In this section of the course,
we're going to discuss routing.
Now, up to this point, we've covered a lot of things,
but our network traffic really hasn't even left
our local area network yet.
Now, once we're ready to leave our local area network,
this is where routers
are going to really start coming into play.
When we start talking
about connecting to subnets within our network
or connecting our internal network with an external network,
we have to use a router.
Now, the first thing you must understand
is the function of a router,
which is simply to route traffic.
Now, I know that sounds really silly,
but essentially it is going to forward traffic
in one direction or the other
between different subnets,
between internal and external networks,
or between two different external networks.
Now, each subnet or external network
is going to be on its own broadcast domain
because routers are used to separate broadcast domains.
Unlike a switch which are layer 2 devices,
a router will be able to split up a broadcast domain.
With a switch, you only have one broadcast domain
for everything that you're going to connect to it.
But with routers, since they're layer 3 devices,
they can separate and break those apart
into smaller broadcast domains and these smaller portions
give us more efficiency and security in our networks.
Now, let me take a quick detour our here for a second
because I need to provide you with an important warning
for the real world versus the exam.
Now, in the real world, you can use a layer 3 switch
or a multi-layer switch, as it's also known,
to perform routing functions too.
Many small office home office networks
or small office and medium sized businesses
are going to opt to use layer 3 switches
because they can handle both layer 2 forwarding
when they act like a switch and layer 3 routing
when they act like a router.
Now, for the exam though, I want you to remember,
if you're using a multilayer switch,
it is functioning as a router,
so we're going to call it a router on the exam.
Now, in this entire section,
whenever you hear me use the word router,
even if you're using a layer 3 switch
or a multi-layer switch to perform those functions,
we are still talking about a router
because that's the part of that device
that we're really covering
when we talk about routing in this section.
Now for the exam, if they simply use the word switch
in any of the questions,
they're referring to a regular switch
that operates at layer 2 of the OSI model.
If on the other hand, they use the term router,
multi-layer switch or layer 3 switch,
they're referring to the layer 3 functionality
of that device, which we refer simply as a router
during both this course and your official exam.
Now, let's talk about how a basic router process works.
Let's consider a small network diagram here
with two networks, one on the left and one on the right.
Now as you can see, PC1 is connected on the left side
and PC2 is connected over on the right side.
To connect these two networks together,
we have two routers with a serial connection
or a wide area network connection between them.
For now, we're not going to focus on the connection
between them as much because we're going to cover that
when we talk about WAN technologies
in a different section of the course.
But for now, I just want you to realize there is
some kind of a connection between these two networks,
whether that's a fiber connection, a serial connection,
a satellite connection, a VPN or some of the link,
it really doesn't matter right now.
Just know there is a connection between those two networks
and we're going to have to use two routers, one on each side
to communicate over that WAN connection.
Now, if I'm sitting on PC1
and I want to send a message over to PC2,
I can't just send it out an ARP like I did with a switch.
If you remember back in switching,
we talked about how PC1 would send
an ARP packet to the switch,
and if it didn't know where to deliver it,
it would send it out to every other switch port
based on the MAC address,
and then PC1 could and will start to do that,
and they'll be able to get from here using the ARP request
to figure out how to get from the switch
and the switch realizes it doesn't know the MAC address
associated with the request,
so it forwards it out to its default gateway
and that default gateway, that's a router.
Then that router is going to respond to the ARP request
on behalf of the destination device.
At this point, we now are going to leave MAC addressing behind
because MAC addresses are only used
internally to our network using layer 2.
As soon as we get to the router though,
we're switching over to IP addresses or logical addressing,
which operates at layer 3.
Now, this is where we begin to make
our layer 2 to layer 3 transition happen.
At this point, PC1 has determined that the MAC address
for where the router is,
because it needs to send any packets there
that it wants to get out of this local network.
The left router in this case,
is going to want to send the information over to PC2
because that's what PC1 asked it to do.
Now, to do that, it's going to pass the information
over to router two,
and then router one will get the data frame,
which is layer 2 from PC1,
the router will repackage it as a packet,
which is layer 3, and then add an IP header.
This allows router one to then push that data
over its default connection as an IP packet,
and then data will be sent across the serial connection
over to router two.
Now, once the packet gets to router two,
router two will strip off that IP header
and bring it back down to layer two data frames
and add the MAC address for PC2 that correlated
with the destination IP of that packet that it received.
Now, because router two knows the MAC address for PC2,
it can call up to PC2 directly over layer 2
and send that data frame over to the switch.
The switch will then take that information
and push it from router two down to PC2,
just as if it was two computers on the same switch
trying to communicate with each other.
Now, whenever PC2 wants to send response back to PC1,
the process will simply be reversed
and the communication continues back and forth
between PC1, its switch, router one, the WAN connection,
router two, its switch, and PC2.
Now a quick summary of what we just saw
in terms of data flows.
We started out with layer 2 information
we called a data frame going from PC1.
When it got to router one,
it was then packaged in a layer 3 thing,
which we call a packet
and sent across our wide area network.
When it got to router two,
it's going to strip it back down to layer 2
and use MAC addresses to deliver it over to PC2.
Essentially, this is how routing works at its simplest form.
Now, at this point,
I've given you a very simple example of how routing works,
but it gets a lot more complicated
as we start to move packets across not just direct WAN links
like the one you saw here, but over other WAN links,
like the internet.
Now, we're going to be discussing routing in this section,
and we'll be focused on Domain 1 Networking Concepts
and Domain 2 Network Implementation.
We're going to cover Objective 1.4,
which states that you must be able to explain
common network ports, protocols, services,
and traffic types.
We'll also cover objective 2.1,
which states that you must be able to explain
characteristics of routing technologies.
First, we're going to look at some routing fundamentals
as we cover the basics of how routing works
inside of our networks.
Then, we'll discuss routing tables,
which are used to determine
which direction your data should be sent
as it attempts to cross from one subnet
or local area network over to another.
Next, you're going to learn
about the different types of routing protocols that we use
inside of our networks, including RIP, OSPF,
BGP, and many others.
After that, we'll cover
how route selection is going to be performed
so that your data can be set over the most efficient route
when it's being transferred between two different networks.
Then, we'll discuss the concept of address translation
by covering Static NAT, Dynamic NAT and PAT.
Next, we'll explore some routing redundancy protocols
and how these are used to set up a pair or a team of routers
that are going to share a virtual IP address
and provide load balancing or redundancy
for our network devices.
After that, I'm going to demonstrate
how routers are configured for use in our networks,
and then we'll cover multicast routing
and how different modes of PIM are going to operate.
Next, we'll discuss
the Generic Routing Encapsulation protocol known as GRE
and why it's used in some of our networks.
Finally, we'll take a short quiz to see what you learned
during this section of the course and review your answers
to ensure you know why the right answers were right
and the wrong answers were wrong.
So if you're ready,
let's get started with our coverage of routing
in this section of the course.