Now, unlike FedEx or DHL, we're not sending physical packages between network devices.
We're sending packets.
So how does a phone indicate to a switch that its traffic is of great importance?
And how does one switch indicate to another switch that some packets are more important than others?
So this is called classification and marking a phone would classify its own traffic as being important,
but the traffic from a PC would be classified as not being important.
A switch can be configured with access lists, or it could use other technologies such as end bar or
network based application recognition to recognize certain applications such as HTTP or voice traffic
or video traffic, and then tries the traffic accordingly.
So essentially a phone sees its own traffic as important, but not the traffic of a PC.
The switch or a router has many options that you can use to put traffic into classes.
Again, you could use an access list or network based application recognition, but once you've put
your traffic into a class, so as an example, the phone says it's traffic is important, it's voice
traffic.
It needs to mark the traffic and send it to the first switch.
So all packets sent from the phone to the switch need to be marked as important.
Now that can be done at a layer two using a 2 to 1 queue.
So on Ethernet, we would require a to two one queue links.
And in the attitude of one queue header, we can change what's called the cost field or class of service
field to indicate the importance of the traffic that is known as ADA 2 to 1, P or ADA two or one priority
bits.
We could also use MPLS experimental bits to indicate that the traffic is important at a layer three.
In the IP header, we've got IP precedence and differentiated services code points or DHCP IP explicit
congestion notification or ECN can also be used, but that's not covered in the Q&A course.
The Layer two and layer three parameters are the ones that you should concentrate on for the CCNA exam.
So at layer two, we have added 2 to 1 Q or eight or 2 to 1 PPI class of service bits.
There are three bits in the header which equated to eight classes of service in the range of 0 to 7.
So let's look at that practically.
I'm going to capture traffic on this link between the phone and the switch.
So why shock is capturing traffic?
Now this phone icon is actually an osteo, not a node, which is a package generator.
In the packet generator, I can specify the priority bits of an 801 Q header.
Notice the values are from 0 to 7 because this is a phone, I'm going to set the priority to five.
Click.
Okay, and then I'm going to start generating packets.
So packets are being sent into the network.
These are UDP packets that I'm sending.
So.
Here's one of the packets that have been generated.
And notice that ethernet too.
We have a source and destination mac address.
We are generating an 8 to 1 Q frame and in the editor to one Q frame notice we've got a priority field.
So the priority field or COS field consists of three binary bits in the range 000 up to 111.
In other words, in the range of 0 to 7, there are eight binary values that we can configure.
And in this case I've set the value to five to indicate voice traffic.
So back in ostinato.
I'll change the cost value to seven.
And what I'll do is generate more frames.
So scrolling down to the latest frames, notice the party bit is three binary ones, which equates to
network control.
In this case, I'll filter for UDP.
In the previous frame, it showed up as video.
In other words, we require low latency and jitter.
Now, that's used by Cisco phones to indicate important voice traffic.
This R&D guide has some recommendations for what you should mark traffic as so at a layer two costs
or MPLS experimental bits.
A cost of five is what you used for voice video.
Cisco recommend using a cost value of four.
Mission critical data should have a cost value of three.
Call signaling to set up telephone calls should have a cost value of three.
And then it goes all the way down to best effort, which has a cost value of zero.
So as an example, if we were sending call signaling something like CIP or H to three, the cost value
should be set to three.
So we should set the cost value here to three.
And you can generate some frames.
I notice over here the cost value is set to three.
Binary is 011 decimal is three.
Now applications should follow the conventions, but sometimes they don't.
And this is where the trust boundary is important.
Would you trust your users to set their quality of service correctly?
Typically not.
So in this case, the switch can trust the markings that it receives from the phone, but not necessarily
the markings that are received from this PC.
So again, they are different ways to mock.
We can market layer two in Ethernet frames using an edited or one Q header.
As I've demonstrated, there were eight classes in the range 0 to 7.
We can also mock type of service, which is at layer three.
So in the type of service header.
There are eight binary bits and in the old days we used to have what was called IP precedence, which
marked the most significant three bits.
Today we use a differentiated service code points or server bits, which is a marking of the most significant
six bits.
I'll talk about this in more detail in a moment.
But just to demonstrate it, what we could do is under the IP header.
We can set a type of service or DHCP value.
So if we send a frame into the network.
Notice at layer two, we have the editor to one Q header set to a cost value of three, but at least
three.
We have a differentiated service code point set in this example to expedited forwarding or 46.
So we've marked one, two, three, four, five, six bits in the IP header, explicit condition notification.
The last two bits here are not used and you don't need to know ECN for the CCNA exam.
But notice the type of service field in the IP four header is eight bits in size.
IP precedence uses the most significant three bits differentiated services code points uses the most
significant six bits.
ECN uses the least significant two bits.
And that's what we can see here.
Here's an example of DHCP and ECN.
Notice this is just a standard IP packet.
We have source address, destination address.
We have the protocol being UDP.
But one of the fields is the toss field or type of service field which consists of DHCP and ECN today.