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[AUDIO LOGO]

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So in this video,
we're going to start

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by just getting an overview of
the different OSPF area types

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that we have available.

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So we'll be talking about the
stub, the totally stubby area,

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the not so stubby area,
and the totally stubby, not

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so stubby area, which we
really just often refer

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to as the total NSSA
just because it's

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a lot easier to say.

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It's a bit shorter.

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So let's jump in and take a look
at these different area types.

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We'll bring up our topology map.

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And we'll talk about which
LSA types are or are not

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allowed into which areas and
how they get filtered and where.

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So let's go ahead and
jump in and get started.

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So to begin, we will talk about
the normal or the regular area.

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Now, there's not
really a whole lot

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to say about these as
it's sort of what we're

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working with normally in OSPF.

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This is an area type that
allows all LSA types.

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Now, we say all, we
mean all normal LSAs.

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So that would
effectively be type

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1's, 2's, 3's, 4's, and 5's.

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Remember that 6's are
used for multicast

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and not supported by Cisco.

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And a type 7 is actually
a very special use.

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We'll see that here in
just a couple of minutes.

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So a type 7 would not
actually be in a normal area.

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Now, the next thing
to take note of

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is the backbone area
must be a normal area.

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Of course, we're referring
to area zero up here.

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You cannot have the backbone
be a non-normal area.

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It must support all
of these LSA types.

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Also note that
virtual links, which

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we'll be talking about
later in this course,

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can only transit a normal area.

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They can't transmit a stub
or a not so stubby area.

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It has to be a normal area.

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So other than these
few attributes,

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there's not really a whole lot
to say about the normal area.

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Again, this is what
we have by default

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if we just create
an area in OSPF.

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So now let's move
on to the stub.

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So the main idea
behind the stub area

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is that we want to filter
out the type 5 external LSAs.

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And at the same time,
since the type 4

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is used to support the type 5,
we'll filter those out as well.

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The idea is a stub area--

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a good example of this
would be something

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like area 200 up here--

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would not need external
routes such as this one coming

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from EIGRP over here.

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We don't need it as long
as we inject a default

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route into that area instead.

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So if we inject this
default into area 200,

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there is no need
for area 200 to get

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external routing information
from other areas such as area

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100.

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We can just default
right to the ABR.

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Now, a side effect of
all of this, of course,

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is that if we cannot have
type 5 LSAs, in other words,

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we cannot have external routes,
then an ASBR, which, of course,

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by nature, is bringing
external routes into OSPF,

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cannot exist in a
stub area simply,

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because type 5 LSAs
are not allowed.

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Also, since having external
routes in the database

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would affect whether
the database is

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the same between two devices,
all of the routers in an area

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must be configured for
it to be a stub area.

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Again, the simple
reason for this,

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all the routers in an area
must have identical databases.

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I can't have one
router, thinking

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that external routes are OK,
and another router saying,

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no, they're not.

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We have to agree on
what LSA types are

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allowed in the database.

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So stub areas are good
in situations where

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we don't need external routes.

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And with this default
route coming into area 200,

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we have to ask ourselves, do
we even need the other type

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3 LSAs?

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Do I even need router 8's
loopback address, for example,

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if I can just
follow the default?

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If the answer to
that question is

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no, which would be the case
here because router 6 is

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the only way out of
area 200, well, then we

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can move on to a total stub.

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So any time we see the
word "totally" in relation

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to OSPF areas, just think
about blocking type 3 LSAs.

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If we look at the
rest of these points,

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that we block type 5's,
and we block type 4's.

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And there's a default
route injected,

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and there can be no ASBR.

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And all of the routers have to
be configured for stub area,

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all of these things
are the same things

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we said about the stub
area to begin with.

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So what the real difference
here is the type 3 LSA.

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And what we're
going to do is we're

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going to stop things like
router 8's loopback that

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would normally come in
to area 200 as a type 3.

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We're going to, in fact,
filter that and not

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send it in at all since
we're getting the default.

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So the difference between a
stub area and a total stub

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is, simply, we don't
send the type 3 LSAs

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into the total stub.

