All right, let's jump into this challenge together.
If you haven't already done so, be sure to grab a piece of paper and a pencil
and do
some of this math to make sure we can translate some of these values back to
decimal.
It's going to be very important that we can do so.
But first of all, let's answer this question about COS tags.
What are two situations where a COS tag ends up getting lost?
Well, the COS tag is inside not just the Ethernet header, but also specifically
the 802.1q part
of the header, which is optional.
So anytime we lose this, either one of these elements, we lose the COS tag. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So anytime we lose this, either one of these elements, we lose the COS tag.
The two primary situations we'd be talking about here would be non-trunk links,
in which
case we lose the 802.1q header.
And so we lose the COS tag.
The other would just be non-ethronet links, such as T1s.
Anytime we send a packet across one of those types of connections, we're going
to lose
either the Ethernet header or the 802.1q header.
In either case, the COS tag is lost.
So with that out of the way, let's go ahead and do some math. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So with that out of the way, let's go ahead and do some math.
So we're going to start with CS6.
Class selector six, this is only using, remember, our diagram there with the
bits.
It's only using these first three bits.
And so a value of six here would be 110.
And we could do our binary math and say, okay, well, here's a bunch of zeros
here, and we
try to figure that all out.
Or we could simply realize that again, this is the eights.
And we're essentially saying that we have six of these eights.
So math is simply six of the eights, and that would be a value of 48. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So math is simply six of the eights, and that would be a value of 48.
So DSCP 48 here would be the answer for CS6.
Now things get a little bit fun.
We're going to go into the Assured Forwarding World, and we're going to start
with AF13.
Well in order to do this math calculation, we want to remember our formula 8x
plus 2y.
And so we're going to look at 1 and 3.
And so the math formula simply says 8x1 plus 2x3.
And so that would be 8 plus 6. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so that would be 8 plus 6.
And the answer here is 14.
Then we go to AF32.
Same deal.
We're going to take 8x3 this time plus 2x2.
So this is 24, this is 4, and the answer is 28.
And so we see how we can calculate pretty seamlessly to decimal simply using
this formula.
And as we explained earlier, it's really just playing off of the binary and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
this situation
over here where we have an eights and we have a twos because this is always a
zero with
a shared forwarding.
Now lastly here, or not lastly lastly, but we do have the next one which is
asking us
to work our way backwards, which is kind of interesting.
We're saying we want to start with decimal 36.
And we want to go back to an AF number.
So how would we go about doing this?
This is a pretty interesting exercise and there's lots of ways to do this.
We could translate 36 into binary and then look at it in this format here using WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We could translate 36 into binary and then look at it in this format here using
6 binary
digits and extract the x and y values.
Or we could actually use this formula in reverse.
We could pull out a number of eights knowing that there's going to be a
remainder probably.
And then the rest of that would be our twos.
So for example, 36, if I think about multiples of 8, 8, 16, 24, 32, I could
pull 32 out of
this and that's 4 of our eights.
And so that means that x is going to be 4. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so that means that x is going to be 4.
Now after I take 32 out of 36, there should be a remainder of 4 and so that's
how many
twos?
Well that would be two separate twos.
And so that means my y is a 2.
That means that the answer to this should be AF4 for x2 for y.
And then I can turn around and insert this back into the formula in order to
make sure
it works.
4 times 8 plus 2 times 2.
That would be 32.
That would be 4 and the answer is 36 which is what we started with. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
That would be 4 and the answer is 36 which is what we started with.
So AF42 is our answer.
Now the last question here is simply asking us do we know the decimal value of
EF expedited
forwarding?
This is always going to have a value of DSCP 46.
It's simply something to memorize.
It's essentially the highest even value before we get into CS6, CS7 area which
we typically
see reserved for network traffic.
And so that would be the number that it is if that helps us to remember it. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so that would be the number that it is if that helps us to remember it.
But ultimately it's simply a number that we're going to have to memorize at
some point.
So this brings us to the end of this skill.
That's a great look at how we perform classification and marking.
And in the end we do want to be able to come away from this being able to
understand the
structure of our COS and our DSCP tags and to be able to perform some math
operations
to make sure that we can calculate out the per hot behavior or decimal values
while also
understanding what those per hot behaviors really mean. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
understanding what those per hot behaviors really mean.
I hope this has been informative for you and I'd like to thank you for viewing.