[AUDIO LOGO]
So now that we've gotten some more detail on multiarea OSPF,
now let's jump in and look at our different LSA
types and the role that they play in our multiarea SPF
topology.
So we're going to start off, of course,
by looking at type 1 and type 2 LSAs that work within the area.
Then we'll look at our type 3 LSAs between the areas.
We'll look at our external LSAs, our LSAs for our ASBR--
so our summary ASBR entries.
And then we'll finally wrap up by taking a look at LSAs
that we use in a not so stubby area or an NSSA.
So we'll take a look at each of these.
And then in our next video, we'll
actually jump in and see these in action on the routers.
So for now, let's jump in and take a look
at the role of each LSA in the multiarea OSPF topology.
So our first LSA type is of course
going to be LSA type 1, otherwise known as the router
LSA.
So the type 1 LSA is generated by all routers in OSPF.
And as such, this serves as the fundamental building
block for our OSPF database.
This is what contains all the information
about the router and its links.
So this is going to include things like the router ID
to identify the router, as well as the current status of all
of its interfaces.
Now, since this is the detailed information for the database,
this is going to be contained within an area.
So if we were to, for example, pick on router 7 here,
it's type 1 LSA will be sent over here to router 8.
It would be sent up to router 6, to router 1,
and over to router 3.
However, it won't go any further than that.
Router 6 would not flood it into area 200.
Router 1 and router 3 would not flood it into area 100.
Type 1 LSAs are completely confined within the area.
And again, they're used to define the routers themselves.
Next is the type 2, or the network LSA.
Now, this is generated by the DR, or the Designated Router,
on a broadcast network type.
In our example, we actually have two broadcast network types.
So let's take a look at the one in area 0.
We have three routers connected to this--
router 6, router 7, and router 1.
These are all connected to this common subnet 50.0.167.0/24.
One of these routers will be elected as the DR.
Keep in mind that the default for this election
is to go with the router with the highest priority.
All routers, by default, have a priority of one.
So if we as administrators don't take control over this,
it will then go to the router with the highest router ID.
The router ID is generally based on its loopback interface.
So therefore, it would probably go
to the router with the highest loopback address.
In our example, that would, in fact, be router 7 here.
So let's go ahead and label that as our DR,
assuming that that's the router that would have won.
So if router 7 becomes the DR, that
means that router 7 is going to be the one
to generate this LSA.
This would get sent to router 3, to router 6, to router 1,
and of course, over here to router 8.
Once again, it gets flooded through the whole area.
This contains information about that multiaccess network
segment.
So this contains information about this 50.0.167.0/24
network.
Now, just like the type 1 LSAs, these
are also going to be contained within the area, which means,
once again, router 6 will not send this into area 200.
And router 1 and router 3 will not send this into area 100.
The main purpose of doing this is
so that all of the routers connected
to this multiaccess segment don't
have to generate the type 1 LSAs for that link individually.
So the idea is we send this information out.
The DR tells everybody else which routers are connected
to this multiaccess segment.
We'll see this in the next video when
we look at these in a little more detail.
Keep in mind, as well, in our example,
this same thing would happen for this segment over here
in area 100, which is also a multiaccess segment.
In other words, it's network type broadcast.
Next up is the type 3 LSA, or the summary LSA.
This is the LSA that's actually generated by ABRs to represent
the prefixes in this area.
So say, for example, router 7 has a loopback interface.
That interface will get sent with a type 1 LSA to router 6.
Since router 6 is in fact an ABR,
it will then generate a type 3 LSA into area 200,
representing router 7's loopback.
At the same time, router 7 is also
going to send this to router 1 and router 3 with type 1 LSAs.
And both router 1 and router 3 will generate a type 3 LSA
into area 100, again, representing
router 7's loopback.
This happens, of course, for every prefix in every area.
We're just picking on router 7's loopback.
But we would have the exact same thing
going the other direction.
So if we were talking about router 4's loopback,
for example, once again, it would go as a type 1
over here to router 3.
And router 3, of course, would generate that type 3
into area 0.
So again, this is going to be for all prefixes
in all directions between all the areas.
And that's what this point is that it's
used to advertise the prefixes from one area
into another area.
As we just demonstrated, this is going
to be flooded both into the backbone and into other areas.
Now, remember what we learned in our previous video
that when we get these type 3 LSAs in another area,
yes, we have to update our database,
either adding or removing the prefix.
And of course, that causes us to update our routing table,
either adding or removing the route.
But we do not run SPF, Shortest Path First,
or Dijkstra's algorithm in response
to receiving these type 3 LSAs.
Also keep in mind that although these are called summary LSAs,
it's referring to the fact that it's a database summary.
Don't look at this as a network summarization.
This is where we can create a network summarization,
but we have to do that manually as administrators.
Now, we can do that to help reduce or control
the flooding of these type 3 LSAs into other areas.
And we'll look at that in a future skill
when we get to summarization and filtering between areas.
But keep in mind that that's going
to be talking about controlling these type 3 LSAs.
By default, all prefixes are sent into all other areas
with a type 3 LSA.
Next is the type 5 LSA, or the external.
Maybe you're thinking to yourself, wait a minute,
what happened to the type 4?
We're going to get to the type 4.
But since the type 4 is to support the type 5,
it actually sort of makes sense to talk about the type 5 first,
even though, numerically, that's not exactly in order.
So let's talk about the type 5 first.
