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[AUDIO LOGO]

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So now that we've gotten some
more detail on multiarea OSPF,

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now let's jump in and
look at our different LSA

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types and the role that they
play in our multiarea SPF

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topology.

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So we're going to
start off, of course,

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by looking at type 1 and type 2
LSAs that work within the area.

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Then we'll look at our type
3 LSAs between the areas.

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We'll look at our external
LSAs, our LSAs for our ASBR--

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so our summary ASBR entries.

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And then we'll finally wrap
up by taking a look at LSAs

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that we use in a not so
stubby area or an NSSA.

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So we'll take a look
at each of these.

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And then in our
next video, we'll

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actually jump in and see these
in action on the routers.

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So for now, let's jump
in and take a look

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at the role of each LSA in
the multiarea OSPF topology.

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So our first LSA
type is of course

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going to be LSA type 1,
otherwise known as the router

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LSA.

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So the type 1 LSA is generated
by all routers in OSPF.

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And as such, this serves
as the fundamental building

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block for our OSPF database.

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This is what contains
all the information

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about the router and its links.

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So this is going to include
things like the router ID

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to identify the router, as well
as the current status of all

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of its interfaces.

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Now, since this is the detailed
information for the database,

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this is going to be
contained within an area.

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So if we were to, for example,
pick on router 7 here,

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it's type 1 LSA will be
sent over here to router 8.

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It would be sent up to
router 6, to router 1,

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and over to router 3.

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However, it won't go
any further than that.

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Router 6 would not
flood it into area 200.

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Router 1 and router 3 would
not flood it into area 100.

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Type 1 LSAs are completely
confined within the area.

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And again, they're used to
define the routers themselves.

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Next is the type 2,
or the network LSA.

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Now, this is generated by the
DR, or the Designated Router,

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on a broadcast network type.

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In our example, we actually have
two broadcast network types.

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So let's take a look
at the one in area 0.

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We have three routers
connected to this--

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router 6, router
7, and router 1.

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These are all connected to this
common subnet 50.0.167.0/24.

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One of these routers will
be elected as the DR.

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Keep in mind that the
default for this election

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is to go with the router
with the highest priority.

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All routers, by default,
have a priority of one.

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So if we as administrators
don't take control over this,

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it will then go to the router
with the highest router ID.

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The router ID is generally
based on its loopback interface.

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So therefore, it
would probably go

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to the router with the
highest loopback address.

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In our example, that would,
in fact, be router 7 here.

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So let's go ahead and
label that as our DR,

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assuming that that's the
router that would have won.

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So if router 7
becomes the DR, that

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means that router 7
is going to be the one

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to generate this LSA.

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This would get sent to router
3, to router 6, to router 1,

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and of course, over
here to router 8.

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Once again, it gets flooded
through the whole area.

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This contains information
about that multiaccess network

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segment.

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So this contains information
about this 50.0.167.0/24

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network.

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Now, just like the
type 1 LSAs, these

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are also going to be contained
within the area, which means,

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once again, router 6 will
not send this into area 200.

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And router 1 and router 3 will
not send this into area 100.

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The main purpose
of doing this is

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so that all of the
routers connected

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to this multiaccess
segment don't

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have to generate the type 1
LSAs for that link individually.

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So the idea is we send
this information out.

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The DR tells everybody else
which routers are connected

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to this multiaccess segment.

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We'll see this in
the next video when

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we look at these in
a little more detail.

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Keep in mind, as
well, in our example,

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this same thing would happen
for this segment over here

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in area 100, which is also
a multiaccess segment.

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In other words, it's
network type broadcast.

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Next up is the type 3
LSA, or the summary LSA.

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This is the LSA that's actually
generated by ABRs to represent

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the prefixes in this area.

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So say, for example, router
7 has a loopback interface.

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That interface will get sent
with a type 1 LSA to router 6.

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Since router 6 is
in fact an ABR,

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it will then generate a
type 3 LSA into area 200,

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representing router
7's loopback.

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At the same time,
router 7 is also

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going to send this to router 1
and router 3 with type 1 LSAs.

