[MUSIC PLAYING]
Next up, let's jump in and take a look
at the not-so-stubby area, or the NSSA.
Remember, of course, that the big advantage to NSSA
over the stub area is, although it does still block Type 5
LSAs, and therefore doesn't allow
external routes from other areas,
it does allow us to bring in external routes into the NSSA.
We take that through the NSSA with a Type 7.
And then that gets translated to a Type 5 on one of the ABRs.
So we're going to jump in and take a look
at this in much more detail.
As always, let's start on the topology,
and talk about exactly what we're going to be configuring.
In this demonstration, we will now be focusing on area 100.
Since area 100 already has a router doing redistribution,
Router 2, which is of course pulling in our routes
from EIGRP, we already have an ASBR.
So we've already seen what happens if we
try to convert this to a stub.
We of course know that Router 2 would stop being an ASBR.
So instead, of course, we're going to choose NSSA.
So we'll take a look at how this works.
We'll see how these LSAs come in as a Type 7.
And then we'll see how either Router 1 or Router 3
becomes our translator, and translates that
into a Type 5 going out into the rest of the environment.
And we'll explore how to figure out which
one became the translator.
So let's jump over to our command line.
Let's begin by exploring the routing table on Router 7.
So if we look at our routing table here.
And what we see here is we are in fact
getting two external Type 2 routes coming from the EIGRP
environment.
If we were to say show ip ospf database external to look
at those just a little closer, what we'll see
is they're both being advertised by Router 2.
That means that we should actually
understand that Router 2 is in fact a border.
We do a show ip ospf border.
We do in fact see that Router 2 is an ASBR.
So we're seeing that information.
If we do a show ip ospf database ASBR,
we should actually have an entry for Router 2
being sent to us from both of our ABRs, Router 1 and Router
3.
Remember, this is the Type 4 LSA that's supporting the Type 5.
We're looking at this because, as we convert to an NSSA,
we're going to see that this structure is
going to change significantly once we have translators
involved.
Now let's also do some verification
from a router inside of area 100.
So let's go to Router 5.
Let's start with our show ip ospf database external.
Once again, we see the advertising router is 10.2.2.2.
We do our show ip ospf border.
We again see that Router 2 is in fact an ASBR.
Also note, by the lowercase i at the beginning,
this is an intra area route, which
means we're learning about this ASBR within our own area.
It's from a Type 1 LSA, not a Type 4.
We can prove this by asking for the ASBR entries.
And there aren't any.
There wouldn't be, because it's within the same area.
So now that we've seen what it looks
like before we convert to an NSSA, let's go ahead
and do that conversion.
We'll start on Router 1.
So we'll simply go into the routing process,
under area 100, and simply configure NSSA.
We will see neighborships going down as we can figure this.
That's going to be fine, as long as they all come back up.
Let's do Router 2 next.
On IOS XR, we can also configure this by saying area 100 NSSA.
You can put it on the same line or two different lines.
If we do a show run however, notice
that it puts it on the two separate lines,
regardless of how we type it in.
So next over to Router 3.
This is the same configuration, so we'll just
go ahead and paste it in.
And the same with Router 4.
Router 5 is running IOS XE, but it's still
the same basic configuration.
It's still just area 100 NSSA.
The only difference is, you do put it on just one line here.
You don't put it on two separate lines.
We'll go ahead and exit out of configuration.
And we see that our neighborships are already
coming back up.
So now that we've done our conversion to NSSA,
now let's jump back over to our Router 7, our Router 5,
and let's take a look at how these things have changed now
that it's an NSSA.
So since we're already on Router 5,
let's just start here, and see what things
look like inside of the NSSA.
If we do a show ip router ospf, the first thing to note
is that our externals are now listed
as NSSA Type 2 externals, indicated by the N2.
Other than that, they look significantly similar.
If we do a show ip ospf database external,
this will now be empty, because Type 5 LSAs are not
allowed in the NSSA.
Therefore, there are no external entries.
Instead, we have to ask for NSSA.
These are going to be our Type 7s.
As we can see, these look almost identical to the Type 5.
The big difference here is the Type 7/5 translation.
The Type 7/5 translation flag is essentially
the ASBR asking the translator to translate this to a Type 5
when it receives this update.
