So let's get started in this course by taking a look at what
OSPF is and how it is used on a network.
While I will assume that the majority of those people watching this course
already understand at least the high-level purpose of OSPF,
I want to run through some of the basics for those who don't already know.
The Open Shortest Path First or OSPF is an interior gateway protocol,
meaning that it is used to control how IP and IPv6 traffic is
routed within a single organization's network.
Other common interior gateway routing protocols include the enhanced
interior gateway routing protocol or EIGRP and the intermediate
system to intermediate system or is IS-IS.
Each one of these has their own advantages and disadvantages,
but I'll provide a high-speed routing solution.
These interior gateway routing protocols are contrasted with
exterior gateway protocols that are used to control how traffic is
routed between organizational networks.
The only protocol that is commonly used for this purpose
is the border gateway protocol or BGP.
Within the realm of interior gateway routing protocols there are two
different types; distance vector and link state.
Distance vector protocols exchange a distance or a metric
and a vector or direction between devices.
Each device will then make a decision based on this collected information.
This collected information is based only on the current
reachability known to the neighboring devices.
Because of this,
each device has a limited view of the network and assumes
its neighbors' information is correct.
The alternative is a link state protocol.
Link state protocols exchange the state of each of the connected lengths.
In this case,
every link state device forms a neighborship with other directly attached
devices and exchanges their copy of the link state database,
which contains how the network is connected and the
cost to reach each connecting link?
If the neighbor's update contains additional information,
then it is appended to the known database.
This way every device will eventually have a mirror
copy of the link state database.
Because of this, link state devices have a better view of the network overall,
but are also more complex than their distance vector cousins.
OSPF is an example of a link state protocol.
With OSPF the information that is exchanged is performed using
different types of link state advertisements or LSAs.
These blocks of information are received and stored
inside a link state database.
This information is then used by OSPF via shortest path first algorithm,
to determine the best route of each of the specific known destinations.
Now let's take a quick look at how this works using a simple topology.
In this topology we have six different routers that
are labeled from R1 through R6.
In this case all of the links are gigabit Ethernet and
the default metric formula is used.
When we do this with OSPF, all of the links are given a metric of 1.
So now let's move into the lab and see how this looks in a real environment.
Now if we look here at our lab environment,
we'll see that I have this set up in MTPuTTY where each
of the routers is given a tab on here,
and for the purposes of this lab we are going to be using Cisco devices.
For this early lab,
all I'm going to be doing is showing you how the different devices
are neighbored together and how OSPF shows that.
Now if we can see from R1,
if we remember from the topology that was just on the previous slide,
we'll know that it was connected to two different devices; R1 was
connected to R2 and R1 was connected to R3.
Now if we see this command here, which is just neighborship command,
which displays what are the currently established
neighborships between the devices,
we can see that we have two interfaces which is Gigabit 0/1 and Gigabit 0/2,
and if we look, we have established full states between R1 and R2 and R1 and R3.
Now if we look at our routing table, so these are all displayed as OSPF routes,
and you can see that certain ones of these can be equally
reached via either one of R1's interfaces,
like this one which is the link between R2 and R3,
and then there's several other ones here,
depending on where they are where there's only one
route that has the lowest cost,
and at the very end of the topology off of our six,
I have one LAN configured with the 100.100 .100 /24 network.
And we can see this at the very end here,
which is saying that R1 has two paths to reach to equal
cross paths to reach this network.
Each of them has a cost of 4, and if we count through all the links,
since we had previously stated,
each one of these links with the default metric formula has a cost of 1.
So if we follow through, we'll see that R1 can take either one of its interfaces,
equally costing to reach that network.
So we're going to move back to the slide for a second and walk
through the topology and see which ones are available.
Okay,
so let's move back to the slides for a second and take a look at the
different paths that are available but would cost the same.
This includes from R1 from R2, from R2 to R4,
and from R4 to R6, from R1 to R2, R2 to R5,
and R5 to R6, R1 to R3, R3 to R4, and R4 to R6,
or finally from R1 to R3, R3 to R5, and from R5 to R6.
As you can see, each one of these paths,
if you add them up, all equal the same metric of 4.
So now let's jump back into the lab here,
and as I showed in the previous short little lab there,
there are currently two entries to reach the 100.100 network from R1,
and the only reason there are two here,
and we talked about four different paths that have the same equal cost,
but from R1's perspective it only has two links that connect into the networks,
into the network that connects to R6 eventually.
So from R1's perspective we have two available options,
from R1 to R2 and from R1 to R3.
Both of these are available.
Okay, so let's jump into R2 for a second,
and we'll take a look and see what his path says to get to that 100 network.
And as you can see, R2 also has two available paths that would cost the same.
One would go from R2 to R4, which is this guy,
and the other one would be from R2 to R5,
which is this one; this is that second one we talked about.
Now R3 as you would imagine,
since everything's the same and the network's all symmetrical,
this routing table for that network looks very similar,
so it has two paths that are available.
Again, this includes through R4 and R5 so this is through R4,
this is through R5, and if we keep jumping,
now if we go to R4, now R4 is going to look different,
because R4 only has one path that's available to it that has the lowest cost.
It has other paths that are available to it if you look at the topology,
but those would cost more.
So if we see his smallest path includes a metric of 2.
So his link between R4 and R6 is 1,
and then it also costs 1 to be on the LAN side of R6 so that's the other 1.
So he has a path of 2.
Jump into R5 for a second.
As you would imagine,
R5 looks almost identical to R4 where it'll have R1 path from R5 to R6.
It also has a cost of 2, and of course if we jump to the final end here,
this path from our 6s perspective, it's a connected so it only shows it here.
So that's just sort of a quick walkthrough,
but for now let's move back to the slides.
And now that we have gone over this,
this is just sort of a brief introduction into OSPF and what
it looks like in the routing table and a really brief
introduction into the OSPF metric.
Let's move on from this section and let's move into a section on the OSPF metric specifically.