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[AUDIO LOGO]

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So we've mentioned IS-IS
network type broadcast, or LAN,

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quite a few times now.

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We've talked a little bit
about its hellos and such.

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And in a previous video,
we also did, in fact,

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mention that it uses
something called

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a Designated Intermediate
System or a D-I-S or a DIS.

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This is very similar to OSPF's
DR, or designated router.

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The purpose of
this is to prevent

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the mass flooding of updates
on a multi-access segment.

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It's going to be far
more efficient to have

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all of the updates sent
to 1 central source,

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and then that central source
disseminates that information

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out to all of the other
neighborships on the LAN.

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So let's jump in and take a
look at how the DIS actually

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operates in IS-IS.

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And then we'll jump into both
Wireshark and the command line,

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and get a look at what this
looks like, both on the wire

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and in our show commands
and so on in the iOS.

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Let's jump in and get started.

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So, as we've mentioned,
on a broadcast,

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or we would also call it
multi-access network type,

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such as what we're
showing over here

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in our diagram as
an Ethernet segment,

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IS-IS will elect a DIS, or that
Designated Intermediate System.

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Now, the job of the DIS is to
actually be responsible for

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and handle the pseudonode.

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The pseudonode is
being represented over

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in our diagram by this
sort of grayed out router

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here in the middle.

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So the way this works
is the pseudonode

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is created by the DIS.

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It's not, of course,
a real entity.

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However, it acts
as a virtual router

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to be connected to
all the other nodes.

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So the first thing to note
is that the pseudonode

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will be logically connected
to the other nodes.

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And again, that's being
shown by these neighborships

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here, where each
of the other nodes

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is connecting to the
pseudonode regardless

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of the actual physical underlay.

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The whole purpose
of this pseudonode

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is to represent the LAN Itself.

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The idea here is, again,
we have one device

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acting on behalf of
the network segment,

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rather than all
of these devices,

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all having to exchange
databases with each other.

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It's just a much more
organized system.

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Some other things to note
about this device DIS

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is it can be different for
the level 1 and the level 2

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databases.

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So just as an example, maybe
this router is the DIS level 1,

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and maybe this router
is for level 2.

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Again, they can be different
for the two different databases.

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So the next question
might be, well,

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how do each of these routers
get elected as the DIS

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to begin with?

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Well, the DIS is elected
based on three criteria.

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The first criteria is
that the router actually

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has to have adjacencies.

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You are not going
to be a DIS if you

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don't have any neighborships.

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So in other words,
if it were just

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a single router
on a segment, it's

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not going to elect
itself as a DIS.

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Next, it will go to the
highest interface priority.

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The default is 64.

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So if you wanted to force
which router would be the DIS,

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the easiest thing to
do would be to raise

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that router's interface
priority to be above 64.

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If all of the devices have
the same interface priority,

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it will then use the
subnetwork point of attachment,

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or the SNPA, as the tiebreaker.

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Now, so far, this might sound
a whole lot like an OSPF DR.

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And while the fact
of the matter is,

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we've said it a
few times now, it

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is very similar, both
in concept and in

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how it actually operates.

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However, it does have two
fundamental differences, which

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are our last two points here.

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Let's take a look.

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So the first of
these is that the DIS

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is a preemptive process.

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That means that if a
new router comes online

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that would win based
on these criteria,

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it will actually take
over and become the DIS.

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Keep in mind that the DR in
OSPF does not do preemption.

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So this is not something
that happens with OSPF.

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If a new router comes on in
OSPF, maybe it becomes the BDR,

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maybe it becomes
nothing, but it does not

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take over just because based on
the scenario it theoretically

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should be the DR. It's not
preemptive where the DIS is.

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And, of course,
the other big one

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here is there is no backup DIS.

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Of course, we know
that in OSPF we

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have the BDR, which is the
Backup Designated Router.

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And again, we're just saying no
such equivalency here in IS-IS.

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It's just the DIS.

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And if the DIS fails, we
simply elect another one.

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So now that we've seen a
bit about how this works.

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Let's actually jump into the
command line and Wireshark,

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and take a look at this in
a little bit more detail

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in its actual operation.

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Let's start by looking
at our topology,

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so we know what's connected
to what, and well,

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theoretically,
which router should

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be the DIS to begin with.

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And again, we'll jump in
and see how that's working.

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Again, for this
demonstration, we

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will be using IS-IS
Basic Topology,

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and this is simply our
five routers, 1 through 5,

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all connected, as shown here.

