Alright, let's jump into quality of service now.
So, first question, which quality of service tagging methodology is used by SD-WAN?
Well, we talked about how class of service is Layer 2 and IP precedence is legacy Layer 3.
We don't really use it anymore. Those bits were wrapped up into the DSCP bits.
And so, it's really all about DSCP.
So, DSCP is something that a lot of us in the networking world are used to seeing,
but it gets a little tricky sometimes, especially on Layer 2 devices when we're also running COS values.
We might even see some IP precedence out there.
We're definitely not using VLAN tagging for quality of service.
But the idea here is that packets will be tagged in some way, usually with DSCP.
And so, maybe something like DSCP for voice over IP would be DSCPEF.
And if that shows up, we don't need to try to figure out what this packet is.
I just have to look at the tag and know that I'm going to break it out into the top priority queue based on the tag.
So, it's very important for us to understand some of these basic QoS mechanisms.
And certainly, DSCP tagging is going to be one of those.
So, recall that we are going to, especially in the SD-WAN world, have to worry about this
because our service provider circuits typically are restricted in some way.
So, maybe I get a 200 megabit per second service to my service provider.
Well, that's great. They're going to have a router on site, probably in some way.
And so, we're going to have to have an actual physical cable between these two devices.
And how do I connect a 200 megabit per second cable? I can't.
Ethernet only allows me to connect a 1 gigabit per second connector.
And so, if I'm getting tons of traffic in, my edge, by default, is just going to send 300 megabits per second.
And it's not going to worry about the fact that there's 200 megabit per second policer on this side.
And that's the idea here is that it's a policer dropping traffic.
It's just going to send all the traffic because I have plenty of link bandwidth.
So, the idea here would be not to send, well, here we'll leave that in place,
not to send 300 megabits per second, but to shape that traffic down to my SLA.
And the idea here is that even if I have to drop traffic to make this happen, at least I can control the traffic that gets dropped.
So, all my DSCP EF traffic is going to be allowed to be shaped down and sent out,
whereas I'm going to have to end up dropping maybe some AF13 traffic or something that's a little lower rated.
And that at least I decide what gets dropped because if the policer has to drop it,
it's going to do so without any kind of factoring in the DSCP tag.
So, all that to say, the answer here is going to be D, to prevent policers from dropping traffic.
All right, now we get into DSCP values here.
So, what are the decimal values for each of the following DSCP per hop behaviors?
So, remember, just like with URLs being human-readable names that are better than us trying to remember IP addresses,
DSCP PHBs are human-readable names that do end up getting tied back to, in our case, a decimal number.
And so, we need to be able to do the math for this.
So, do you remember the equation for how to go about doing AF values here as well as CS values?
EF is just a memorization factor.
So, let's start down here.
EF is a memorization.
We just have to know it's decimal 46.
There's no way to calculate that based on running any kind of math operation here.
It's just two letters.
CS4 starts to give us a number.
If we do see a single number there, we know that this is a PHB that's tied back to IP precedence.
We're only going to use the 8th bit and above.
And so, we basically just multiply this by 8 as a result.
So, I multiply it by 8, and the decimal value is going to be 32.
Assured forwarding, or AF, this is where things start to get a little more interesting.
Because, remember, we have the format AFXY, where X is a value that represents priority, and Y is a value that represents droppability.
Now, we're not asking about that in this question.
We just want to know what the decimal values are.
And so, here's that equation.
And that's all we have to do.
So, AF13 is 8 times 1 plus 2 times 3.
So, that would be 8 plus 6 is 14.
AF22 is 8 times 2 plus 2 times 2.
So, that's 16 plus 4.
That would be 20.
And then, 8 times 4 plus 1 times 2, or 2 times 1, I guess.
Either way.
It would be 32 plus 2, and that would be 34.
So, that is how we are able to take per-hop behaviors, look at them, figure out what decimal value they represent.
We should always be able to calculate it based on the numbers that are there, knowing that we do have to memorize expedited forwarding as being decimal 46.
