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[MUSIC PLAYING]

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Next up, let's jump
in and take a look

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at the not-so-stubby
area, or the NSSA.

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Remember, of course, that
the big advantage to NSSA

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over the stub area is, although
it does still block Type 5

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LSAs, and therefore
doesn't allow

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external routes
from other areas,

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it does allow us to bring in
external routes into the NSSA.

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We take that through
the NSSA with a Type 7.

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And then that gets translated
to a Type 5 on one of the ABRs.

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

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at this in much more detail.

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As always, let's
start on the topology,

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and talk about exactly what
we're going to be configuring.

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In this demonstration, we will
now be focusing on area 100.

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Since area 100 already has a
router doing redistribution,

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Router 2, which is of
course pulling in our routes

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from EIGRP, we
already have an ASBR.

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So we've already seen
what happens if we

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try to convert this to a stub.

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We of course know that Router
2 would stop being an ASBR.

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So instead, of course,
we're going to choose NSSA.

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So we'll take a look
at how this works.

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We'll see how these LSAs
come in as a Type 7.

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And then we'll see how
either Router 1 or Router 3

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becomes our translator,
and translates that

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into a Type 5 going out into
the rest of the environment.

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And we'll explore how
to figure out which

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one became the translator.

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So let's jump over
to our command line.

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Let's begin by exploring the
routing table on Router 7.

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So if we look at our
routing table here.

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And what we see here
is we are in fact

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getting two external Type 2
routes coming from the EIGRP

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environment.

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If we were to say show ip
ospf database external to look

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at those just a little
closer, what we'll see

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is they're both being
advertised by Router 2.

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That means that
we should actually

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understand that Router
2 is in fact a border.

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We do a show ip ospf border.

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We do in fact see that
Router 2 is an ASBR.

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So we're seeing
that information.

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If we do a show ip
ospf database ASBR,

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we should actually have
an entry for Router 2

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being sent to us from both of
our ABRs, Router 1 and Router

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3.

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Remember, this is the Type 4 LSA
that's supporting the Type 5.

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We're looking at this because,
as we convert to an NSSA,

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we're going to see
that this structure is

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going to change significantly
once we have translators

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involved.

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Now let's also do
some verification

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from a router
inside of area 100.

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So let's go to Router 5.

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Let's start with our show
ip ospf database external.

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Once again, we see the
advertising router is 10.2.2.2.

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We do our show ip ospf border.

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We again see that Router
2 is in fact an ASBR.

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Also note, by the lowercase
i at the beginning,

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this is an intra
area route, which

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means we're learning about
this ASBR within our own area.

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It's from a Type 1
LSA, not a Type 4.

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We can prove this by asking
for the ASBR entries.

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And there aren't any.

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There wouldn't be, because
it's within the same area.

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So now that we've
seen what it looks

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like before we convert to
an NSSA, let's go ahead

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and do that conversion.

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We'll start on Router 1.

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So we'll simply go into
the routing process,

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under area 100, and
simply configure NSSA.

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We will see neighborships going
down as we can figure this.

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That's going to be fine, as
long as they all come back up.

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Let's do Router 2 next.

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On IOS XR, we can also configure
this by saying area 100 NSSA.

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You can put it on the same
line or two different lines.

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If we do a show
run however, notice

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that it puts it on the
two separate lines,

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regardless of how we type it in.

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So next over to Router 3.

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This is the same
configuration, so we'll just

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go ahead and paste it in.

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And the same with Router 4.

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Router 5 is running
IOS XE, but it's still

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the same basic configuration.

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It's still just area 100 NSSA.

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The only difference is, you do
put it on just one line here.

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You don't put it on
two separate lines.

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We'll go ahead and exit
out of configuration.

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And we see that our
neighborships are already

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coming back up.

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So now that we've done
our conversion to NSSA,

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now let's jump back over to
our Router 7, our Router 5,

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and let's take a look at how
these things have changed now

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that it's an NSSA.

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So since we're
already on Router 5,

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let's just start here,
and see what things

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look like inside of the NSSA.

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If we do a show ip router
ospf, the first thing to note

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is that our externals
are now listed

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as NSSA Type 2 externals,
indicated by the N2.

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Other than that, they look
significantly similar.

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If we do a show ip
ospf database external,

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this will now be empty,
because Type 5 LSAs are not

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allowed in the NSSA.

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Therefore, there are
no external entries.

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Instead, we have
to ask for NSSA.

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These are going
to be our Type 7s.

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As we can see, these look
almost identical to the Type 5.

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The big difference here is
the Type 7/5 translation.

