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[AUDIO LOGO]

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So now that we understand
the IS-IS hierarchy,

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now let's jump in and look
at some good design practices

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that we can use with
this both with our IP

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addressing and using
this layer 2 hierarchy.

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Let's jump in and take
a look at the details.

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So one of the primary
things we want

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to be concerned with really
with any protocol design--

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this isn't necessarily
specific for IS-IS--

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but we need to have
good addressing design.

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And this is really because
we want to summarize.

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We don't want all of the
routers and all of our network

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to know about all the
routes, and therefore, we

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need to summarize.

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Now with a link-state
protocol, it's

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important that we have a good
design for our addressing

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to follow the protocol
because we can only really

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summarize at these
hierarchical boundaries.

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So, for example, in OSPF, we
can summarize between areas.

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And really, in IS-IS,
it's the same thing.

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We can summarize going
from Level 2 into Level 1

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and Level 1 into Level 2.

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We can't summarize
within the areas.

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That's why it becomes
important here

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to make sure that our IP
addressing design follows

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our network design.

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Now with IS-IS, we do
have one extra thing we

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need to be concerned
with, and that

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is to plan out our
CLNP addressing.

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Now we'll be talking
more about the addressing

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itself in an
upcoming video here,

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but for now, just remember
that this does need

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to be part of our planning.

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It's not a difficult task, but
it does need to be planned out.

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Now again, we're almost
always, except in the smallest

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of environments, we're
going to want to use

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this two-layer hierarchy.

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And as we discussed
in the previous video,

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this, of course, is to
limit that LSP flooding,

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and it also, of course,
as I just mentioned,

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gives us a place to actually
do that summarization.

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And that's why it's so
important that our IP addressing

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design follow our
layer 2 hierarchy so

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that we can summarize at the
places that we're able to.

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Now it's quite
common in networking

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to use a three-layer
architecture.

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Now, this isn't exactly
the same as the hierarchy.

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The architecture is how we're
going to be building out

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the physical network.

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So what we're going
to do is we're

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going to take a look at how the
three-layer architecture goes

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with the layer 2 hierarchy
in a typical design.

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So just to go over a
design here a little bit.

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As we mentioned,
the devices should

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be grouped into areas based
on the IP address and scheme

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again because of where we
need to do our summarization.

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And let's talk about how this
fits into the architecture.

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So typically, we will
have a core layer.

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In this case, the
core layer is going

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to be these routers
up here at the top.

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These are going to be
acting as the backbone area.

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Now again, as we mentioned
in our previous video,

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you don't have to have
a dedicated backbone

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area in IS-IS.

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However, for good scalability,
it's generally a best practice.

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These are generally going to
be level two-only devices.

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As you can see in
our Area 100 here,

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we're only using these
Level 2 connections up

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here in Area 100.

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All of the other areas would
then connect to this area.

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In this case, we're
only picturing Area 10.

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But if there were
more areas, they

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would just be connected
in here to Area 100,

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acting as our backbone.

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And again, this is
just a design decision.

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As we've mentioned,
you do not have

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to have a dedicated
backbone area in IS-IS.

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Then, of course, we have
the distribution layer.

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In our design, that would
be these routers right here.

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These are the routers
that, of course, connect up

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to the core layer.

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We can see that in our diagram.

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These are going to be using
Level 2 to go to the core

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but Level 1 inside of the area.

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We can, of course, see
here that Level 1 is all

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of these connections down here.

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And to go back to our
design and the whole idea

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that the IP addressing
needs to follow our design,

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this is where we would
do that summarization.

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This is the boundary
between the Level 1

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and the Level 2 environments
for this particular area.

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And again, if we had more
than Area 10, this, of course,

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would be the same in
all the other areas.

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Last but not least, of course,
we have the access layer.

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That would, of course,
be these devices

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down here at the bottom.

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And this is where all
of the other devices

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connect in to the
rest of our network.

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So this is the connection
point for everything else.

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So this is where PCs,
printers, servers, everything,

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connections to other
routing domains.

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Everything comes in
at the access layer.

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Keep in mind that this, of
course, is not set in stone.

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But this is a very
typical design

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to use our two-tier
hierarchy in IS-IS

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in a three-tier architecture
for our network,

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where we have core
distribution and access.

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Just some final things to keep
in mind in your IS-IS designs.

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Most large
environments are going

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to be using those wide
metrics that we discussed.

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Remember, they're
required for IPv6 anyway.

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So again, as I mentioned
when we talked about them

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the first time,
you're probably going

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to want wide
metrics in any case.

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So you probably just
want to turn them

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on right from the beginning.

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Other considerations.

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IOS does not dynamically set
the metric on the interfaces.

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All of these interfaces actually
default to a cost of 10,

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which unfortunately makes it act
a little bit like a hop count.

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So if you do have varying
speeds in your environment

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for your interfaces, which
most environments do,

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you're going to have
to set this manually.

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And when I say you, I, of
course, mean the administrator.

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So this does need to
be manually configured

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on most of our IOS devices.

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One final thing to consider
with our IS-IS designs

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is IS-IS does use the
LSPs very efficiently.

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What that ultimately
means is this

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allows us to have around
1,000 routers in an area

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without too much trouble.

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This would actually be pushing
the limits in an OSPF design.

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But since IS-IS does handle this
a little bit more efficiently,

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it does make this a
little more scalable

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than OSPF, which, again,
is one of the reasons we

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like this in a service
provider environment.

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In this video, we
jumped in, and we

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looked at a little
bit of IS-IS design.

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Specifically, what
we focused on is

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how we take a two-tier
hierarchy like IS-IS uses,

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and we fit that into our
three-tier network architecture

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that we generally use in
our networking designs

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where we have core
distribution and access.

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And when we overlay
IS-IS on top of that,

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we end up with the core usually
acting as a backbone area.

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Although, again, just
one final reminder.

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IS-IS does not need
a backbone area,

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but we do typically use
the core as a backbone.

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Then we have our
distribution layer,

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which is where we do our
summarization, acting

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as the border between our Level
1 and our Level 2 routing.

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Then, of course, we
have the access layer

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where everything else connects
in to the IS-IS environment.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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