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[AUDIO LOGO]

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So we've already
mentioned the fact

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that IS-IS actually
runs on top of CLNS,

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and we mentioned in
the previous video

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that that means we have to
design our CLNS addressing.

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Well, that's what we're going
to take a look at in this video.

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How is that address formatted?

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How do we put it on the router?

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And ultimately, how do we
design it for our environment?

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So let's go ahead and
jump in, take a look

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at some of the details.

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We'll jump on to
the command line

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and see how this is
configured on the routers.

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And then just for fun, we'll
even jump into Wireshark

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and see what this actually
looks like on the wire itself.

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So let's go ahead
and get started.

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So first, let's talk
about the overall IS-IS

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addressing format,
and then we'll

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talk about how Cisco uses this.

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So this is an example of
what the headers look like.

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Let's go over these one by one.

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So the first thing
to note here is

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that the ISO NSAP
addresses actually

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consist of two elements.

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The first is the Initial
Domain Part or the IDP.

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We can see on the diagram--

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that would be this
part right here--

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this is then divided
into two other parts.

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The Authority and Format
Identifier, AFI, this

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specifies the format of the
address and the authority

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it's assigned to.

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Then you have the Initial
Domain Identifier or the IDI.

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This is the subdomain
under the AFI.

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So that would, of course,
be these two fields

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up here, the AFI and the IDI.

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Next up, is the Domain-Specific
Part, or the DSP,

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and that's, of course, this part
of the addressing over here.

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Now, as we can see, the DSP
is broken up into three parts.

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The first is the high-order DSP.

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This is what subdivides
the domain into areas.

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Then we have the system ID.

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The system ID is sort of
like the OSPF router ID.

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This is a unique identifier
for each device in the area.

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And then finally,
we have the NSEL,

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which is sort of like
an IP port number.

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This is generally going to be 00
when we're configuring the net

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number.

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Look at it like when
you're putting an IP

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address on an interface.

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You don't put a
port number on it,

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you just put on the address.

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So the NSEL, acting
like the port number,

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we would use this in part of the
address for a specific process,

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such as IS-IS.

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And we'll see this when we
get to our Wireshark packet

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captures, where this NSEL is not
00, it's set to another number.

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If it's 00, it's referring to
the device itself or the net

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number.

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Now, the way Cisco has chosen
to implement all of this

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is we usually set the AFI to 49.

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This simply means that
it's locally administered

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addresses--

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in a way, sort of
like RFC1918 addresses

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that we have in IP version 4.

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This would then be followed by
the area ID, which, of course,

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we have to make the same on
all the devices in the area.

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Next, we have the
system ID, which

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would be the next 6 bytes.

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This, as we said, is a unique
identifier for the device.

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And then the NSEL, again, set
to 00 to indicate the device

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itself.

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That's what we're going
to be configuring.

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An example of this
would look something

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along these lines, where
this portion of the number

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is that AFI.

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This portion of the
number is the area.

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This is the system ID.

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And the last section
here is that NSEL.

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But this is an example of what a
net number that we put into one

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of our devices might look like.

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Now, technically you could
use the same system ID

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on devices in different areas.

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But as we're saying
here at the bottom,

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this is generally
not recommended, OK?

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Best practice is to make sure
that all of the system IDs

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are unique in the entire domain.

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Again, every net number
has to be unique.

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But I could theoretically, if
the area number is different,

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I could reuse system IDs.

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And we're simply saying that's
generally not recommended.

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I wouldn't do that.

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Keep all of your
system IDs unique.

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This is only one number
we have to come up

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with for each device.

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So let's jump into the
command line here real quick,

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take a look at what this looks
like when we're configuring it

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on the device.

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So we just have a real
simple topology here,

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and we're just
going to be taking

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a quick look at router 1.

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I just wanted to show
you the topology so you

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can see that it does,
in fact, have devices

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that it's talking to out
its different interfaces.

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So we're going to look first at
the configuration on router 1.

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And then we're going to fire up
Wireshark and we'll take a look

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at a packet capture here
on the G0/0/0/0 interface.

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And we'll look at an
IS-IS hello that's

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headed over towards router 2.

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So let's start by jumping
into the command line.

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So here on router 1,
let's just do a show run.

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We'll begin with IS-IS
and we can see right here

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an example of the net number.

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So you can see how we're
leading with that 49

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for the private addressing,
and then this portion

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is specifying the area.

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So this is putting
router 1 into area 1,

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and then this portion
is the system ID.

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So what this net
number is doing is

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it's making router 1
system ID 1 in area 1.

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And as you can see here,
that NSEL cell number is 00

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because we're not going to
be specifying a port here

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for anything.

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So let's also do
a show isis prot,

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and here we can actually
see how that's broken up.

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We can see where
the system ID is 1,

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and we can see where the
area is also set to 1.

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So we can see in this output
how it sort of breaks it up

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into the two different parts.

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Now, this may be just
a little bit confusing

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just because both the
system ID and the area

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are both 1 on this router.

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Let's just take a look at
the same thing on router 4

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real quick.

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On router 4, we'll also
do the show isis prot.

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This is a little easier to see,
where the system ID is now 4

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but it's still in area 1.

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And as we mentioned, let's take
a look at this in Wireshark

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as well.

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So here's our Wireshark capture.

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Again, this is
G0/0/0/0 on router 1,

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and what we can see here is that
we've actually exchanged hellos

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between system ID
1 and system ID 2.

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So we've gotten a
hello going both ways.

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So rather than looking at router
1's hello, which again might

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be a little bit confusing,
let's take a look

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at the one coming from router 2.

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If we look at the one
coming from router 2,

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we can open up the
ISIS HELLO, and we

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see that this is coming from
the system ID of all zeros

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and then a 2.

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Also, we have area
addresses down here.

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If we open up area addresses,
scroll down a little bit,

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we also see the area
address here listed

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as the all zeros and the 1.

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Now, as you can see, the 49
is technically listed here

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as part of the area address.

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But again, we generally discard
that because it's just used

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for the private addressing.

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So we would just refer
to this as area 1.

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So we can see this information
being exchanged in the hellos

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where each device is sending
their system IDs and the area

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that they belong in.

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In this video, we jumped
in and we took a look

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at our CLNS addressing.

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We saw how the different
parts are broken up,

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and then we saw how to enter
this information on the Cisco

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CLI.

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We also, of course, took
a look here at Wireshark,

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and we can see that this
information is being

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exchanged in the IS-IS hellos.

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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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