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 P.C. would be classified as not being important.
A switch can be configured with access lists or it could use other technologies such as Anbar or network
based application recognition to recognize certain applications such as HDP or voice traffic or video
traffic and then pro tries that traffic accordingly.
So essentially a phone sees its own traffic as important but not the traffic of a P.C. a 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 its traffic is important.
It's voice traffic.
It needs to mark the traffic and send it to the first switch.
So all packets are sent from the phone to the switch need to be marked as important.
Now that can be done at a layer to using editor at 1 Q So on Ethernet we would require attitude at one
Q links and in the editor to one Q header we can change what's called the Cost field or plus of service
field to indicate the importance of the traffic that is known as 82 two to one P or edited to 1 priority
but we could also use MPEG less experimental bits to indicate that the traffic is important at least
three in the IP header we've got IP precedents and differentiated services code points or DCP IP explicit
congestion notification or ECM can also be used but that's not covered in the CCN a course the layer
two and layer three parameters are the ones that you should concentrate on for the CCMA exam so layer
two we have added to the one Q or editor the one PE class of service but there are three bits in the
header which equate it to eight classes of service in the range 0 to 7.
So let's look at that practically.
I'm gonna capture traffic on this link between the phone and the switch
so why shock is capturing traffic
now this phone icon is actually in austere not a node which is a packet generator
in the packet a generator I can specify the protein bits of an attitude at one Q header.
Notice the values of from 0 to 7 because this is a phone I'm going to set the priority to 5 click okay
and then I'm going to start generating packets of packets of 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 to we have a source and destination MAC Address we are generating an attitude at one Q
frame and in the edit to the one Q frame.
Notice we've got a priority field so the priority field or cost field consists of three binary bits
in the range 0 0 0 up to 1 1 1.
In other words in the range 0 2 7 there are eight binary values that we can configure and in this case
offset the value to five to indicate voice traffic.
So back in Austin Noto 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 but is three binary ones which equates to network control in this case a 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 the SRM D guide has some recommendations
for what you should mock traffic as so act layer to causal NPL is experimental but a course of five
is what she used for voice video Cisco recommend using a cost value for mission critical data should
have a cost value of three calls 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 calls signaling something like SAP or a should 3 the car's value
should be set to 3.
So we should set the course value here to 3 and generate some frames.
And it is over here that cost value is set to three binary 0 1 1 decimal is 3.
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 in this case the switch can trust the markings that it receives from the phone but not
necessarily the markings that it receives from this P.C..
So again they are different ways to mock.
We can market layer two in Ethernet frames using an editor the one Q header.
As I've demonstrated there are eight classes in the range 0 to 7.
We can also mark type of service which is at least three so in the type of service header.
There are eight binary bits and in the old days we used to have what is called IP precedents which marked
the most significant three bits.
Today we use differentiated services code points or serve 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 DCP Valley so if we send a frame into the network notice
at least two we have the editor of one Q header set to a cost value of 3.
But at least 3 we have a differentiated services code point set in this example to expedited forwarding
or 46.
So we've marked one two three four five six but in the IP header explicit congestion notification the
last 2 bits here are not used and you don't need to know ECM for the CCMA exam but notice the type of
service field in the IP for header is 8 but since size IP precedence uses the most significant three
bits differentiated services code points uses the most significant six bits.
ESPN uses the least significant two bits and that's what we can see here.
Here's an example of DCP and ESPN.
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 tops field or type of service field which
consists of DCP and ECM.
Today.