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[AUDIO LOGO]

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In this video, we're
going to start off

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by looking at single area OSPF.

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We'll take a look at what
the database looks like,

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as well as what
happens if there's

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a failure in single area OSPF.

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We'll also, of course,
talk about some

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of the advantages and
disadvantages of running

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OSPF in a single area.

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So let's go ahead
and jump in and get

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started by talking about some
of the attributes of single area

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OSPF.

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So the first thing
we want to consider

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when talking about using,
particularly single area

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OSPF is it's main issues come
into play with scalability.

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The real issue here
is when our network

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starts getting very
large, single area just

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really doesn't cover it.

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There are, of course,
a few reasons for this.

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The first is that the
Dijkstra algorithm really

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can cause high CPU utilization.

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Remember, it has to run
against the entire database

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for the area.

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And because of that, it
actually makes the running

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of OSPF proportional
to the square

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of the number of routers.

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So the more routers
there are in an area,

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that means the database
is going to be larger.

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And overall, it
means that Dijkstra

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is going to take a
lot more horsepower

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to run on our devices.

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And going along with that,
since each router must

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hold the complete link state
database for that area,

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it means that it can also
cause high memory use.

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Now, just as a note on this
topic, when we say high memory

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use on today's devices,
honestly this is probably not

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going to be an issue.

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But it is something to keep in
mind, particularly if you have

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a very large environment
or if maybe you

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have some dated routers that
are still in your environment.

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But overall, really neither
one of these two issues

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is really going to be a huge
concern on today's routers

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until our network starts
getting very large.

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But these are some
things we need

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to keep an eye on as our
OSPF environment gets larger.

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So we're going to jump
over to the command line

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and take a look at what
the database looks like

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and what failures look like
in an OSPF single area.

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To do that, we're going
to be using this topology.

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As we can see, this
is all of our routers,

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router 1 through router 10, all
being in a single area zero.

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So this is not an exceedingly
large environment.

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Again, it's just 10 routers.

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However, we'll still
see how every single one

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of these devices is going to
be receiving all of the LSAs

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from all of the other routers.

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So every single router is going
to have every other routers LSA

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information in its database.

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So we'll see this as we look at
some of our different routers.

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Let's jump over to the
command line and take a look.

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So let's start off
here on router 10,

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and let's just start with
a show IP route OSPF.

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And what we see here is that
we have all OSPF intra area

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routes.

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We can tell because
our key here is just

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an O where it would be followed
by something else like an IA

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or an E1 or any E2, if this
was external and so on.

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So we can see by the fact that
there's nothing after the O,

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that these are all routes
from within the same area.

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And this applies
to all the routes.

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If we do a show
IP OSPF database,

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we can see that all we have
are type 1 and type 2 LSAs.

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These are known as router links
states and network link states.

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We don't have any
LSAs that would

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be coming from other areas.

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This is, of course, because
everything's in this area.

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We will talk more
about these link

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state types in an
upcoming video.

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So now let's take a look
at what happens if we

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trigger SPF to actually run.

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Let's start with a show ip ospf.

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And what we want to note
here is that for area zero,

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currently SPF has
executed 17 times.

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And this is simply
how many times

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it executed while this
particular lab environment was

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being built. So
let's take a look

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at what happens when a
change occurs in OSPF.

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Let's turn on a
debug ip ospf spf.

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So we have this on.

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Let's now switch
over to router 5.

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Here we'll go into
configuration mode.

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And we'll simply go in
to say it's loopback

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and shut that interface down.

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We'll switch back
over to router 10,

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we'll scroll up a
little bit here.

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And what we see as
we start going down

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through the database,
obviously a lot is going on.

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There are several pages here.

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We're not going to look
at every detail of this.

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But what we want to see is that
SPF is definitely executing,

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and it's checking every
single route in the database.

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So we can see here, as we
scroll down through this,

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this is covering the
entire OSPF domain

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because it's all in one area.

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So we're forcing SPF to run
against the entire database

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here.

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And as you can see, this
is quite a bit of work.

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There's quite a bit going
on here in the debug.

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And if we look at
the very end, we

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can see that this actually
took on this equipment

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17 milliseconds.

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Now, that's of course
not a whole lot.

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But let's see how that stands up
when we get to multi-area OSPF

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next, and we'll see if it's
any faster when it's converging

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on a much smaller area.

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Now, it might not be
much faster, again,

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this is a relatively
small lab environment.

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But let's see if we get
any performance increase

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when we go multi-area.

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The main thing, of course,
however, regardless

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of how long it actually
took, is if we do our show ip

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ospf again, notice that now,
SPF algorithm has executed

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18 times, which means
it did in fact run again

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in response to this change.

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And of course, if we bring
that interface back up, so

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back over here on router 5,
we'll simply do a no shut.

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Back over on router 10, we
can see that it once again

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executed SPF.

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We don't have to scroll
through the whole thing again.

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I think you get the idea.

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This time it only
took 14 milliseconds,

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so it was a little bit faster.

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And again, if we look
at our show ip ospf,

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we see that it's now
executed 19 times.

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So this is definitely executing
SPF any time anything changes

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in the area.

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And this is part of what leads
to that scalability problem.

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In this video, we talked
about, well, mostly

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the disadvantages of running
a single area in OSPF, mostly

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focused around that
scalability issue.

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But keep in mind,
like most things,

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there are advantages to running
single area OSPF as well.

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First, it's definitely
easier to configure.

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And it's certainly
easier to design,

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since it's literally
just a single area.

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And it also has some advantages
because certain technologies

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such as MPLS traffic engineering
actually function better

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in a single area environment.

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And when you go multi-area, it
gets a bit more complicated.

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So like most
things, it certainly

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has advantages
and disadvantages.

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And we'll talk about
this a little bit more

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in our next video as we
jump into multi-area OSPF.

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I hope this has been
informative for you.

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And I'd like to thank
you for viewing.

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