In this video, you and I are going
to do a quick review of the OSPF area types,
the devices that connect those areas together,
and also a review of of some of the LSAs
that we're going to find in OSPF version 2 for IP version 4.
And so let's start off by listing a few of the LSA types
that we're going to find in OSPF version 2.
And that would be a type 1, which is a router LSA.
And let's also take a moment to remember together
what that does. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
what that does.
A router LSA is generated by each router in the area who's
participating in OSPF.
So they're going to generate a type 1.
A type 2 LSA, they call that the network LSA.
That's generated by a designated router on a segment
if-- and only if--
there is a another OSPF speaker on that same network segment.
If we have a designated router, like, R1 on this segment
right here, with nobody else there,
he's not going to bother creating an LSA type 2
because he's the only guy there.
That information about that network
is already in his LSA type 1. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
is already in his LSA type 1.
So it doesn't bother creating an LSA type 2 for a segment
where he's the only guy on that segment.
And then we have an LSA type 3, which is called a summary LSA.
And a summary LSA is generated by the area border routers.
We'll point those out here.
So this is an area border router.
It's between the backbone and another area.
That's where ABRs exist.
And also here, over R4 is an ABR.
And all the routes that are on in area zero and all the routes
that are here in area two, what's going to happen WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that are here in area two, what's going to happen
is the area border router is not going
to forward those LSAs specifically over
into the other areas.
What it will do, it will generate a summary LSA
for each of those networks.
It will generate the summary LSA, an LSA type 3,
and inject that down.
So R5 and R7, when they learn about these LSA type 3s,
those will show up as inter-area routes in the routing table
on those devices.
They're summaries that are generated by the area border
router about the rest of the network for the benefit
of these routers right here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
of these routers right here.
It also goes the other direction.
R3 is going to generate LSA type 3s regarding
all these networks in area 1 for the benefit of the backbone
area here as well.
Then we have a type 5.
And you might say, hey, Keith, 4 comes after 3.
Well, let's do a type 5 first.
And then we'll talk about why we would even need to type 4.
A Type 5 is an external.
So if we have a router, let's pick on R1.
And if R1 is bringing additional routes into OSPF,
but they're not native OSPF routes.
It's doing redistribution, let's say
from EIGRP or from static routes or somebody else. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
from EIGRP or from static routes or somebody else.
And it's redistributing them into OSPF.
Those redistributed routes are coming in as an external
or a type 5 LSA.
And the type 5 is one of the few LSAs
that actually makes it all the way through the network.
So if R1 brings in LSA type 5s, it's
going to be replicated here in the backbone area.
They're also going to be forwarded by the ABRs
to the other normal areas, in this case, area 1 and area 2,
unless we start getting stubby or not so stubby with them.
We'll take a look at that in just a moment. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We'll take a look at that in just a moment.
So that's the LSA type 5.
And then secondly, if we have an LSA type 5,
let's say we have a route for 10.0.11.0/28 and that's been
redistributed on R1.
And that route shows up here.
Well, these routers don't know anything
about reaching R1 because R1 is the ASBR, the autonomous system
boundary router that redistricted
those external routes in.
And so what the area border router will do,
it will generate an LSA type 4 that describes, basically, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
it will generate an LSA type 4 that describes, basically,
how to get to this guy, R1, who's the ASBR.
So we'll just put describe ASBR.
And that's what it does.
So that's injected by the area border router.
If it's passing along the LSA type 5s,
they'll also generate an LSA type 4 for that area.
So they'll have a clue on how to reach that guy, that ASBR.
And the rule for OSPF is that we're
going to have a backbone area.
And then we're going to have other areas that are connected
to the backbone by an ABR.
But you you can't go two or three layers deep. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But you you can't go two or three layers deep.
For example, you can't have area--
we'll draw it over here-- area 5 and some router--
let's call it router 6-- that's connected to this network.
You can't have that in area 5 because area 5 can't be, like,
hanging off of area 2.
Area 5 could if it had an ABR that was also
connected to the backbone area.
That would work.
But you can't have, without some trickery,
which we'll talk about in a separate video
with virtual links, you can't have an area, an OSPF area,
that doesn't have an ABR that's directly connected WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that doesn't have an ABR that's directly connected
to the backbone area.
So that's a big no-no and it would not
work as far as full reachability if we did include some networks
in an area that didn't have an ABR that was also
directly connected or logically connected to the backbone area.
So let's discuss the idea of these external LSAs for LSA
type 5s.
Let's discuss preventing them from going into one
of our areas, area 1 or area 2.
If we wanted to avoid having to learn those routes inside
of area 2, we have two options. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
of area 2, we have two options.
If we want to avoid those routes being propagated by the area
border router into this area, we could go ahead make this area
a stub area.
And what a stub area does, it simply says, I hate--
I'm going to use the word hate; it's a little bit strong--
I hate LSA type 5s.
I hate the external routes and I won't let them in.
Blah.
He's just going be totally prejudiced against LSA type 5s.
And as a result, the routers in this area
won't have to learn anything about LSA type 5s WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
won't have to learn anything about LSA type 5s
because the area border router doesn't let them in.
And it won't generate LSA type 4s
as well because they're no longer needed.
Because these routers, if they don't have the LSA type 5s,
they don't need the LSA type 4s that describe
how to get to the ASBR.
Now, in addition to that, if we create
a stub area, what's going to happen
is by default this area border router, if this whole area is
a stub, it's going to generate a default route.