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This is further filtering
that's happening on the ABR.

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Now, the default it injects
will still be a type 3.

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So it's not the type 3's
are not allowed in the area.

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It's just the ABR is
filtering them out.

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Now, this isn't going to be
perfect in every environment.

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If there's more than
one exit from the stub,

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we may prefer one ABR over
another ABR coming out

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of that area.

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In that case, we
would want the type

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3's so that we can prefer
specific prefixes through one

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ABR versus another.

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So in that case, you
would want to just do

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a stub, not the total stub.

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But in the case of our area 200
here, it only has one way out.

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So there's no need to
have anything other

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than that default.

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Now, one of the big limitations
here of a stub area is that

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an ASBR cannot exist
in this area type.

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So maybe we just had
a few external routes

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we wanted to bring in.

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Well, we can't do
that in a stub.

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And because of that, we
have another area type,

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the Not So Stubby
Area, or the NSSA.

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So the idea behind the NSSA
is that although we are still

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not allowing type
5's and type 4's,

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we can still bring
in external routes.

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Now, one other fundamental
difference here with a stub

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is that a default route is
not injected automatically

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into the NSSA.

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And we'll see that
more when we actually

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look at this in action.

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However, the biggest difference
is that an ASBR actually

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can exist in this area.

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It's going to use LSA type 7
for those external prefixes.

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So a good example
would be area 100 here.

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If we were going to try
to make this a stub,

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we would lose the routes from
router 9 coming in from EIGRP.

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Well, that's not what
we would want to do.

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We do need those routes.

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So if we still want
to bring those routes

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into this area,
meaning, of course,

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we still want router
2 to be an ASBR,

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the only choice would
be to make this NSSA.

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What happens is these
updates come in as type 7's.

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That type 7 will then get
translated into a type 5

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by one of the ABRs and sent
into the rest of the network.

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So the type 7 only exists
inside of the NSSA.

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Now, at the end of the
day, this is still a stub.

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So keep in mind those type 5's
and those type 4's are still

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not allowed.

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And, again, just
like we said before,

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since that's affecting
what's in the database,

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all the routers are going to
have to be configured for NSSA

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just like they did for the stub.

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Now, the NSSA is actually
probably our more complicated

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of the area types.

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So we're going to jump in
and take a look at this.

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And we're actually
going to spend

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a little bit more
time on this one

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than we do on stub
and total stub.

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NSSA does get a little
more complicated.

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So we'll see that in
an upcoming video.

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But just like we have for stub,
there's a counterpart here.

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So as we had stub
2 totally stubby,

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we also have NSSA
to the total NSSA.

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Let's take a look at that one.

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So just like with the total
stub, the totally NSSA,

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or the total NSSA, as it's
more commonly referred to,

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simply adds that additional
filtering of the type 3.

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Again, all the rest
of these points

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are going to be exactly
the same as the NSSA.

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The only real difference
is, once again, on the ABRs.

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Any type 3's that would
be sent into this area

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are going to be filtered out.

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A couple of things to note
about the use of total,

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be it for the stub or the NSSA.

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Technically, the total
stub and the total NSSA

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are Cisco proprietary.

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However, since it's just the
ABR doing additional filtering,

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the other routers in that
area don't know anything

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about that total flag.

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They don't know
anything about the fact

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that the ABR is doing
extra filtering.

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So even though it is technically
Cisco proprietary, that

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doesn't really
affect compatibility

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with other vendor's
routers because,

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again, they don't
know it's happening.

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And, also, on that
note, the total

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only has to be
configured on the ABRs.

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And there's nothing
that stops those

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from still forming neighborships
with other devices that

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are just configured as
either stub or NSSA.

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So in this video,
we just took a look

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at the basics of the
different OSPF area

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types, their attributes, which
LSAs are sent between which

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areas, and a little bit more
look at where that type 7

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LSA is used in the NSSA.

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In the next video, we're going
to jump into the command line

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and get started looking
at this, starting

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with the stub and
the total stub.

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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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