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[AUDIO LOGO]

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So now that we've
seen a little bit more

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about how our IS-IS
adjacencies work,

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let's take a little
bit of a closer

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look at that communication.

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Now, we did look a
little bit, of course,

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at the hello packets themselves.

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We did see that it uses
a different type of hello

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for a LAN or a point-to-point
interface or network type.

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But in this video, we're going
to focus more on the packets

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that it uses for the
actual database exchange.

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So let's get in and take
a look at the formatting

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of those packets and
the different types

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that we have to support,
both for IPv4 and for IPv6.

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So as we jump into the
IS-IS packet types,

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the first thing
to keep in mind is

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that these are called PDUs,
or Protocol Data Units.

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That's just the generic
name for the packet type.

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Now, these PDUs are broken down
into three major categories.

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The first is the hello packets,
which we have sort of looked

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at a little bit already.

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We know that there's
actually two different types

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of the hello packets--

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one type for LAN and one
type for point to point.

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Then we have the Link-State
Packets, or the LSPs.

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These are the ones that
actually distribute

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the routing information.

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So these are the ones
actually carrying

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the bulk of the information
for the IS-IS database.

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Then there's the
sequence number packets.

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And these are really
just to control

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the distribution of the LSPs.

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It's to make sure that
we get all of them,

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we get them in order,
we don't miss any.

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So these are really just
for checking the LSPs.

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So like we said,
the hello packets

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are broken down into, well,
sort of two major categories

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for the LAN.

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And then those are broken up
into level one versus level two

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hellos.

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And these are PDU
types 15 and 16.

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If you remember, we saw
these in the previous video.

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And then, of course,
there's the hello

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for the point-to-point
network, which is the type 17.

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Again, we saw these as well.

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But there's three total
different hello types--

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two for LAN, one for
point to point, but two

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overall categories of LAN
versus point to point.

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Now, the LSPs also
have subcategories.

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There's the LSPs for
level one, and these

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are going to be type 18.

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And then we have the level
two LSPs that are type 20.

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So these are for
carrying, of course,

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the database information
for the different levels.

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The sequence number packets are
also broken into subcategories.

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As you can see, there's,
again, sort of a major category

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where we have level one and
level one versus, of course,

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the level two and the level two.

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And what makes these different,
of course, from each other is

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there's a complete for
each one, and there

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is a partial for each one.

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Each of these, of course,
have their own types--

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24, 25, 26 and 27.

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So as we see, these
are sequential.

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These are, again, used to
verify the receipts of the LSPs.

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The way this works is if I'm
getting the complete database,

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essentially, I will do a
complete sequence number.

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And I'll do a
partial if we're just

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updating certain parts of the
database that have changed.

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Now, all of these PDUs do
follow the same overall packet

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structure.

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The first thing we have
is a common header.

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This is followed by
a specific header.

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This is going to be, of course,
based on the type of PDU.

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And then we have the TLVs,
which is where we're actually

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carrying the data that this
PDU is supposed to be carrying.

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So the common header,
to break it down

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in a little more
detail, contains

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all of these fields, each
one of these being 1 byte.

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The first is the intradomain
routing protocol discriminator.

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This is going to be set to 0x83.

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Next is the length indicator.

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Now, this is going
to, of course,

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be the length of the header.

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And it's going to be in bytes.

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So this could vary, of
course, because there's going

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to be different packet sizes.

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That's why we need
the length field

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to begin with is because it
can be variable header lengths.

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So this is giving
that information.

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Next is the version
and protocol ID.

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This is currently going
to be set to 0x01.

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Then we have the ID length.

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This is going to tell us
the length of the NSAP

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address and the NET ID.

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Again, those can vary,
so we need something

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to tell us the length.

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This is followed
by a reserved byte.

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This is, of course,
going to be set

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to zero since it's reserved.

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Next is the PDU type.

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This is what's going to
tell us what comes next.

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So if you want to compare this
to TCP/IP operation, in a way,

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this is sort of like
the next header field.

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This is going to be talking
about what type of PDU

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this is based on what we talked
about on the previous slide.

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Next is the version.

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Again, this is currently
going to be set to 0x01.

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And then the maximum
area addresses.

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And this is the maximum
number of area addresses

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that we're going to be
carrying in the PDU.

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Now, each PDU type has
its own specific headers.

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This is talking about
this field right up here.

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There's a lot of
information here,

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which we probably don't
really have to have memorized.

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But I will give you a reference
to that towards the end

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of the video here.

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Now, again, the last
section of the header

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are the Type Length
Values, or the TLVs.

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And again, there are a
lot of these as well.

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These are some of the major
ones, things like the area

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address of the source node, the
neighboring routers, if it's

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intradomain routes.

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What networks are
actually supported?

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So for example, again, are
we supporting IPv4, IPv6,

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maybe just CLNS, which
wouldn't be very likely?

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But that is what
this was created for.

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Whether it's an external route.

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What are the actual
addresses of the interfaces?

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There's, of course, a
corresponding one for IPv6

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as well.

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We'll be focusing
in more on the TLVs

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for IPv6 in the skill based
around IPv6 IS-IS support.

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As I mentioned a
few moments ago,

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you can refer to ISO
Standard 10589 and RFC

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1195 for more
information, if you really

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want to know all
those different TLV

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values and those
PDU-specific headers

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for every single packet type.

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Let's wrap up this video
with just a quick look

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at a Wireshark
capture so we can see

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where some of this
type information

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is actually carried in
those IS-IS packets.

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So let's start off looking at
packet three, which is actually

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a point-to-point hello
coming from router one.

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If we expand the IS-IS
hello and then scroll down

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a little bit, some of
the things we can see,

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we can see the protocol
supported being 129.

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So if we open that
section up, we

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can see that as of right
now, it is a type 129,

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and it is currently
supporting just IP.

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So there's no IPv6 on
here at the moment.

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It's just IP.

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But you can see
where it's carrying

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the information about what's
actually supported here.

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Also, notice our type 132, where
it's carrying the actual IP

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interface addresses.

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And if we open that up,
scroll down a little bit,

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we can see where it's
actually advertising router

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one's, IPv4 interface address.

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The other thing we
can look at here

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is packet 17, which is a
complete sequence number

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packet.

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So if we highlight packet 17,
we can open up the ISO header.

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And we go to LSP entries.

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We can actually see all
of the entries being

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exchanged in this complete
sequence number packet.

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So in this video,
we jumped in, and we

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looked at a little
bit more detail

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on the different packet
types that are used by IS-IS

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as well as a little bit of
what's in those packets.

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Then, of course, we jumped
in here to Wireshark.

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And we took a look at
some of those packets,

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how they look on the
wire, and what's actually

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in some of those headers.

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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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