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So now, let's jump in and talk about multi-area OSPF.
This is going to be used two address our scalability issues
with OSPF.
Now understand, this still won't scale as large as something
like BGP.
It still can't carry hundreds of thousands or millions
of routes.
However, it will let us scale it up to a much larger environment
than we could with just a single area.
It also brings to the table the ability
to do route summarization.
Keep in mind as we talked about in the previous video,
all of the routers in an area have
to have the same exact link-state database.
And, of course, a side effect of that is I can't have one router
have a /16 for a network while another router has a /24.
So since they have to have identical databases,
we cannot summarize within an area.
So we have to have multi-area OSPF in order
to do proper summarization.
That also implies that our summarization
points in our network also have to fall on area boundaries.
So let's go ahead and take a look at some of these things.
We'll start by taking a look at some
of the attributes of multi-area OSPF.
And then, of course, we'll jump in
and take a look at what our databases look
like with multiple areas.
Let's get started.
So as we said during the introduction,
the biggest thing that multi-area OSPF brings to OSPF
is really that scalability.
And again, the reason this helps is because remember,
that all of the routers in a given area
have to have identical link-state databases.
And by having multiple areas, that
means that SPF is only run for database changes
within that area.
So the way that it works in between areas
is we only carry prefix information
between those areas.
We do not carry the full database information.
Now, we'll talk a lot more about these LSAs, the different types
of LSAs within the area and between the areas
in an upcoming video.
But for now, just realize that the LSAs sent between areas
carry less information.
And we don't actually run Dijkstra against those LSAs.
And we'll see this in action in just a few moments.
Now, one of our requirements when using multiple areas is we
do have to follow a very strict hierarchy.
The reason for this is as we mentioned
during the introduction, the only place
we can summarize in OSPF is at these area borders.
And because of that, this has to follow our IP structure
as well.
So that where our areas cross, this
is also the place that we can actually
do our network summarization.
So this does require a very specific design.
Now, not only does this have to follow
our IP addressing scheme, but this hierarchy
does have some rules of its own.
The first, of course, is that there must be a backbone area.
This is an area that all of the other areas have to connect to.
This has a few guidelines.
First, it must be area zero.
This is not an option in OSPF.
As soon as you have more than one area, one of them
has to be area zero.
It also must be contiguous.
In other words, you cannot have two separate area zeros.
So you cannot have, for example, an area zero here,
have a net router between them, have this connected to, say,
area 21 with another router.
And then over here, do another area zero.
This would be a split area zero.
This is not allowed.
Now, there are ways we can make this work, which we will
talk about in future skills.
But in making it work, what we're really doing
is fixing the fact that it's split
and making it so that it's not split.
That's the actual fix for it.
And the last point here is not so much a rule as a guideline.
We don't generally put end user devices or resource networks,
such as servers, or printers, or things
like that in the backbone area.
It certainly can be done.
Area zero is a full blown OSPF area.
So you can do this.
But area zero is very often reserved to just be a backbone
and, therefore, not actually have any resources in it.
All of the other areas are then non-backbone areas.
Now this, of course, can use any 32-bit number.
They all, as we said earlier, must be connected to area zero.
And this is usually where we provide those connections
for the end users.
And those resource networks that we spoke of up above.
So quite commonly, what you would have,
you would have an area zero, connect it out
to some other area, say, 21 again.
And this is where we would actually
connect in our resources.
Let's take a look at this in a little bit more
of a structure diagram.
So here, we have a typical multi-area OSPF design
where, of course, we have our backbone area, area zero,
as well as two other areas, area 100 and area 200.
So the main thing we want to look at here
are the different roles of the routers when we start getting
into a multi-area topology.
So the first thing we have is something
called a backbone router.
That would be this router here, which
is a router that has all of its interfaces in the backbone
area.
Other routers would be things like internal routers, which
are routers such as these that have all of their interfaces
in an area other than area zero.
And in case you're wondering here, yes, a backbone router
is also an internal router.
We just give it a more specific name
because all of its interfaces happen
to be in the backbone area, area zero.
But yes, it is still an internal router.
Now, the other routers here are a little more specialized.
The first thing we have is an ABR or an Area Border Router.
And that would be this router and this router.
These are routers that have one interface at least in area zero
and one interface in some other area, possibly more than one.
But nonetheless, these are then performing
the job of an ABR or an area border router.
This, by the way, is where we can actually
do our network summarization.
So the ABR is where we can summarize internal routes.
Now, we have another type of router here bringing
in routes from the outside.
So maybe, for example, this would be something like BGP.
Maybe this would be something like EIGRP
or something like IS-IS.
The router performing these functions
is called an ASBR or an Autonomous System Boundary
Router.
This is a router that's doing some form of redistribution
from these external protocols into OSPF
bringing external routes into our network.
This is where we can actually summarize external routes.
So technically, we can summarize at the ABR or at the ASBR,
depending on which type of route we're talking about.
The ABR, again, is for internal routes going between areas.
The ASBR can only summarize the external routes
as they're being brought into OSPF during the redistribution.
So now, let's wrap up with a look at some advantages
and disadvantages of multi-area versus single-area OSPF.
So let's begin with the advantages.
First, we have that it allows for filtering and summarization
between the areas.
Now, we've been talking a bit about the summarization
as we go here.
We haven't mentioned a whole lot of filtering.
But for the exact same reason that we can only
summarize between areas, the ABR is also
the only place we can filter.
So again, with multi-area we can do
route filtering between the areas as well as summarization.
It also localizes the impact of any topology changes.
So if an interface goes up, an interface goes down,
or more importantly, if an interface is flapping,
it won't affect the entire OSPF domain.
It would be localized just to the single area where
the problem is occurring.
Part of that also relates to the containing
of the flooded detailed LSAs.
But we're really talking about containing
the flooding of those detailed LSAs regardless
of whether it's because of a topology change,
but just in general.
This is what causes that larger database
and for the longer execution of Dijkstra's algorithm.
Now, these are really good advantages.
Very, very important that we keep these in mind.
And quite frankly, these are most of the reasons
that we're going to go with multi-area OSPF.
Now, of course, scalability is also a huge advantage.
But I was trying to go for some more specifics here.
Now, there are some concerns, however,
with running multi-area OSPF as well.
You could also refer to these as advantages
to running single area, however you want to look at this.
But the first one is that the routers no longer have
full visibility of the network.
Now, we'll get into this more as we go through future skills.
But since the routers no longer have full visibility,
this can actually lead to routing loops.
Link-state routing protocols are immune to routing loops.
Because they have that full visibility.
But without that full visibility,
you lose that immunity.
So as we start digging into more detail with multi-area
and how our ABRs work and such, we'll
get more into how this actually has to do loop prevention
and implement some rules on our ABRs.
And further talking about those ABRs,
we need to make sure that they don't become
a single point of failure, which means
we have to worry about redundancy with those ABRs.
Meaning, we should have at least two ABRs between our areas
in most cases.
And finally, there are some technologies
that just don't work as well without having a single area.
One of those as an example would be MPLS traffic engineering.
Can we get it to work between areas?
Yes.
Is it much more difficult to configure and set up?
A bit.
So it's certainly more preferred to have
a single area for things like traffic engineering,
if it can be accomplished.
In this video, we jumped in.
And we took a look at some of the advantages
and a few of the disadvantages of running multi-area OSPF.
Keep in mind its main advantage is scalability.
Because at a certain point, we simply
can't run Dijkstra against a really large database
and still have an efficient network.
I hope this has been informative for you.
And I'd like to thank you for viewing.