[AUDIO LOGO]
In this video, we're going to start off
by looking at single area OSPF.
We'll take a look at what the database looks like,
as well as what happens if there's
a failure in single area OSPF.
We'll also, of course, talk about some
of the advantages and disadvantages of running
OSPF in a single area.
So let's go ahead and jump in and get
started by talking about some of the attributes of single area
OSPF.
So the first thing we want to consider
when talking about using, particularly single area
OSPF is it's main issues come into play with scalability.
The real issue here is when our network
starts getting very large, single area just
really doesn't cover it.
There are, of course, a few reasons for this.
The first is that the Dijkstra algorithm really
can cause high CPU utilization.
Remember, it has to run against the entire database
for the area.
And because of that, it actually makes the running
of OSPF proportional to the square
of the number of routers.
So the more routers there are in an area,
that means the database is going to be larger.
And overall, it means that Dijkstra
is going to take a lot more horsepower
to run on our devices.
And going along with that, since each router must
hold the complete link state database for that area,
it means that it can also cause high memory use.
Now, just as a note on this topic, when we say high memory
use on today's devices, honestly this is probably not
going to be an issue.
But it is something to keep in mind, particularly if you have
a very large environment or if maybe you
have some dated routers that are still in your environment.
But overall, really neither one of these two issues
is really going to be a huge concern on today's routers
until our network starts getting very large.
But these are some things we need
to keep an eye on as our OSPF environment gets larger.
So we're going to jump over to the command line
and take a look at what the database looks like
and what failures look like in an OSPF single area.
To do that, we're going to be using this topology.
As we can see, this is all of our routers,
router 1 through router 10, all being in a single area zero.
So this is not an exceedingly large environment.
Again, it's just 10 routers.
However, we'll still see how every single one
of these devices is going to be receiving all of the LSAs
from all of the other routers.
So every single router is going to have every other routers LSA
information in its database.
So we'll see this as we look at some of our different routers.
Let's jump over to the command line and take a look.
So let's start off here on router 10,
and let's just start with a show IP route OSPF.
And what we see here is that we have all OSPF intra area
routes.
We can tell because our key here is just
an O where it would be followed by something else like an IA
or an E1 or any E2, if this was external and so on.
So we can see by the fact that there's nothing after the O,
that these are all routes from within the same area.
And this applies to all the routes.
If we do a show IP OSPF database,
we can see that all we have are type 1 and type 2 LSAs.
These are known as router links states and network link states.
We don't have any LSAs that would
be coming from other areas.
This is, of course, because everything's in this area.
We will talk more about these link
state types in an upcoming video.
So now let's take a look at what happens if we
trigger SPF to actually run.
Let's start with a show ip ospf.
And what we want to note here is that for area zero,
currently SPF has executed 17 times.
And this is simply how many times
it executed while this particular lab environment was
being built. So let's take a look
at what happens when a change occurs in OSPF.
Let's turn on a debug ip ospf spf.
So we have this on.
Let's now switch over to router 5.
Here we'll go into configuration mode.
And we'll simply go in to say it's loopback
and shut that interface down.
We'll switch back over to router 10,
we'll scroll up a little bit here.
And what we see as we start going down
through the database, obviously a lot is going on.
There are several pages here.
We're not going to look at every detail of this.
But what we want to see is that SPF is definitely executing,
and it's checking every single route in the database.
So we can see here, as we scroll down through this,
this is covering the entire OSPF domain
because it's all in one area.
So we're forcing SPF to run against the entire database
here.
And as you can see, this is quite a bit of work.
There's quite a bit going on here in the debug.
And if we look at the very end, we
can see that this actually took on this equipment
17 milliseconds.
Now, that's of course not a whole lot.
But let's see how that stands up when we get to multi-area OSPF
next, and we'll see if it's any faster when it's converging
on a much smaller area.
Now, it might not be much faster, again,
this is a relatively small lab environment.
But let's see if we get any performance increase
when we go multi-area.
The main thing, of course, however, regardless
of how long it actually took, is if we do our show ip
ospf again, notice that now, SPF algorithm has executed
18 times, which means it did in fact run again
in response to this change.
And of course, if we bring that interface back up, so
back over here on router 5, we'll simply do a no shut.
Back over on router 10, we can see that it once again
executed SPF.
We don't have to scroll through the whole thing again.
I think you get the idea.
This time it only took 14 milliseconds,
so it was a little bit faster.
And again, if we look at our show ip ospf,
we see that it's now executed 19 times.
So this is definitely executing SPF any time anything changes
in the area.
And this is part of what leads to that scalability problem.
In this video, we talked about, well, mostly
the disadvantages of running a single area in OSPF, mostly
focused around that scalability issue.
But keep in mind, like most things,
there are advantages to running single area OSPF as well.
First, it's definitely easier to configure.
And it's certainly easier to design,
since it's literally just a single area.
And it also has some advantages because certain technologies
such as MPLS traffic engineering actually function better
in a single area environment.
And when you go multi-area, it gets a bit more complicated.
So like most things, it certainly
has advantages and disadvantages.
And we'll talk about this a little bit more
in our next video as we jump into multi-area OSPF.
I hope this has been informative for you.
And I'd like to thank you for viewing.