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Well, as we've said, eventually wired packets are going to land on the wireless

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, and wireless

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packets are going to land on the wire.

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And so we really have this back and forth relationship where we're using user

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priorities on one

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side and DSCP on the other side.

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Classic service is not necessarily a play here because a lot of access points

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connect

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via access ports and not trunks, and furthermore, we don't actually have

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Classic service.

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On the wireless side anyway.
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On the wireless side anyway.

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And so we really do need to worry about this translation between user priority

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and DSCP.

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So let's go ahead and start by looking at DSCP to user priority translation.

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As we said in the last video, we're going to need this right from the very

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beginning

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when a station wants to send a frame, because the application is going to pick

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the DSCP

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tag, and then we as the wireless station need to decide which user priority to

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use.

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So how do we go about doing this?
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So how do we go about doing this?

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Well, the user priority, keep in mind, this has three bits.

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And as we were called, DSCP has sort of this three plus three mechanism where

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the original

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three bits used for IP precedence are now made a little bit deeper by adding

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these three

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additional bits.

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And so what we end up doing is we take these three bits here, and we translate

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them directly

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into the user priority.

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And from a math perspective, this essentially says that we're going to take D
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And from a math perspective, this essentially says that we're going to take D

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SCP, divide

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it by eight, and then scrap the remainder.

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Because the remainder is going to be effectively these three bits here.

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So we're only going to take the how many eights there are in a DSCP tag.

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So for example, let's say that we have a DSCP value of 45.

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So I'm going to take 45, divide it by eight, and we know that that's going to

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be five

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and some change.
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and some change.

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And so five points, I guess six to five, something like that, five eighths.

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And so we're going to scrap that part and just call it a user priority of five.

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So I can essentially round this down to the nearest power of eight, and then

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divide by

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eight, and there's my answer.

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So when I do this over the course of a lot of different values, we'll see a

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pattern emerge

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where zero through seven on the DSCP side translates over to a user priority of

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zero,

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and then eight through 15 translates over to a one, 16 through 23 goes to two,
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and then eight through 15 translates over to a one, 16 through 23 goes to two,

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and so

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on and so forth.

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So that is how we take a DSCP value and establish the user priority.

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Now we'll notice here, and this is going to come up later, that what about the

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range that

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we get to that, let's just say it's 40 through 47.

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So 40 through 47 ends up translating to a user priority of five.
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So 40 through 47 ends up translating to a user priority of five.

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Well what do we use for voice over IP?

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Voice over IP is going to use 46, which is known as that expedited for it.

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So voice over IP ends up with five, which is great when we're talking about

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class of

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service, we like to use a tag of five for voice over IP with class of service.

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So remember, voice is supposed to use a six or a seven.

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And so this is where the translation can start to lose itself a little bit, if

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we do nothing

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other than take the direct translation.
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other than take the direct translation.

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We are going to need to step in and intervene in other words, in order to get

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our voice over

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IP using the voice access category, because five is going to use that vi or

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video access

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category.

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We don't want our voice packets getting into the wrong access category, that's

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for sure.

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So next we need to consider the reverse.

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What happens if I have a user priority, I need to translate it back to DSCP.

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So as we said, when a wireless receiver here, in this case, this access point
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So as we said, when a wireless receiver here, in this case, this access point

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receives a

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packet, is going to be able to do one of two, well, a few things, it can take

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the DSCP and

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just trust that.

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It can override it, as we said, we can just say, I don't trust anything you're

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sending

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me.

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And so I'm going to override it and do it myself.

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Or I can perform this user priority and translation over to DSCP.

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And so if we're going to do this last option, how exactly do we go about doing
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And so if we're going to do this last option, how exactly do we go about doing

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it?

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Well, it's pretty simple.

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We're going to take user priority, multiply it by eight, and that's going to

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give us our

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DSCP value.

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And so if I have a user priority of three, that's going to be a DSCP value of

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24.

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If I have a user priority of five, it's DSCP of 40, which once again, if I'm

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looking at

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things like, hey, maybe I get a voice packet that has a user priority of six,

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well, that's

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going to translate to a DSCP of 48, which is not quite right for my network.
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going to translate to a DSCP of 48, which is not quite right for my network.

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In fact, six and seven would both cause a lot of problems because going as high

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as 48 and

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56 on the wired network, that's going to start to interfere with some important

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network control

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traffic.

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But understand how these values get translated back and forth.

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And we're going to talk about some of the nuances here coming up.

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Now one other thing to consider on the wired side, once this translation

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happens, again,

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we're talking about like a wireless station, transmitting to an access point
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we're talking about like a wireless station, transmitting to an access point

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wirelessly,

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and then this access point is communicating with the switches and ultimately

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the rest

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of the local area network.

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So we have gotten to this point, but there is an important major caveat with

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how we do

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our wireless networking, which is that we have these capwap tunnels that go

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through

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the network to the wireless LAN controller.
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the network to the wireless LAN controller.

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And so when we're running capwap or any tunneling mechanism, we now have two

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sets of headers.

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So when we look at our capwap mechanism, we have the inner packet here.

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And the inner packet is going to include its DSCP value.

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But then we're going to have the outer headers and these outer headers.

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It's going to have its own, at this point, let's just say it's an IPv4 header,
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It's going to have its own, at this point, let's just say it's an IPv4 header,

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it's

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going to have its own IPv4 header.

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And we're about to send this packet in and it could get thrown through a bunch

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of switches

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and a bunch of routers and get go across WAN links.

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We don't know where this wireless LAN controller is going to be.

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It could be anywhere in the network.

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So how do I make sure that I have the right level of classification in the

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middle of all

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of this?

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Because the inner DSCP tag, let's say we did translate that, let's just say we
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Because the inner DSCP tag, let's say we did translate that, let's just say we

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successfully

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translated it to DSCP 46, which is again expedited forwarding.

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So it's a voice over IP packet and it's inside a capwap tunnel inside capwap

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encapsulation.

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But what I'm going to do is fairly simple, I'm simply going to copy this DSCP

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tag over

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to the outer header.

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So that is a pretty critical aspect of all of this.

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And even though the answer is fairly simple in the end, it is something we need
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And even though the answer is fairly simple in the end, it is something we need

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to make

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sure we understand and that we address.

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Because whatever the access point here receives from the station, once it does

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its DSCP translation,

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that is only going to go in the inner header.

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So we need to make sure from a networking and just a network architecture

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perspective that

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this tag, whatever is decided, ends up getting mapped into the outer header in

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the capwap

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tunnel.

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And fortunately, there's nothing we need to do for that that's automatically

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going to

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happen.

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This is an automatic part of the process.
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This is an automatic part of the process.

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But understanding that that happens and that I need to build my policy around

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these DSCP

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tags that ultimately come from these translations, that is going to be an

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important part of building

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a network architecture.

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