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So now that we understand the IS-IS hierarchy,
now let's jump in and look at some good design practices
that we can use with this both with our IP
addressing and using this layer 2 hierarchy.
Let's jump in and take a look at the details.
So one of the primary things we want
to be concerned with really with any protocol design--
this isn't necessarily specific for IS-IS--
but we need to have good addressing design.
And this is really because we want to summarize.
We don't want all of the routers and all of our network
to know about all the routes, and therefore, we
need to summarize.
Now with a link-state protocol, it's
important that we have a good design for our addressing
to follow the protocol because we can only really
summarize at these hierarchical boundaries.
So, for example, in OSPF, we can summarize between areas.
And really, in IS-IS, it's the same thing.
We can summarize going from Level 2 into Level 1
and Level 1 into Level 2.
We can't summarize within the areas.
That's why it becomes important here
to make sure that our IP addressing design follows
our network design.
Now with IS-IS, we do have one extra thing we
need to be concerned with, and that
is to plan out our CLNP addressing.
Now we'll be talking more about the addressing
itself in an upcoming video here,
but for now, just remember that this does need
to be part of our planning.
It's not a difficult task, but it does need to be planned out.
Now again, we're almost always, except in the smallest
of environments, we're going to want to use
this two-layer hierarchy.
And as we discussed in the previous video,
this, of course, is to limit that LSP flooding,
and it also, of course, as I just mentioned,
gives us a place to actually do that summarization.
And that's why it's so important that our IP addressing
design follow our layer 2 hierarchy so
that we can summarize at the places that we're able to.
Now it's quite common in networking
to use a three-layer architecture.
Now, this isn't exactly the same as the hierarchy.
The architecture is how we're going to be building out
the physical network.
So what we're going to do is we're
going to take a look at how the three-layer architecture goes
with the layer 2 hierarchy in a typical design.
So just to go over a design here a little bit.
As we mentioned, the devices should
be grouped into areas based on the IP address and scheme
again because of where we need to do our summarization.
And let's talk about how this fits into the architecture.
So typically, we will have a core layer.
In this case, the core layer is going
to be these routers up here at the top.
These are going to be acting as the backbone area.
Now again, as we mentioned in our previous video,
you don't have to have a dedicated backbone
area in IS-IS.
However, for good scalability, it's generally a best practice.
These are generally going to be level two-only devices.
As you can see in our Area 100 here,
we're only using these Level 2 connections up
here in Area 100.
All of the other areas would then connect to this area.
In this case, we're only picturing Area 10.
But if there were more areas, they
would just be connected in here to Area 100,
acting as our backbone.
And again, this is just a design decision.
As we've mentioned, you do not have
to have a dedicated backbone area in IS-IS.
Then, of course, we have the distribution layer.
In our design, that would be these routers right here.
These are the routers that, of course, connect up
to the core layer.
We can see that in our diagram.
These are going to be using Level 2 to go to the core
but Level 1 inside of the area.
We can, of course, see here that Level 1 is all
of these connections down here.
And to go back to our design and the whole idea
that the IP addressing needs to follow our design,
this is where we would do that summarization.
This is the boundary between the Level 1
and the Level 2 environments for this particular area.
And again, if we had more than Area 10, this, of course,
would be the same in all the other areas.
Last but not least, of course, we have the access layer.
That would, of course, be these devices
down here at the bottom.
And this is where all of the other devices
connect in to the rest of our network.
So this is the connection point for everything else.
So this is where PCs, printers, servers, everything,
connections to other routing domains.
Everything comes in at the access layer.
Keep in mind that this, of course, is not set in stone.
But this is a very typical design
to use our two-tier hierarchy in IS-IS
in a three-tier architecture for our network,
where we have core distribution and access.
Just some final things to keep in mind in your IS-IS designs.
Most large environments are going
to be using those wide metrics that we discussed.
Remember, they're required for IPv6 anyway.
So again, as I mentioned when we talked about them
the first time, you're probably going
to want wide metrics in any case.
So you probably just want to turn them
on right from the beginning.
Other considerations.
IOS does not dynamically set the metric on the interfaces.
All of these interfaces actually default to a cost of 10,
which unfortunately makes it act a little bit like a hop count.
So if you do have varying speeds in your environment
for your interfaces, which most environments do,
you're going to have to set this manually.
And when I say you, I, of course, mean the administrator.
So this does need to be manually configured
on most of our IOS devices.
One final thing to consider with our IS-IS designs
is IS-IS does use the LSPs very efficiently.
What that ultimately means is this
allows us to have around 1,000 routers in an area
without too much trouble.
This would actually be pushing the limits in an OSPF design.
But since IS-IS does handle this a little bit more efficiently,
it does make this a little more scalable
than OSPF, which, again, is one of the reasons we
like this in a service provider environment.
In this video, we jumped in, and we
looked at a little bit of IS-IS design.
Specifically, what we focused on is
how we take a two-tier hierarchy like IS-IS uses,
and we fit that into our three-tier network architecture
that we generally use in our networking designs
where we have core distribution and access.
And when we overlay IS-IS on top of that,
we end up with the core usually acting as a backbone area.
Although, again, just one final reminder.
IS-IS does not need a backbone area,
but we do typically use the core as a backbone.
Then we have our distribution layer,
which is where we do our summarization, acting
as the border between our Level 1 and our Level 2 routing.
Then, of course, we have the access layer
where everything else connects in to the IS-IS environment.
I hope this has been informative for you,
and I'd like to thank you for viewing.