- We mentioned earlier
when we were talking about OSPF terminology,
what an LSA was.
An LSA, we said was information
that was used to help build a Link-State Database.
And that information that LSA was sending inside
of an LSU packet, Link State Update.
And when we're talking about LSAs,
we need to understand that there are different types
and it's important that you understand
the different LSA types
we're going to be talking about in this video.
And let's start our discussion of LSA types
with this basic topology,
and then we're going to build on to it a little bit.
Here we've got a couple of OSPF areas.
And we learned that the name that we give to a router
that has an interface in more than one area,
that's called an ABR or an Area Border Router.
But within each area, area zero and area one in this case,
we have a Link-State Database for each area.
So the ABR, it actually has a couple
of Links-State Databases.
One for area zero and one for area one.
Let's talk about how LSAs are used to educate everybody
in an area about the networks available in that area
and in other areas.
To begin with, let's talk about a Type 1 LSA,
that's called a Router LSA.
If a router is connected to a network
and we assume it's connected to
at least one network segment,
otherwise it's not going to be talking to any other routers,
but a router is going to use Type 1 LSAs
to advertise directly connected networks.
So in this case, R1 is going to be advertising that link
between itself and R2.
R2 is going to be advertising the same thing,
but everybody knows it's the same link.
So we're going to have a single Type 1 LSA
in our Link-State Database representing that link
between R1 and R2, same thing in area one,
we've got one LSA Type 1,
representing the network between R2 and R3.
By the way, notice that there sort of a lightning bolt icon
between R2 and R3 that indicates a WAN link.
And specifically, I want you to know that
I'm representing a point-to-point WAN link.
And on a point-to-point link,
we do not elect a day Designated Router, a DR.
That's something that we do on broadcast networks,
like ethernet networks.
We elect a DR and a Backup Designated Router or BDR.
But on that point-to-point WAN segment,
there's really no point in electing a DR
because there's no issue with
forming a full mesh of adjacencies.
And the reason I emphasize
that there's no DR on that segment
is that having a DR on a segment is one of the criteria
for generating a Type 2 LSA.
We're going to have a Type 2 LSA
to represent every network segment within an area.
If that network segment meets two conditions,
condition one is the network has to be a transit network.
In other words, it's interconnecting
a couple of OSPF speaking routers.
It's not just going off to a switch
and connecting to end users,
but it's linking together a couple of a wispy of routers.
The other criteria is that
there be a DR elected on that link.
Now we're assuming we have some sort of Ethernet interface,
like a Gig Ethernet Interface
coming out of all R1 going over to R2.
So a DR would be elected there,
but remember a point-to-point WAN segments
like we have between R2 and R3,
not going to be electing a DR there.
So there's no Type 2 LSA to represent that WAN link
between R2 and R3.
Next step, let's talk about a Type 3 LSA.
And I think a Type 3 LSA has an unfortunate name,
because I think it can lead to some confusion.
Specifically, I mean that a Type 3 LSA
is called a Summary LSA.
And oftentimes in networking, when we hear the word Summary,
we think of network summarization,
we look at all these networks and we say,
Hey, they've got the first 19 bits in common, for example.
And we'll advertise just one network
with a 19 bit subnet mask.
That is not what we're talking about here.
In this context, a Summary refers to summary information.
Remember within an area we know about how every network,
every router is interconnected with another,
we're able to run the Dexter algorithm
on this very detailed map with cost of all those links,
but we might want to get to a network in another area.
Do we need a map of that other area?
No, that would kind of defeat the point of having another area
to break up the Link-State Databases.
So instead we're going to have the Area Border Routers,
and summarized information,
instead of saying, here's a map to area one,
R2 is going to tell all the routers in the area zero,
here is a listing of Network Scenario one.
So we're not giving any sort of a detailed map.
We're saying, here's a list
of networks available in the area one.
And if you want to get to one of those networks,
come to me and I'll forward you on to that network.
And for every network we have in any area,
there's going to be a single Type 3 LSA
that sent into the other area to advertise that network.
But let's say that this is the network of a company
that merges with another company,
and this other company instead of running OSPF,
maybe they were running a EIGRP,
and we need to connect into that Autonomous System.