This is going to be generated by the ASBR That's
doing redistribution into OSPF.
In our example, that would be R2.
This is our ASBR.
And what it's doing is taking the EIGRP prefixes
and redistributing them into OSPF.
This is going to come in as a type 5.
So again, this is going to be advertising those prefixes
from an external protocol in this example, such as EIGRP,
into the OSPF domain.
This is then going to be flooded through the OSPF domain.
Well, what that means is when R2 sends this in,
it's going to go to R4.
It's going to go to R5.
It's going to go to R1.
4 is going to send it over here to 3.
Router 1 is going to send it into 6.
6 is going to send it to 10.
1's going to send it to 7, and so on.
Of course, 7 sends it to 8.
3 sends it to 7.
You get the idea.
It's going to be flooded everywhere.
Now, the reason this is important to understand
that we're flooding the type 5 LSA
unaltered through our network is this
helps us understand why we need the type 4.
For example, in this case, this being flooded from router 2,
it's going to have an advertising router
ID of router 2's loopback, which, in our example,
would be 10.2.2.2.
That's all fine when it gets to router 1 and router
3 and router 4, router 5 even, because they're
in the same area as router 2.
And they would have a type 1 LSA for router 2.
But what happens when that information
goes into another area, such as, in our case, area 0?
It gets to say router 7.
Router 7 has no idea where router 2 is.
And that is what we need the type 4 LSA for.
Let's take a look.
So again, our next type is the LSA type 4.
This is called an ASBR summary.
Now, this is going to be generated
by the ABRs for the area, much like a type 3, actually.
However, instead of advertising prefixes
from one area to another, it's instead
advertising the router ID of the ASBR.
Remember what we said when we were talking about the type
5's.
When this type 5, again, reaches, say, router 7,
it's going to have on it the router ID of 10.2.2.2.
The problem is router 7 literally has
no idea where 10.2.2.2 is.
So how could I possibly use this type 5 LSA
if I don't know where the advertising router is?
Because it's in a different area.
And this is where the type 4 comes in.
The type 4's are going to be generated by the ABRs
and sent out into the area, advertising the router ID
of the ASBR.
This will get forwarded through the whole network.
Once I receive this type 4 LSA--
and again, everybody will get it because it is flooded,
just like the type 3's into the backbone and the other areas.
Now, all of a sudden, router 7 knows exactly where
10.2.2.2 is.
It's no longer a question.
And now router 7 can use the type 5 LSA.
Our next LSA type is the LSA type 7.
This is NSSA external.
Now, this one is going to be generated by an ASBR
but specifically in a not so stubby area.
So for example, let's say that area 200 up here is an NSSA.
Router 10 has a loopback interface on it
that it is redistributing into OSPF.
This will be sent as a type 7 to get it through the NSSA.
Now, once it gets to router 6, which is acting as the ABR--
remember, router 10 is acting as the ASBR in this case--
router 6, being the ABR, will then
translate this and send it into the rest of the network
as a typical type 5 LSA.
So again, this is to advertise the prefixes
from that external protocol into the NSSA itself.
And this one is contained within the NSSA
because it's translated to a type 5 at the first ABR.
Now, I realize we haven't really gotten into OSPF area types
yet.
And you may not fully understand what an NSSA is at this point.
And that's OK.
I only bring it up here so that we've
at least seen that there's something called a type 7 LSA
and that we use it in an NSSA.
NSSA will be covered in a future skill.
And we will be jumping into it in much more detail.
Now, you might have noticed as we
were going through our discussion
there that we actually skipped quite a few LSAs.
For example, we skipped right over type 6.
And that's because type 6 is used for multicast.
Quite frankly, it's not a very popular implementation.
And Cisco has actually never supported multicast with OSPF.
We're not talking about OSPF using multicast.
We're talking about multicast using OSPF.
Type 8 is used for BGP interaction
in the case of OSPF version 2.
And in the case of OSPF version 3,
these are used for link LSAs.
Remember that OSPF version 3 is specifically for IPv6.
As such, it has to deal with link-local addresses.
Type 8's are used to carry link-local information just
between directly connected neighbors.
Type 9's, in the case of OSPF version 2,
these are just referred to as opaque LSAs.
Opaque LSA is really just a way of saying
that it has no specific or specified function in the RFCs.
Now, it can be used for other things.
And we'll see some uses for these opaque LSAs
as we move through this entire course.
Now, the type 9's in the case of OSPF version 3,
these are your interarea prefixes.
The type 9 LSA in IPv6 actually carries
information that's usually carried in a type 1
in OSPF version 2.
One of the big changes that they made in OSPF between version
2 and version 3 is they actually separated
the advertising of the topology from the advertising
of the prefixes.
So in OSPF version 2, the prefixes are advertised,
of course, in a type 1, where in OSPF version 3 for IPv6,
we actually separate that out into these type 9's.
So the type 1's advertise the topology.
The type 9's advertise the prefixes for that topology.
Now, you might notice that type 10 and type 11 for both OSPF
version 2 and OSPF version 3, they're
all listed as opaque just because, again, they
don't have a specific use according to the OSPF RFCs.
In this video, we went through and we
talked about all of the different OSPF LSA types,
their general function on our network,
and where they get flooded as far as whether they
cross area boundaries, what routers generate them,
and so on.
So hopefully, we now have a clearer understanding
of how these LSAs and the different types
operate in our OSPF network.
I hope this has been informative for you,
and I'd like to thank you for viewing.