP precedence is going to understand.
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something that IP precedence is going to understand.

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As a result, we're essentially saying we're only going to use the first three

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bits.

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This assures the network that with some device doesn't support DSCP, then we

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can still support

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it even though we are only looking at the IP precedence bits.

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So the range for this is CS0, where CS stands for class selector, through CS7.

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And this is going to match the value that shows up here in those first three
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And this is going to match the value that shows up here in those first three

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bits.

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So for example, CS4 is going to be a binary of 100 because that's binary for

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four, and

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then 0, 0, 0, we're always going to have zeros here, regardless of what happens

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in the first

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three bits.

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So 0, 0, 0, there we go.

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So if we want to do a calculation to convert it to decimal, which is going to

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be a big part

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of all of these conversations, is what's the actual decimal number in here,
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of all of these conversations, is what's the actual decimal number in here,

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well, we're

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simply going to take the CS value and we're going to multiply it by eight for

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the reason

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that we just mentioned up here with the fact that from a six bits perspective,

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we're essentially

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only changing the eights and up.

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So for example, if I've got a CS of four, like we just said, we can say, all

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right, well,

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four times eight is equal to 32.

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And if we look at what we just said here, this is the ones to the zero power,
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And if we look at what we just said here, this is the ones to the zero power,

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but ones,

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two's, four's, eight's, sixteens, and yes, that is the 32's column.

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And so it happens to be 32.

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So that means that by extension, CS six is simply six times eight, which must

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be a decimal

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value of 48.

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So we can more or less use class selector to align ourselves with COS and IP

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precedence.

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That is what class selector is for.
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That is what class selector is for.

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If we just want to use eight different values in our network and always align

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with COS and

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IP precedence, then class selector is exactly what we're going to use for our

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per hot behaviors.

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Now next up is assured forwarding.

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This is a little bit more involved of a conversation because it's going to use,

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I'll say most of

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the bits.

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It's actually going to use five of the bits, but we're going to see this here

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in a moment.

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This gives us 12 different classes.

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So class selector gave us eight classes assured forwarding gives us 12 and it
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So class selector gave us eight classes assured forwarding gives us 12 and it

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follows a specific

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format of a F for assured forwarding, then X and Y.

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And so X can be between one and four.

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And this indicates the priority level.

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The higher the priority, the higher the number or vice versa, maybe the higher

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the number,

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the higher the priority of priority of four should take precedence over a value

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of one.

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For a Y, this is going to be between one and three.
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For a Y, this is going to be between one and three.

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And this indicates the drop probability.

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And again, the higher the number, the higher the drop priority, which I feel a

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little bit

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backwards, because that means one actually has the least drop priority.

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So in theory, it is treated better than a three.

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So keep that in mind that Y sort of is a little bit of reversed of what we

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might expect.

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So let's look at something like AF31.
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So let's look at something like AF31.

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So remember, the first digit can be from one to four.

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The second one can be from one to three.

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When we're looking at this, we see that three indicates a high priority.

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And the one indicates a low drop probability.

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That is how we interpret this.

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And because of this, because we've got these 12 different classes, like we said

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, we can

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simply look at a particular value like AF31, and we can compare it to an AF33,
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simply look at a particular value like AF31, and we can compare it to an AF33,

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and we can

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understand how this traffic should be treated relative to one another.

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Specifically, they should have the same priority values, but this AF33 should

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get dropped before

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AF31 traffic, because it has a higher drop probability.

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Likewise, we could look at AF41 and say that, all right, this actually should

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out prioritize

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both of these other assured forwarding classes.

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Again, we're giving names, so to speak, if you want to call this a name, AF31,
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Again, we're giving names, so to speak, if you want to call this a name, AF31,

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we're

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giving names to DSCP values so that we as humans can look at it and understand

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how we're

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supposed to treat it.

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So here's the question, though.

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These have to translate to DSCP values and ultimately binary bits.

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So what values are used?

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Well, when we break it down, let's just draw it out over here again.

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So we've got our first three bits.

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We've got our second three bits.

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What we're going to do is assured forwarding is always going to set zero here
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What we're going to do is assured forwarding is always going to set zero here

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at the end.

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We can use this value for any, you know, we can set a one or a zero there

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normally if

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we're just using raw DSCP values.

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But for assured forwarding, we just always set a zero there.

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X is therefore going to be these first three bits and Y is simply going to be

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the next

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two bits.

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So how do we calculate out the value?

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Well, let's say that we have AF31 like we just said.
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Well, let's say that we have AF31 like we just said.

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So AF31 would be a three here, 011, and then a one over here, 01.

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So then we have to remember that it still ends with a zero.

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Now what if we want to work backwards?

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What if we have something like 010, 010?

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I guess it's the same thing at the end.

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But either way, when we work our way backwards, we'd say, okay, this is binary

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for two.

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So that'd be AF2.
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So that'd be AF2.

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What's the second value?

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Well, if we look at this, we might say that the value is two because this is

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binary for

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two.

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However, again, we ignore the fact that that zero is there.

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We're only looking here at these first two digits.

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So 01 is simply binary for one, and therefore it's AF21.

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Now we can do the math on this to figure out what exactly these decimal values

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are.

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But fortunately, there's a very simple calculation that we can use, and it is 8
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But fortunately, there's a very simple calculation that we can use, and it is 8

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X plus 2Y.

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In other words, if I take these two numbers and I plug them into here and here,

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I can

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figure out what the decimal value is.

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So AF31 would be 8X3 plus 2X1.

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Again the 8 and the 2 are from our formula, the 3 and the 1 are taken from AF31

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.

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And so that would be 24 plus 2, and this is 26.
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And so that would be 24 plus 2, and this is 26.

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So a DSCP value of 26 is equivalent to AF31.

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Same thing with AF21.

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We could say it's 8X2 plus 2X1, and so that'd be 16 plus 2, and this is 18.

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So a DSCP value of 18 aligns with AF21.

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Now from a human perspective, we care more about this up here.

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As humans, we want to look at what we're trying to say.

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How does AF31 compare to AF21, and how do we execute that in our network?
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How does AF31 compare to AF21, and how do we execute that in our network?

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So that's what we care about is the name and the AFXY part.

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But from a computer perspective, at some point, it has to look at the binary

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numbers, and

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hey, we might be asked something on an exam, let's say, about what decimal

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value is equivalent

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to specific assured forwarding values, and so we need to make sure we can go

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back and forth

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between these.

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Now last but not least, we have what we already called out a couple of times,
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Now last but not least, we have what we already called out a couple of times,

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which is expedited

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forwarding.

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Expedited forwarding is a whole lot more simple than assured forwarding and

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even class selector,

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because expedited forwarding is exactly one class.

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And that one class is a DSCP value of 46, which is known as EF or expedited

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forwarding.

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This is intended for voiceover IP traffic.

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That is pretty much the only time wherever we're going to see it and use it,
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That is pretty much the only time wherever we're going to see it and use it,

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but it's

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very intentionally set aside so that we can recognize the highest priority

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traffic on

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the network, place it into a priority queue and reserve network bandwidth.

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So these three different classes of PHBs are going to need to be familiar with

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them,

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how they're used, what they're used for, and how to ultimately calculate out

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the decimal

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and binary values behind each one.