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For this particular
demonstration,

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we will be focusing on the LAN
segment between R1, R3, and R5.

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Now, given that nothing has been
configured as far as priority,

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all of these should right now
have a 64 for their interface

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priority, because, again,
that's the default.

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That means it's going to come
to the SNPA for the tiebreaker.

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That means that router 5
should be our current DIS

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on this segment.

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Let's jump into the command
line and check it out.

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And again, like we said,
we'll take a look at Wireshark

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as well.

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So we'll start off
here on router 1.

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Let's start with a show
isis adjacency detail now

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router 2 here, of course, is a
point-to-point link going over

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to router 2.

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That's not our
concern right now.

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We're concerned with the gigabit
0001 interface going to router

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3 and router 5.

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And, as we can see here, we
see that the DIS priority

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is currently 64
for both of these,

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the local priority on
each of them is also 64,

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and the neighbor priority is 64.

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So they're all the defaults.

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However.

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We can also see here that router
5 has won the DIS selection,

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because it says right here,
under the neighbor priority,

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that this is, in fact, the DIS.

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So let's jump into
Wireshark real quick.

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And see what this actually looks
like on the wire in the hellos.

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So we'll just capture on
gigabit 0001 on router 1,

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and let's see what we can see.

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So here in our
Wireshark capture,

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well just look at
packet 1 here, which is

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a hello coming from router 3.

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Now, if we open up the
ISIS HELLO field on this,

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we can see here under System
ID that the designated

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IS, or the DIS, is
in fact router 5.

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Let's actually look at
a hello from router 5,

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and see if that
looks any different.

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That would be packet 3.

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Notice that with packet 3
highlighted, the fields update,

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but it's still
showing the system

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ID of the designated IS as 5.

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And, of course,
we wouldn't expect

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that to be any different.

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If we look at a hello from
router 1, which in this case

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would be packet 7, we
see this looks the same.

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And, of course, that's
absolutely critical

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because, at the moment,
everybody agrees

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who the DIS is, router 5.

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Now, of course, we could verify
this on router 5, as well.

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The commands a little bit
different, since it's IOS XE.

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So let's jump over
and check there.

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So if we do a show clns
is neighbors detail,

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we can get a little
bit of a look here.

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Notice for the
gigabit 1 interface,

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both from router 1 and router 3,
they're listing the circuit ID

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as router 5.01.

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And, again, this is
because the DIS is

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responsible for this segment.

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So this is another
indicator that router 5

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is, in fact, the DIS.

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But let's take some control of
this and let's make router 1

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the DIS on this segment.

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So on router 1, we'll
go into configuration.

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We're going to do our
router is-is process

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into the interface.

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And let's set the priority.

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You can see here that
the range is 0 to 127.

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And we know that
the default is 64,

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so let's just pick
something like 120.

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We'll commit our changes.

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We'll exit configuration mode.

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And let's do the show ISIS
adjacency detail again.

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Now notice under
router 3 and router 5,

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the DIS priority is listed
as 120, which is what we just

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set router 1 to.

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And notice it's saying
that our local priority

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towards that neighbor is 120,
and therefore we are the DIS.

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Notice It's saying
that the DIS is local.

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Both of those neighbors
have a priority of 64.

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So this is an indicator that
router 1 is, in fact, the DIS.

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Now, let's go back to
Wireshark and see if it agrees.

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So packet 1 on this capture is
actually a Hello from router 1.

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Let's go ahead and expand
the ISIS HELLO header.

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And what we see here is that the
system ID of the designated IS

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is now router 1.

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Of course, all the
routers should agree.

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Let's look at packet 5,
which is from router 5.

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We see that also agrees
that the DIS is router 1.

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And let's also verify
router 3. router 3

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also shows the system ID of
the designated IS as router 1.

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So we have successfully switched
over our DIS to be router 1.

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So in this video, we
took a closer look

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at the Designated Intermediate
System or DIS operation

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in IS-IS.

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We actually saw how it was
pretty similar to OSPF's DR

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concept.

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However, it does have
some differences.

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One of which being, if you
notice during our little lab

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demonstration here, we made
a change to the priority

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on router 1, and it immediately
took over as the DIS.

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So, clearly, it's a
preemptive process.

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Then, of course, we
took a look at what

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this looks like on
the wire, and we

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can see how all of the
routers on that segment

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agree who the DIS is.

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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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