In this video, we're going to jump in and take
a look at the IS-IS hierarchy.
Much like OSPF, this is also based on a two-layer hierarchy.
Now in OSPF, that would be area 0
being the backbone and all of the other areas being connected
to area 0.
That's your two-layer hierarchy.
In IS-IS, it's done a little bit differently.
We have level 1 and level 2 adjacencies.
Now some adjacencies can be both,
and we'll get into all of that as we go through this video.
But just be aware that there's essentially level 1
and level 2.
The level 1 adjacencies are generally for within an area,
and the level 2 adjacencies are for between areas.
Therefore, the level 2 adjacencies
are effectively acting as the backbone.
Now since this is just done with a string of adjacencies,
it is a bit more flexible than OSPF's area 0.
But all in all, it serves the same basic function.
Now, just like OSPF area 0 has to be contiguous,
well, we have the same thing with level 2 adjacencies
in IS-IS.
The level 2 adjacencies have to be connected
through our entire environment.
So that's what we're going to be focusing on in this video.
Let's jump in and take a look at some of the facts, figures,
and specifications and some of the rules
that go along with all of this.
So let's jump in and take a look at this in some more detail.
So it doesn't really make much sense to talk about a hierarchy
unless we understand that, of course,
we can partition our entire environment into areas, again,
just like OSPF.
And we're doing it for the same reason.
So what happens is we use something called LSPs or Link
State Packets to exchange the link state information.
This is the equivalent of LSAs in OSPF.
This builds the link state database or the LSDP,
which we're pretty much calling the same thing
as we did in OSPF.
And then of course, we run Dijkstra's Shortest Path First
algorithm against that database to populate our routing table.
That's just the general idea behind how this works.
And for the exact same reason we do it in OSPF,
we want to split up the database here in IS-IS.
If the database gets too large, well, Dijkstra's
going to take a really long time to run.
It could cause a total failure, although on today's equipment
that would take a pretty large area.
But even so, it's going to slow down convergence times.
So a bigger area wouldn't be better
just because the equipment can necessarily handle it.
We still have to be careful that we
don't make our convergence times be too high.
So again, we're breaking this up into areas
for the exact same reason that we do in OSPF.
The next thing to discuss is just a little bit
of terminology here.
So we already mentioned during the introduction
that this is a two-level hierarchy.
And well, let's talk about its components.
First is the domain itself the domain
itself, of course, is just referring to the entire IS-IS
environment, just like we refer to an OSPF domain.
So it's the environment as a whole.
Then of course, as we've mentioned,
we split this up into areas.
This is, of course, just going to be
a set or a subset of routers that's
going to be defined by you.
This is going to be handled by the administrator.
You have to determine for each environment based
on the number of routes how big each area should be.
Then we have the level 1 routing.
Again, this is the routing within an area
or the intra-area routes.
Then we have level 2 routing, which again this
is between the areas.
This is going to be your inter-area routes.
So again, just to compare this a little bit to OSPF,
your level 1s are like your type 1 and type 2 LSAs.
These would be the LSBLK A's that stay within the area.
In the case of OSPF, we have type 3s
that go between areas that's essentially
the equivalent of level 2s.
They function very much in the same way.
So this is an idea of what that hierarchy may look like.
Now one thing to note here, and of course,
we could have used any number we wanted,
but I essentially made the backbone here area 1.
But a couple of things to note here about the IS-IS topology.
First off, you don't have to have an area 0,
just like we're picturing here.
But the other thing is the backbone
doesn't have to be an area.
This area 1 here that we're using as a backbone
could very easily have level 1 connections and routers in it
as well.
So there's not a dedicated area that's going
to be the backbone, be it area 0 or not.
This is all about how you choose to design your backbone.
But in this case, what we've done
is we've used area 1 here sort of as a backbone area.
First thing to note is that IS-IS areas cross on links, not
on routers.
Remember in OSPF, we had that special router, the ABR,
the area border router.
Well, as we can sort of see here, in IS-IS,
the area borders are happening here
on the links between the routers.
Now again, these links are going to be level 2.
