Well, as we've said, eventually wired packets are going to land on the wireless
, and wireless
packets are going to land on the wire.
And so we really have this back and forth relationship where we're using user
priorities on one
side and DSCP on the other side.
Classic service is not necessarily a play here because a lot of access points
connect
via access ports and not trunks, and furthermore, we don't actually have
Classic service.
On the wireless side anyway. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
On the wireless side anyway.
And so we really do need to worry about this translation between user priority
and DSCP.
So let's go ahead and start by looking at DSCP to user priority translation.
As we said in the last video, we're going to need this right from the very
beginning
when a station wants to send a frame, because the application is going to pick
the DSCP
tag, and then we as the wireless station need to decide which user priority to
use.
So how do we go about doing this? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So how do we go about doing this?
Well, the user priority, keep in mind, this has three bits.
And as we were called, DSCP has sort of this three plus three mechanism where
the original
three bits used for IP precedence are now made a little bit deeper by adding
these three
additional bits.
And so what we end up doing is we take these three bits here, and we translate
them directly
into the user priority.
And from a math perspective, this essentially says that we're going to take D WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And from a math perspective, this essentially says that we're going to take D
SCP, divide
it by eight, and then scrap the remainder.
Because the remainder is going to be effectively these three bits here.
So we're only going to take the how many eights there are in a DSCP tag.
So for example, let's say that we have a DSCP value of 45.
So I'm going to take 45, divide it by eight, and we know that that's going to
be five
and some change. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and some change.
And so five points, I guess six to five, something like that, five eighths.
And so we're going to scrap that part and just call it a user priority of five.
So I can essentially round this down to the nearest power of eight, and then
divide by
eight, and there's my answer.
So when I do this over the course of a lot of different values, we'll see a
pattern emerge
where zero through seven on the DSCP side translates over to a user priority of
zero,
and then eight through 15 translates over to a one, 16 through 23 goes to two, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and then eight through 15 translates over to a one, 16 through 23 goes to two,
and so
on and so forth.
So that is how we take a DSCP value and establish the user priority.
Now we'll notice here, and this is going to come up later, that what about the
range that
we get to that, let's just say it's 40 through 47.
So 40 through 47 ends up translating to a user priority of five. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So 40 through 47 ends up translating to a user priority of five.
Well what do we use for voice over IP?
Voice over IP is going to use 46, which is known as that expedited for it.
So voice over IP ends up with five, which is great when we're talking about
class of
service, we like to use a tag of five for voice over IP with class of service.
So remember, voice is supposed to use a six or a seven.
And so this is where the translation can start to lose itself a little bit, if
we do nothing
other than take the direct translation. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
other than take the direct translation.
We are going to need to step in and intervene in other words, in order to get
our voice over
IP using the voice access category, because five is going to use that vi or
video access
category.
We don't want our voice packets getting into the wrong access category, that's
for sure.
So next we need to consider the reverse.
What happens if I have a user priority, I need to translate it back to DSCP.
So as we said, when a wireless receiver here, in this case, this access point WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So as we said, when a wireless receiver here, in this case, this access point
receives a
packet, is going to be able to do one of two, well, a few things, it can take
the DSCP and
just trust that.
It can override it, as we said, we can just say, I don't trust anything you're
sending
me.
And so I'm going to override it and do it myself.
Or I can perform this user priority and translation over to DSCP.
And so if we're going to do this last option, how exactly do we go about doing WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so if we're going to do this last option, how exactly do we go about doing
it?
Well, it's pretty simple.
We're going to take user priority, multiply it by eight, and that's going to
give us our
DSCP value.
And so if I have a user priority of three, that's going to be a DSCP value of
24.
If I have a user priority of five, it's DSCP of 40, which once again, if I'm
looking at
things like, hey, maybe I get a voice packet that has a user priority of six,
well, that's
going to translate to a DSCP of 48, which is not quite right for my network. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
going to translate to a DSCP of 48, which is not quite right for my network.
In fact, six and seven would both cause a lot of problems because going as high
as 48 and
56 on the wired network, that's going to start to interfere with some important
network control
traffic.
But understand how these values get translated back and forth.
And we're going to talk about some of the nuances here coming up.
Now one other thing to consider on the wired side, once this translation
happens, again,
we're talking about like a wireless station, transmitting to an access point WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
we're talking about like a wireless station, transmitting to an access point
wirelessly,
and then this access point is communicating with the switches and ultimately
the rest
of the local area network.
So we have gotten to this point, but there is an important major caveat with
how we do
our wireless networking, which is that we have these capwap tunnels that go
through
the network to the wireless LAN controller. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the network to the wireless LAN controller.
And so when we're running capwap or any tunneling mechanism, we now have two
sets of headers.
So when we look at our capwap mechanism, we have the inner packet here.
And the inner packet is going to include its DSCP value.
But then we're going to have the outer headers and these outer headers.
It's going to have its own, at this point, let's just say it's an IPv4 header, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It's going to have its own, at this point, let's just say it's an IPv4 header,
it's
going to have its own IPv4 header.
And we're about to send this packet in and it could get thrown through a bunch
of switches
and a bunch of routers and get go across WAN links.
We don't know where this wireless LAN controller is going to be.
It could be anywhere in the network.
So how do I make sure that I have the right level of classification in the
middle of all
of this?
Because the inner DSCP tag, let's say we did translate that, let's just say we WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Because the inner DSCP tag, let's say we did translate that, let's just say we
successfully
translated it to DSCP 46, which is again expedited forwarding.
So it's a voice over IP packet and it's inside a capwap tunnel inside capwap
encapsulation.
But what I'm going to do is fairly simple, I'm simply going to copy this DSCP
tag over
to the outer header.
So that is a pretty critical aspect of all of this.
And even though the answer is fairly simple in the end, it is something we need WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And even though the answer is fairly simple in the end, it is something we need
to make
sure we understand and that we address.
Because whatever the access point here receives from the station, once it does
its DSCP translation,
that is only going to go in the inner header.
So we need to make sure from a networking and just a network architecture
perspective that
this tag, whatever is decided, ends up getting mapped into the outer header in
the capwap
tunnel.
And fortunately, there's nothing we need to do for that that's automatically
going to
happen.
This is an automatic part of the process. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This is an automatic part of the process.
But understanding that that happens and that I need to build my policy around
these DSCP
tags that ultimately come from these translations, that is going to be an
important part of building
a network architecture.
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