Well, a router that connects between Autonomous Systems
is called an ASBR, an Autonomous System Boundary Router.
And we're receiving all these EIGRP networks
coming in from the EIGRP cloud.
And R1 is going to advertise each of those EIGRP networks
with a Type 5 LSA.
So it's going to send all these Type 5 LSAs into area zero,
but when R2 advertises those Type 5 LSAs into area one,
it needs to include an extra piece of information.
For example, R3 sees a Type 5 LSA,
and R3 says, yeah, I'd really like to go to that network,
but I don't know what IP address is going to get me
out of my Autonomous System into that network.
So R2, since it does know about the ASBR,
it's sharing an area with the ASBR,
R2 is going to send a Type 4 LSA.
A Type 4 LSA is called a Summary ASBR LSA,
that says to another area,
Hey, here is how you get to the ASBR.
Here's the IP address
of the Autonomous System Boundary Router
that you'll need to communicate with,
if you're wanting to reach one of those networks,
I just advertise to you with a Type 5 LSA.
But as we look at this, we might be concerned that
there are a lot of EIGRP routes being flooded
into our OSPF Autonomous System.
Instead of having all those Type 5 LSAs going to area one,
we could look at this typology and say,
well really, if anybody in area one
wanted to get to an EIGRP route,
all it would have to do is go to R2
and R2 would get it there.
So instead of sending all those individual Type 5 LSA,
what if we do this?
What if instead, we send a default route using a Type 3 LSA.
We can say, if you don't know explicit information
about how to get to this network, go here.
So R3 is going to say, I don't have explicit information
about how to get to this network
that lives in the EIGRP Autonomous System.
I haven't learned about it
via one of the Type 3 LSA is coming from area zero.
So I'll use my default route
that I learned via this Type 3 LSA,
which is going to get me over to the ASBR and out to EIGRP.
I mean, there's no other place to go.
It must be out there,
if I don't have that network learned locally
or in areas zero.
So if we do that, we eliminate the need to flood
all of those Type 5 LSAs into area one,
and even the Type 4 will say,
doesn't need to go into area one.
Instead, we're going to send a Type 3 default LSA.
And this is called a Stub Area,
implying that this area is just hanging off
of the OSPF backbone area, area zero.
It's not connecting us to any other areas.
It's not connecting us to any other Autonomous Systems.
It just kind of hanging off on the edge.
It doesn't get us anywhere else.
And in this Stub Area,
we're still receiving individual Type 3 LSAs
from networks in area zero.
A lot of people miss that, make sure you hear me on that,
that if we had 10 networks in areas zero,
we would be sending 10 Type 3 LSAs
advertising those networks.
In addition to those 10 Type 3 LSAs,
we would still be sending the Type 3 default LSA,
representing all those EIGRP routes.
And this is a Stubby Area.
But if we think about it,
if the Stubby Area really only has one way
to get out to the rest of the world,
including an area zero networks,
why don't we just send that single Type 3 default LSA.
That's going to reduce the number of Type 3 LSAs
that we need to flood into area one.
And if we do that, this Stub Area becomes
a totally Stubby Area.
Now we're sending this default Type 3 LSA into area One
saying, if you don't have it in your area, go here.
But let's take this concept a step further.
And to do that, let's back off just a little bit,
let's go back to where we had a Stub Area.
Remember we're sending individual Type 3 LSAs
from area zero into area one.
Let's say we had a Stubby Area,
and then we had another company merger
and R3 connected to a network
that was running rip as its routing protocol.
So now R3 has become an ASBR
and this previously Stub Area is no longer very stubby.
In fact, it's literally called a Not-So-Stubby Area
or NSSA for short.
But there's a rule with a Stub Area,
and that rule is, thou shall not send a Type 5 LSAs
into a Stub Area.
Remember those Type 5 LSAs, those are AS External LSAs.
We send one of those for each network
in that external Autonomous System.
And since we're not allowed to send Type 5 LSAs
into area one from R3,
I think this was sort of cheating personally,
but here's what happens, R3 says,
huh! I've got all these RIP routes,
and I'd like to send them into area one using Type 5 LSAs,
but area one is that pesky Stub Area.
So how do I do that?
