-: Routing fundamentals. 2 2
All right, we have covered a lot of information 3 3
up to this point in the course, 4 4
but we still haven't left our local area network yet. 5 5
Once we're ready to leave our network, 6 6
this is where routers are really going to start 7 7
coming into play. 8 8
When we start talking about connecting 9 9
to subnets within our network 10 10
or connecting our internal network and an external network, 11 11
we have to use a router. 12 12
So in this section of the course, 13 13
we're going to be talking all about routing fundamentals. 14 14
The first thing you must understand 15 15
is the function of a router, which is to route traffic. 16 16
Now, I know that sounds kind of silly, 17 17
but essentially that's what it's going to do. 18 18
It's going to forward the traffic in one direction or another 19 19
between different subnets or between an internal 20 20
and external network or between two different 21 21
external networks. 22 22
Each subnet or external network is going to be 23 23
its own broadcast domain because routers 24 24
are used to separate broadcast domains. 25 25
Unlike switches, which are layer two devices, 26 26
and therefore can only maintain one broadcast domain 27 27
for everything that's connected to the switch, 28 28
the routers are going to be layer three devices. 29 29
And they're used to separate and break apart 30 30
those broadcast domains into smaller portions to provide 31 31
additional efficiency and security to your networks. 32 32
Now, before we go too far in this lesson, 33 33
it's important to note that there's a difference 34 34
between the real world and the exam. 35 35
In the real world, 36 36
you may often find yourself using a layer three switch, 37 37
also known as a multi-layer switch. 38 38
When you do this, it can perform routing functions for you. 39 39
Many small office home office networks 40 40
or small and medium sized businesses 41 41
will often opt to use a layer three switch 42 42
because it can handle both functions. 43 43
It can handle layer two forwarding 44 44
when it's acting like a switch 45 45
and layer three routing when it acts as a router. 46 46
For the exam though, remember, 47 47
if you're using a multi-layer switch, 48 48
it is functioning as a router. 49 49
And so we're going to treat it as a router, 50 50
or a layer three device. 51 51
Now in this entire section, 52 52
we're going to be referring to these devices as a router, 53 53
even if I'm using a multi-layer switch in the real world. 54 54
Now for the exam, if they say switch, 55 55
they're talking about a standard layer two switch. 56 56
If they say multi-layer switch or router, 57 57
they're referring to layer three functionality of a device 58 58
that we call a router during this course and the exam. 59 59
So how does the basic router perform this work? 60 60
Well, let's consider this diagram here. 61 61
I have two networks, one on the left side 62 62
and one on the right side of the screen. 63 63
As you can see, PC one is connected to our left network, 64 64
and PC two is connected to our right network. 65 65
Now, to connect these two networks together, 66 66
we have two routers and a serial connection 67 67
or wide area network connection between them. 68 68
For now, we're not going to focus too much on that connection 69 69
because we're going to cover that 70 70
in a different section of the course 71 71
when we talk about all the different WAN technologies. 72 72
Right now just realize that there's connection between them, 73 73
and it's a long-distance one. 74 74
This could be a fiber connection, a serial connection, 75 75
a satellite connection, a VPN or any other kind of link. 76 76
It really doesn't matter right now. 77 77
As long as you understand that there is connectivity between 78 78
these two routers and they can communicate to each other 79 79
over that WAN connection, that's all we need to know. 80 80
Now if I'm sitting at PC one and 81 81
I want to send a message to PC two, 82 82
I can't just send it through ARP like I did with a switch. 83 83
If you remember back in switching, 84 84
we talked about how PC one would send an ARP packet 85 85
to the switch. 86 86
And if it didn't know where to deliver it, 87 87
it would send out every single switch port 88 88
based on that MAC address. 89 89
Well, PC one can and will still do that. 90 90
But what's going to happen here is that our ARP request is 91 91
going to go up to the switch and the switch is going to 92 92
realize that it doesn't know the MAC address associated with 93 93
that request. 94 94
So it's going to forward it out as default gateway. 95 95
The default gateway is the router. 96 96
Now that router is then going to respond to the ARP request 97 97
on behalf of the destination device. 98 98
At this point, 99 99
we're now going to have to leave MAC addresses behind 100 100
because MAC addresses only work internally in our network. 101 101
Once we get to the router, 102 102
we're going to switch to using IP addresses 103 103
or logical addressing at layer three. 104 104
This is where we begin to make our layer two 105 105
to layer three transitions. 106 106
At this point, 107 107
PC one has determined the MAC address 108 108
for where the router is 109 109
because it needs to send the packet to get out of this local 110 110
area network and leave to go find this device. 111 111
The left router in this case wants to send 112 112
the information to PC two, 113 113
because that's what PC one told it to do, to do that, 114 114
it's going to pass the information over to router two, 115 115
and then router one is going to get the data frame, 116 116
which is layer two from PC one. 117 117
The writer will repackage it up as a packet, 118 118
which is layer three, and then add an IP header to it. 119 119
This allows router one to then push the data over its 120 120
default connection as an IP packet. 121 121
And the data will then be sent across the serial connection 122 122
to get to router two. 123 123
All right, 124 124
once that packet gets to router two, 125 125
router two is going to 126 126
strip off the IP header and bring it back down to layer two 127 127
data frames by adding a MAC address for PC two. 128 128
This way it's going to correlate PC two's MAC address with the 129 129
destination IP of the packet, 130 130
because router two knows who PC two is, it's on its network. 131 131
Now, since router two knows the MAC address for PC two, 132 132
it can call to PC two directly over layer two 133 133
and send that data frame through the switch. 134 134
The switch is going to take that information and push it 135 135
from router two down to PC two 136 136
just as if the two of them were computers sitting on the 137 137
same switch, trying to communicate locally. 138 138
When PC two wants to send a response to PC one, 139 139
this process is going to be reversed. 140 140
And that way the communication can continue back and forth 141 141
between PC one, its switch, router one, 142 142
the WAN connection, router two, its switch, and PC two. 143 143
Alright, so a quick summary 144 144
of what we just saw in terms of data flow. 145 145
We started out with layer two pieces of information called a 146 146
data frame coming from PC one. 147 147
When it got to router one, 148 148
it got packaged up as a layer three packet 149 149
to cross the wide area network. 150 150
When it got to router two, 151 151
it stripped it back down to layer two, 152 152
to make it a data frame again, 153 153
using MAC addresses to deliver it to PC two 154 154
on its local network. 155 155
Essentially, this is how routing works. 156 156
Now at this point, it's pretty simple, 157 157
but it's going to get a lot more complicated as we start to 158 158
move packets across not just a direct way in link 159 159
like this one, 160 160
but over the world's largest wide area network, 161 161
the internet. 162 162
Now in this section of the course, 163 163
we're going to talk about all the different protocols 164 164
that are used in routing, 165 165
including static and dynamic routing 166 166
and address translation and multicast routing. 167 167
We're going to spend some time in three different domains in 168 168
this section of the course, including domain one, 169 169
networking fundamentals, domain two, 170 170
network implementations and domain five, 171 171
network troubleshooting. 172 172
Within these domains, 173 173
we're going to focus on objectives 1.4, 2.2 and 5.5. 174 174
Objective 1.4 states that given a scenario, 175 175
configure a subnet and use appropriate 176 176
IP addressing schemes, 177 177
objective 2.2 states that you must be able to compare and 178 178
contrast running technologies and bandwidth management 179 179
concepts, and objective 5.5 states that given a scenario, 180 180
you must troubleshoot general networking issues. 181 181
So let's get started talking all about routing our packets 182 182
in this section of the course.