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And both router 1 and router
3 will generate a type 3 LSA

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into area 100,
again, representing

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router 7's loopback.

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This happens, of course, for
every prefix in every area.

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We're just picking on
router 7's loopback.

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But we would have
the exact same thing

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going the other direction.

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So if we were talking
about router 4's loopback,

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for example, once again,
it would go as a type 1

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over here to router 3.

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And router 3, of course,
would generate that type 3

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into area 0.

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So again, this is going
to be for all prefixes

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in all directions
between all the areas.

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And that's what this
point is that it's

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used to advertise the
prefixes from one area

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into another area.

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As we just demonstrated,
this is going

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to be flooded both into the
backbone and into other areas.

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Now, remember what we
learned in our previous video

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that when we get these type
3 LSAs in another area,

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yes, we have to
update our database,

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either adding or
removing the prefix.

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And of course, that causes us
to update our routing table,

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either adding or
removing the route.

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But we do not run SPF,
Shortest Path First,

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or Dijkstra's
algorithm in response

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to receiving these type 3 LSAs.

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Also keep in mind that although
these are called summary LSAs,

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it's referring to the fact
that it's a database summary.

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Don't look at this as a
network summarization.

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This is where we can create
a network summarization,

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but we have to do that
manually as administrators.

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Now, we can do that to
help reduce or control

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the flooding of these type
3 LSAs into other areas.

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And we'll look at
that in a future skill

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when we get to summarization
and filtering between areas.

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But keep in mind
that that's going

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to be talking about
controlling these type 3 LSAs.

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By default, all prefixes are
sent into all other areas

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with a type 3 LSA.

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Next is the type 5
LSA, or the external.

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Maybe you're thinking to
yourself, wait a minute,

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what happened to the type 4?

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We're going to
get to the type 4.

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But since the type 4 is
to support the type 5,

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it actually sort of makes sense
to talk about the type 5 first,

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even though, numerically,
that's not exactly in order.

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So let's talk about
the type 5 first.

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This is going to be
generated by the ASBR That's

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doing redistribution into OSPF.

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In our example,
that would be R2.

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This is our ASBR.

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And what it's doing is
taking the EIGRP prefixes

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and redistributing
them into OSPF.

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This is going to
come in as a type 5.

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So again, this is going to
be advertising those prefixes

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from an external protocol in
this example, such as EIGRP,

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into the OSPF domain.

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This is then going to be
flooded through the OSPF domain.

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Well, what that means is
when R2 sends this in,

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it's going to go to R4.

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It's going to go to R5.

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It's going to go to R1.

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4 is going to send
it over here to 3.

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Router 1 is going
to send it into 6.

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6 is going to send it to 10.

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1's going to send
it to 7, and so on.

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Of course, 7 sends it to 8.

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3 sends it to 7.

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You get the idea.

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It's going to be
flooded everywhere.

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Now, the reason this is
important to understand

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that we're flooding
the type 5 LSA

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unaltered through
our network is this

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helps us understand
why we need the type 4.

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For example, in this case, this
being flooded from router 2,

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it's going to have
an advertising router

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ID of router 2's loopback,
which, in our example,

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would be 10.2.2.2.

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That's all fine when it
gets to router 1 and router

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3 and router 4, router
5 even, because they're

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in the same area as router 2.

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And they would have a
type 1 LSA for router 2.

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But what happens
when that information

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goes into another area, such
as, in our case, area 0?

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It gets to say router 7.

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Router 7 has no idea
where router 2 is.

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And that is what we
need the type 4 LSA for.

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Let's take a look.

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So again, our next
type is the LSA type 4.

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This is called an ASBR summary.

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Now, this is going
to be generated

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by the ABRs for the area,
much like a type 3, actually.

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However, instead of
advertising prefixes

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from one area to
another, it's instead

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advertising the
router ID of the ASBR.

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Remember what we said when we
were talking about the type

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5's.

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When this type 5, again,
reaches, say, router 7,

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it's going to have on it
the router ID of 10.2.2.2.