Because I'm not allowed to send Type 5 LSAs,
Oh I know, I'll still send the information,
but I'll just call the Type 5 LSA something else.
I'll say, they're a Type 7, that's it.
Type 7 LSAs, isn't that cheating, I kind of think it is,
but it sends those network advertisements
into area one using Type 7 LSAs.
And those go over to R2, and R2 says,
yeah, I see what you're doing.
These are networks in another Autonomous System.
They should be advertised with Type 5 LSAs.
So R2, as it takes those RIP networks
and sends them into areas zero,
it says, I'm going to take those individual Type 7 LSAs
and I'm going to send them into area zero as Type 5 LSAs
like they should have been to begin with.
And of course we have to send information
about how to reach R3,
the Autonomous System Boundary Router.
We have to send that into areas zero,
so routers like R1, they would know
how to get to those RIP routes.
But remember we took a step back to a Stubby Area
and with a Stubby Area, we still have the issue
that we're sending all those individual Type 3 LSAs
for area zero networks into area one.
Well, let's make it a little bit better.
Let's get back to where we had a totally Stubby Area
where we just send a single Type 3 default LSA,
but what does that do to our Not-So-Stubby Area?
Well, if we combine our individual Type 3 LSAs
into that single Type 3 default LSA going into area one,
we now have not making this up.
It's called a Totally Not-So-Stubby Area, a Totally NSSA.
And that's a look of the different LSA types
you might see in an IP version 4 network.
Now to get a better handle on this,
let's go out to some live gear right now,
and let's look at the Link-State Database and a router
and see if it matches what we predict
will be in that Link-State Database.
Let's take a look at (mumbles)
and see if we can predict the different LSA types
that are going to be present
in the OSPF Link-State Database of router R5.
Router R5 It is not an ABR or an SBR,
It's just sitting inside of OSPF area zero,
but we do have another area.
So we'll be getting some Type 3 LSAs from area one,
and we have EIGRP is another Autonomous System.
So we'll probably be getting some Type 5 LSAs from EIGRP,
and we'll probably get a Type 4 LSA
telling us how to get to the ASBR, which is R2.
Well, let's take a look.
Let's go through the LSA types, one at a time.
First of all, how many Type 1 LSAs are we going to have?
Those router LSAs.
Well in areas zero, there are only two segments.
The segment between R4 and R5
and the segment hanging off of R5.
So I expect to see two Type 1 LSAs.
What do you think about Type 2 LSAs?
Well, to be a Type 2 LSA, we had to be a transit network.
So R4 to R5 would qualify, and R5 out to that switch,
switch SW2, that would not qualify.
Even though DRs would be elected on both segments,
there are two criteria.
It has to be a transit link
and it has to be a link on which a DR would be elected.
So I'm expecting, we'll see one Type 2 LSA
and Type 3 LSAs, those Summary LSAs,
that's going to be an announcement
of a network available in another area.
Area one specifically R4, RABR is going to report to us
that there are two network segments in area one,
the segment between R2 and R3
and the one between R3 and R4.
So I'm expecting to see two Type 3 LSAs
and we've got two network segments
in the EIGRP Autonomous System.
The one between R1 and R2,
and the one connecting from R1 to SW1.
So I expect to see two Type 5 LSAs,
but we just have one ASBR, router R2,
So I predict we'll have one Type 4 LSA
to point us to the ASBR of R2.
And on the router R5,
We can do a show IP OSPF database command,
and we can see all the LSAs.
For router LSAs, the Type 1 LSAs as predicted,
we have two one for each segment
and area zero for the Type 2 LSAs.
We only had one qualifying network segment,
the one between R4 and R5 and we see that.
For the Summary LSAs, these are networks available.
And area one, we see these two networks,
they are available in that other area, area one.
And in order to get to the EIGRP Autonomous System,
we have to know how to get to the ASBR that gets us there.
And here is the IP address,
all of that Autonomous System Boundary Router,
and here's the Advertising Router
R4 RABR that can get us to the ASBR.
And we had two network segments
in the EIGRP Autonomous System.
We had network 192.0.O.O and 192.168.1.O
And we see those present in the Link State Database as well.
And lets look at some common OSPF LSA types.