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The Type 7/5 translation
flag is essentially

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the ASBR asking the translator
to translate this to a Type 5

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when it receives this update.

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So the translator
will look at that,

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and will in fact convert
this to a Type 5.

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So I'm sure you can
probably imagine that this

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will almost always be set.

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There is however one exception.

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If this is an ABR
and an ASBR, it

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will think that it will have
already generated a Type 5.

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So I wouldn't need anybody
else to convert it for me.

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So that's the only time
you'll see this not be set.

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And that can
actually be a problem

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if the ABR is connected to say,
hey, discontiguous area zero.

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It would think it had already
translated this into area zero,

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and therefore tell, we'll
say the real translator,

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the one that is connected
to area zero properly,

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to not translate this.

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And this will say,
do not translate.

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And that could actually
create a problem,

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but discontiguous area
zeros cause problems

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in OSPF either way.

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So as long as we don't have
a discontiguous area zero,

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this system actually
works quite well.

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And our last thing here, we'll
do a show ip ospf border.

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And we'll notice that from
Router 5s perspective,

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nothing's really changed here.

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Router 2 is still an ASBR,
and it's of course still

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in the same area as Router 5.

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So of course, what's going
to change a little bit more

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significantly here is what this
all looks like on Router 7 now.

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So the first thing we'll
do is a show ip route ospf.

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This is actually something
that's really not

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going to change at all.

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Notice that our two entries
are still external Type 2.

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That's because they've
been translated.

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However, if we look at
the database detail,

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notice that the advertising
router is now 10.3.3.3

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for both of these entries.

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By the way, we'll verify this
further in just a moment,

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but that's a pretty
good indicator

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right there that Router 3
is in fact our translator.

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It's the one generating these
Type 5 LSAs for the Type 7s.

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Now the other
interesting thing here,

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is if we do a show
ip ospf border,

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Router 2 is no longer
listed as a border router.

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We don't actually see
the real ASBR here,

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because the ABRs are
generating the Type

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5 on behalf of the Type 7.

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So effectively, the
NSSA is actually

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hiding the ASBRs information
from the rest of the network.

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And of course,
since that's hidden,

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it also means that we're not
going to have a Type 4 LSA.

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So if we do our show
ip ospf database ASBR,

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we see here that it
is in fact empty.

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There is no Type 4
because I don't even

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know about the ASBR
inside the NSSA.

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Now, of course, we also want
to explore this on the ABRs

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as well, so let's
hop up to Router 1.

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So the first thing we can
do here is a show ip ospf,

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and begin area 100.

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Now, we can't exactly
highlight something

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that doesn't exist here.

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However, right under where
it says it is a NSSA area,

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it's not saying that it should
be doing the translation, which

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means it's not translating.

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In fact, we can verify this
further by saying show ip ospf

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database external
self-originating.

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And it's not generating
any Type 5 LSAs.

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So Router 1 is clearly
not the translator here.

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Let's go look at
Router 3 in contrast.

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So the same two
commands on Router 3.

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Notice, on this one, it says
that it's actually performing

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Type 7, Type 5 LSA translation.

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So that's a pretty
good indicator

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that we're a translator.

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We can also say show
ip ospf database

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external self-originating.

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And we see that it is in
fact generating those Type 5

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LSAs that are of
course being sent out

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to the rest of the network.

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And we saw them over on
Router 7 a moment ago.

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So we can see how to figure
out who the translator is.

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We see that our
Type 7s are in fact

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being translated to Type 5.

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And if I go to other
routers outside of the NSSA,

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they just look like
standard external routes.

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Now, one final thing I want
to point out on Router 5.

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We've talked a lot about how
the LSAs get either forwarded,

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or filtered, or translated.

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But an interesting
thing about NSSA

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is, if I do the show ip
route here again for OSPF

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on Router 5.

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I want you to note that
the default behavior here

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is to not generate
a default route.

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The reason for this.

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What if the external
route we're bringing

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in as a Type 7 is in fact a
default that we want to follow?

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It gives us the
opportunity to be

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able to follow an external
route that's not part of OSPF.

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If it put an automatic
route in here,

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then that would be a
little more difficult.

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We'll explore that a little
bit more in the next video.

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In this video, we simply
setup and configured

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a not-so-stubby
area, or an NSSA.

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We saw how they
still don't allow

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the external routes to come in.

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And we saw how it allows
external routes within the area

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to be passed through
as Type 7s, and then

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translated into Type
5s, and sent out

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into the rest of the network.

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So again, the
fundamental difference

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here between a
stub and an NSSA is

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that, although they both
don't allow externals

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from other areas,
the NSSA allows

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external routes to be
brought into the NSSA, where

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the stub does not.

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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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