That's what sort of makes them the backbone links.
And just like our type 3 LSAs in OSPF,
these level 2 routing exchanges have less information in them
about the links in other areas.
So they're sort of like the old summary LSAs from OSPF.
And of course, the whole goal of this, as we've mentioned,
is it reduces the load on the routers.
Of course, this is going to allow
for scalability, and that, of course, also
leads to performance and making sure
that our adjacencies won't take too long to come up.
Now the IS-IS adjacencies can be either level 1,
like we're showing here and here,
or they can be level 2, like of course, we're showing up here.
Or they can be level 1, level 2.
We don't have any of those pictured here,
but we could just as easily have a level 1
adjacency between some of these routers
and be connecting these as level 1 and level 2.
The reason we may want them to be both Is we
would use the level 1 to pass routes within the area
because again, maybe there are N systems in this area 1 here,
but we still need to have a contiguous level 2 connection.
So we need to carry both level 1 and level 2 on these.
And again, a lot of this comes down to design
and how we're going to build this, which we'll touch
more on in the next video.
So just to add some more detail to these,
you can have any type of adjacency, again, meaning
either the level 1, the level 2, or the level 1-2.
You can have any of these in the same area.
But again, between areas, we're going
to be limited to only the level 2.
That's its entire purpose in existing,
is that it goes between routers in different areas.
Now another consideration we have to think about
is how the device or the router itself
is going to be configured.
And it can also be configured much like our links.
It can either just be level 1.
It can just be level 2.
Or it could be level 1-2.
In the case of our examples here, this router, for example,
could be level 1 only because all of its adjacencies
are in area 10.
The level 2 devices could again be any device that
only has level 2 adjacencies.
It doesn't have any level 1 adjacencies.
And then of course, last but not least,
the level 1-2 adjacencies would be
any of the routers that have to have both a level 1 and a level
2 adjacency.
So not only should we configure our links
to be what we want them to be, level 1, level 2, or both,
but we should also configure our devices.
It's not strictly necessary, but it is generally best practice
to configure the routers for their role in the network.
So again, we've sort of mentioned this,
but just to make sure we're clear on all of it,
the IS-IS backbone is simply a contiguous string of level 2
routers and adjacencies.
And like we said, this can traverse many other areas.
You don't have to have an area either dedicated
to be the backbone or that is only the backbone.
Just to be clear, I could have run my level 2 adjacencies
between the routers like this and maybe not even
done all of this up here.
I wouldn't even need area 1, and this is still perfectly valid.
This is simply an example of the backbone traversing
many other areas.
It's traversing area 10, 20, 30, and 40 here.
So this is something that can absolutely be done.
You see what I mean when I say you have to actually sort
of design your backbone here.
Again, there is no dedicated backbone area
as we would refer to it in, say, OSPF.
And this does in fact add a lot of flexibility to our design.
And this is why I said you do have to design the IS-IS
hierarchy a bit more than we do with OSPF in that OSPF
is a little more strict, so there's not as much to design.
We may have to work around those restrictions,
but here we have flexibility, and flexibility means
that we have to design it.
In this video, we talked a bit about the IS-IS hierarchy.
We talked about how it is a two-layer hierarchy.
It consists of level 1 and level 2
adjacencies, where the level 1 adjacencies are, of course,
within the area, acting much like OSPF type 1 and type
2 LSAs, where the level 2 adjacencies go
between areas acting sort of like LSA type 3s.
Now again, some major differences here number one.
The area is not dedicated to be a backbone.
Again, it's just a contiguous string of level 2 adjacencies.
It can run all in one area.
It can run between areas.
So we have a lot of flexibility when it comes
to the IS-IS backbone design.
And that we'll be covering a little bit more
in the next video.
But also keep in mind here, the second major difference
is that areas in IS-IS cross on links.
They don't cross on routers, like they do in OSPF,
where we had the ABR.
So keep these differences in mind
as we jump into our next video, where
we're going to talk a little bit about some common designs
that we use with this IS-IS backbone.
I hope this has been informative for you,
and I'd like to thank you for viewing.