So the translator will look at that,
and will in fact convert this to a Type 5.
So I'm sure you can probably imagine that this
will almost always be set.
There is however one exception.
If this is an ABR and an ASBR, it
will think that it will have already generated a Type 5.
So I wouldn't need anybody else to convert it for me.
So that's the only time you'll see this not be set.
And that can actually be a problem
if the ABR is connected to say, hey, discontiguous area zero.
It would think it had already translated this into area zero,
and therefore tell, we'll say the real translator,
the one that is connected to area zero properly,
to not translate this.
And this will say, do not translate.
And that could actually create a problem,
but discontiguous area zeros cause problems
in OSPF either way.
So as long as we don't have a discontiguous area zero,
this system actually works quite well.
And our last thing here, we'll do a show ip ospf border.
And we'll notice that from Router 5s perspective,
nothing's really changed here.
Router 2 is still an ASBR, and it's of course still
in the same area as Router 5.
So of course, what's going to change a little bit more
significantly here is what this all looks like on Router 7 now.
So the first thing we'll do is a show ip route ospf.
This is actually something that's really not
going to change at all.
Notice that our two entries are still external Type 2.
That's because they've been translated.
However, if we look at the database detail,
notice that the advertising router is now 10.3.3.3
for both of these entries.
By the way, we'll verify this further in just a moment,
but that's a pretty good indicator
right there that Router 3 is in fact our translator.
It's the one generating these Type 5 LSAs for the Type 7s.
Now the other interesting thing here,
is if we do a show ip ospf border,
Router 2 is no longer listed as a border router.
We don't actually see the real ASBR here,
because the ABRs are generating the Type
5 on behalf of the Type 7.
So effectively, the NSSA is actually
hiding the ASBRs information from the rest of the network.
And of course, since that's hidden,
it also means that we're not going to have a Type 4 LSA.
So if we do our show ip ospf database ASBR,
we see here that it is in fact empty.
There is no Type 4 because I don't even
know about the ASBR inside the NSSA.
Now, of course, we also want to explore this on the ABRs
as well, so let's hop up to Router 1.
So the first thing we can do here is a show ip ospf,
and begin area 100.
Now, we can't exactly highlight something
that doesn't exist here.
However, right under where it says it is a NSSA area,
it's not saying that it should be doing the translation, which
means it's not translating.
In fact, we can verify this further by saying show ip ospf
database external self-originating.
And it's not generating any Type 5 LSAs.
So Router 1 is clearly not the translator here.
Let's go look at Router 3 in contrast.
So the same two commands on Router 3.
Notice, on this one, it says that it's actually performing
Type 7, Type 5 LSA translation.
So that's a pretty good indicator
that we're a translator.
We can also say show ip ospf database
external self-originating.
And we see that it is in fact generating those Type 5
LSAs that are of course being sent out
to the rest of the network.
And we saw them over on Router 7 a moment ago.
So we can see how to figure out who the translator is.
We see that our Type 7s are in fact
being translated to Type 5.
And if I go to other routers outside of the NSSA,
they just look like standard external routes.
Now, one final thing I want to point out on Router 5.
We've talked a lot about how the LSAs get either forwarded,
or filtered, or translated.
But an interesting thing about NSSA
is, if I do the show ip route here again for OSPF
on Router 5.
I want you to note that the default behavior here
is to not generate a default route.
The reason for this.
What if the external route we're bringing
in as a Type 7 is in fact a default that we want to follow?
It gives us the opportunity to be
able to follow an external route that's not part of OSPF.
If it put an automatic route in here,
then that would be a little more difficult.
We'll explore that a little bit more in the next video.
In this video, we simply setup and configured
a not-so-stubby area, or an NSSA.
We saw how they still don't allow
the external routes to come in.
And we saw how it allows external routes within the area
to be passed through as Type 7s, and then
translated into Type 5s, and sent out
into the rest of the network.
So again, the fundamental difference
here between a stub and an NSSA is
that, although they both don't allow externals
from other areas, the NSSA allows
external routes to be brought into the NSSA, where
the stub does not.
I hope this has been informative for you
and I'd like to thank you for viewing.