And that default route's handy in case
these routers ever need to forward packets, like,
to these external networks.
They can know to forward it up through the ABR who generated WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
They can know to forward it up through the ABR who generated
a default route into the area.
And it gets better.
What if we wanted the ABR not to send all the rules for area 1
and for area 2 as well.
Don't send any of those summary routes into this area.
To do that, we could go ahead, make a totally stubby area.
So totally stubby area simply says, oh, I'm
discriminating now.
I hate LSA type 5s. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I hate LSA type 5s.
And I'm also going to discriminate against LSA type
3s.
So it just won't bother generating
if we have 200 routes over here in the backbone
area and the other areas.
Instead of generating the 200 summary LSAs
to educate this area about those networks,
it simply won't send those LSA type 3s in.
So the routing tables just get reduced.
However, there is one exception, the default
route that the ABR is generating,
it is an LSA type 3.
So there's going to be one LSA type 3, which WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So there's going to be one LSA type 3, which
is the default route.
And, again, all this content we've
covered in greater details including
labs in the EnCORE course.
So if these concepts are brand new to you,
I would encourage you to take a peek
at those sections of the EnCORE course
right here at CBT Nuggets.
Now, another challenge that comes up
is, well, what if we have a fluid design
and we have to make some changes?
If this is a stub area, which doesn't like LSA type 5s, what
if, all of a sudden, we need to redistribute some routes here
on R6 and redistribute them into OSPF. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
on R6 and redistribute them into OSPF.
Well, redistributed routes that go into OSPF
come in as LSA type 5s.
And if this is a stub area, you can't do it.
It won't allow LSA type 5s in.
So to have kind of the same benefits of a stub area,
but also able to inject routes on a router in that area,
we're going to use an--
I'm going to go ahead and do it down here--
we're going to make this an NSSA,
it's a not so stubby area.
And not so stubby area is like a stub area in the fact WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that it doesn't allow LSA type 5s.
However, if we wanted to do redistribution in that area,
if it's an NSSA, we can do the redistribution.
And they'll come in as an LSA type 7.
And those are allowed.
Hey, it's not a type 5.
We're not discriminating.
It'll come in as an LSA type 7 when we do redistribution.
And when the ABR sees that, it will go ahead
and convert the LSA type 7 to an LSA type 5
as it advertises that route to the rest
of the network-- in this case, through the backbone.
And then area 1 would also see that as well. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And then area 1 would also see that as well.
So that's an NSSA area, still not allowing type 5s,
but does allow LSA type 7.
And let's see.
There was one other comment.
Oh, yeah, yeah.
If you also want to go ahead and restrict this area,
if it's an NSSA area, to restrict it
from having all the other summary LSAs
that this ABR is normally generating,
we can make it a totally NSSA area a totally not so stubby
area.
And what that does, it allows LSA type 7s in that area.
It restricts LSA type 5s.
It also does not, the ABR does not, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It also does not, the ABR does not,
inject these summary LSAs from the other networks.
But it will add a default route for the benefit
of this area, which allows these routers with the default route
to forward traffic out of their own local area
in hopes to reach some destination
network that the rest of the OSPF domain knows about.
So to add to the list of things that OSPF devices and routers
have to agree with before they'll form an adjacency,
is they have to agree on the area.
So they'd have to agree on, for example,
these routers up here all have to agree, hey, this is area 0. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
these routers up here all have to agree, hey, this is area 0.
All right.
Thumbs up.
Yes it is.
Great.
These routers down here have to agree that all these networks
are in area 1.
And all these routers have to agree
that all those networks in that area 2 are an area 2.
So they have to agree on the area number.
They also have to agree on the area type.
So if R5 believes this is a stub area.
And R7 believes it's a normal area, not a stubby area,
they won't become neighbors.
And the same thing over here for not so stubby.
If R6 thinks it's a stubby area and R8
thinks it's a not so stubby area, no bueno. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
thinks it's a not so stubby area, no bueno.
It's not going to go.
And the key is, and we'll close it off of this,
the key is that the ABRs themselves
are the key to whether it's a totally or not--
so whether it's totally not so stubby area or a totally
stubby area.
Because it's R3 and R4 in this topology,
the ABRs that would be injecting those additional summary LSAs.
And if you use the word "totally" with stub
or "totally" with not so stubby, they simply
don't add those detailed summary type
3 LSAs to the rest of the area. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
3 LSAs to the rest of the area.
They'll just advertise their default route.
And that reduces the routes in either a stubby area
or a not so stubby area if the ABR has a no-summary option.
So for labs on all of that and the build-up
and the greater explanation for all that, again,
I'd encourage you to check out EnCORE.
What we're going to do next, in the next video,
let me bring up the big camera.
In the next video, I'd like to give you
a lab where you can get familiar with this topology.
And then once we're familiar with this OSPF version
2 for IPv4 topology, then we'll go ahead in the next couple WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
2 for IPv4 topology, then we'll go ahead in the next couple
labs after that, we'll inject problems and actually do
some hands-on troubleshooting.
Now, as before, instead of just doing area mismatches and area
issues, I'm also going to throw in a few additional tweaks
in OSPF.
And that way when you go into the lab,
you can just put your full thinking cap on
and troubleshoot adjacencies and troubleshoot connectivity
and get that hands-on practice.
All right.
So I'll see you in the next video.
We'll introduce the working lab.
And we'll do that right now.
Meanwhile, I hope this has been informative for you. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Meanwhile, I hope this has been informative for you.
And I'd like to thank you for viewing.