Alright, question number 4, and there are 7 questions in this quiz, I believe.
And, yep, just checked.
There are 7 questions.
So, we're roughly halfway there.
Question 4.
So, now we're talking about that X and Y value as having meaning.
Again, X is the priority, and the higher the better.
We want priority to be higher in order to treat it better.
So, AF42 and AF41, this 4 value is much better than the 1, and so these are going to be out.
But, now things get interesting, because we might say, oh, higher numbers are better.
And so, 13 would be treated better than 11.
But, that's where things get a little backwards, because Y represents droppability, and our droppability for a packet being higher is not good.
We're talking about increasing the droppability.
So, we actually want droppability to be lower if we're talking about likelihood of dropping traffic, or just in general being treated better.
So, AF11 here is actually going to be treated better than AF13, because the 3 represents the highest level of droppability within the priority value of 1 class.
So, it's not A, it's actually B.
B is the most likely to start dropping traffic.
There is a little bit of a percent chance based on quality of service configurations.
So, we can't say it's absolutely going to start dropping first, but it has the highest likelihood to start dropping traffic when congestion occurs.
Question number 5.
What SD-WAN configuration is responsible for defining bandwidth reservations in each queue?
So, we're about to unpack a little bit more of quality of service configuration, but it is important for us to understand some of these concepts.
So, when we're looking at bandwidth reservations, that's the role of the policy map in iOS XE, but in the Viptela world, that is a QoS map.
So, we don't have policy map here.
If we saw policy map in the list, they'd both technically be correct.
But, we do know that in SD-WAN manager, I'm going to go in there, configure quality of service maps, I'm going to define the queues, and give each one a percentage.
And, that translates on the back end on iOS XE to policy maps.
So, always remember kind of how this comes together, and QoS maps are a very important part of our quality of service configuration on Cisco SD-WAN.
Alright, question 6.
When performing traffic assignment via localized policy, what mechanism is used?
So, we recall that we are going to have to divide up our QoS configuration a little bit.
We have to do our queue configuration like we just talked about, and that is localized.
But, then I have to also perform traffic assignment.
Which traffic goes into which of these queues?
And so, the traffic assignment can be done localized or can be done centralized.
So, if it's done centralized, then I would do it via a policy.
But, what if I want to do it localized?
Well, at this point, I would deploy an access control list.
So, D here is the answer.
We need something on the interface to receive traffic and say, okay, I'm going to match traffic, and then I'm going to have an action where I set.
And, when I set it, I can place it into a forwarding class, which maps to a queue.
So, again, keep in mind, we have two different parts of quality of service configuration that will help us in the configuration lab coming up.
And, how we do each of these is a little bit different with traffic assignment giving us two options via centralized policy or localized.
All right, last question.
Where does adaptive quality of service configuration belong in the network?
So, adaptive quality of service, very interesting concept here.
What are we talking about?
Specifically, we are referring to shaping.
We already addressed shaping at the start of this quiz, and shaping is a very important part of wide area network configuration and management.
We do not want to have the police or drop any traffic whatsoever.
And so, what I'm trying to do is shape this down.
And, this works really, really well when it's a single target, and I have that 200 megabit per second SLA, and it's a stable environment.
But, what about on unstable WAN links?
That would be where we want adaptive QoS.
Because, this works really well in the MPLS world, and maybe back in the day with other types of circuits.
But, when we have an SLA, that's good.
When we have an internet circuit, this internet circuit, I might have paid for 200 megabits per second.
But, depending on the day, this might drop down to 150.
It could drive up to 250 megabits per second.
The internet is a little bit more notorious for being unstable in this regard.
So, what do I do?
I deploy adaptive QoS, and I target 200 megabits per second, but I allow it to go down to 150 and up to 250 megabits per second.
So, very interesting concept.
Remember that we've got the period option as well, where it's evaluating the bandwidth, effectively saying how much bandwidth do I have.
And, this period is in minutes in the configuration.
So, that is our quiz on quality of service.
So, let's go ahead and jump into a lab together and do some configuration.