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The problem is router
7 literally has

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no idea where 10.2.2.2 is.

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So how could I possibly
use this type 5 LSA

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if I don't know where the
advertising router is?

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Because it's in
a different area.

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And this is where
the type 4 comes in.

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The type 4's are going to
be generated by the ABRs

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and sent out into the area,
advertising the router ID

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of the ASBR.

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This will get forwarded
through the whole network.

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Once I receive this type 4 LSA--

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and again, everybody will
get it because it is flooded,

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just like the type 3's into the
backbone and the other areas.

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Now, all of a sudden,
router 7 knows exactly where

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10.2.2.2 is.

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It's no longer a question.

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And now router 7 can
use the type 5 LSA.

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Our next LSA type
is the LSA type 7.

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This is NSSA external.

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Now, this one is going to
be generated by an ASBR

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but specifically in
a not so stubby area.

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So for example, let's say that
area 200 up here is an NSSA.

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Router 10 has a
loopback interface on it

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that it is
redistributing into OSPF.

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This will be sent as a type
7 to get it through the NSSA.

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Now, once it gets to router 6,
which is acting as the ABR--

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remember, router 10 is acting
as the ASBR in this case--

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router 6, being
the ABR, will then

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translate this and send it
into the rest of the network

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as a typical type 5 LSA.

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So again, this is to
advertise the prefixes

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from that external protocol
into the NSSA itself.

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And this one is
contained within the NSSA

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because it's translated to
a type 5 at the first ABR.

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Now, I realize we haven't really
gotten into OSPF area types

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yet.

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And you may not fully understand
what an NSSA is at this point.

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And that's OK.

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I only bring it up
here so that we've

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at least seen that there's
something called a type 7 LSA

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00:14:05,990 --> 00:14:08,570
and that we use it in an NSSA.

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NSSA will be covered
in a future skill.

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And we will be jumping into
it in much more detail.

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Now, you might
have noticed as we

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were going through
our discussion

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there that we actually
skipped quite a few LSAs.

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For example, we skipped
right over type 6.

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And that's because type
6 is used for multicast.

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Quite frankly, it's not a
very popular implementation.

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And Cisco has actually never
supported multicast with OSPF.

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We're not talking about
OSPF using multicast.

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We're talking about
multicast using OSPF.

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Type 8 is used for
BGP interaction

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in the case of OSPF version 2.

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And in the case
of OSPF version 3,

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these are used for link LSAs.

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Remember that OSPF version
3 is specifically for IPv6.

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As such, it has to deal
with link-local addresses.

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Type 8's are used to carry
link-local information just

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between directly
connected neighbors.

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Type 9's, in the case
of OSPF version 2,

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these are just referred
to as opaque LSAs.

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Opaque LSA is really
just a way of saying

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that it has no specific or
specified function in the RFCs.

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Now, it can be used
for other things.

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And we'll see some uses
for these opaque LSAs

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as we move through
this entire course.

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Now, the type 9's in the
case of OSPF version 3,

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these are your
interarea prefixes.

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The type 9 LSA in
IPv6 actually carries

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information that's usually
carried in a type 1

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in OSPF version 2.

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One of the big changes that they
made in OSPF between version

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2 and version 3 is
they actually separated

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the advertising of the
topology from the advertising

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of the prefixes.

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So in OSPF version 2, the
prefixes are advertised,

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of course, in a type 1, where
in OSPF version 3 for IPv6,

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we actually separate that
out into these type 9's.

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So the type 1's
advertise the topology.

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The type 9's advertise the
prefixes for that topology.

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Now, you might notice that type
10 and type 11 for both OSPF

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version 2 and OSPF
version 3, they're

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all listed as opaque
just because, again, they

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don't have a specific use
according to the OSPF RFCs.

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In this video, we
went through and we

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talked about all of the
different OSPF LSA types,

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their general function
on our network,

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and where they get flooded
as far as whether they

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cross area boundaries,
what routers generate them,

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and so on.

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So hopefully, we now have
a clearer understanding

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of how these LSAs and
the different types

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operate in our OSPF network.

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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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