[AUDIO LOGO]
So we've mentioned IS-IS network type broadcast, or LAN,
quite a few times now.
We've talked a little bit about its hellos and such.
And in a previous video, we also did, in fact,
mention that it uses something called
a Designated Intermediate System or a D-I-S or a DIS.
This is very similar to OSPF's DR, or designated router.
The purpose of this is to prevent
the mass flooding of updates on a multi-access segment.
It's going to be far more efficient to have
all of the updates sent to 1 central source,
and then that central source disseminates that information
out to all of the other neighborships on the LAN.
So let's jump in and take a look at how the DIS actually
operates in IS-IS.
And then we'll jump into both Wireshark and the command line,
and get a look at what this looks like, both on the wire
and in our show commands and so on in the iOS.
Let's jump in and get started.
So, as we've mentioned, on a broadcast,
or we would also call it multi-access network type,
such as what we're showing over here
in our diagram as an Ethernet segment,
IS-IS will elect a DIS, or that Designated Intermediate System.
Now, the job of the DIS is to actually be responsible for
and handle the pseudonode.
The pseudonode is being represented over
in our diagram by this sort of grayed out router
here in the middle.
So the way this works is the pseudonode
is created by the DIS.
It's not, of course, a real entity.
However, it acts as a virtual router
to be connected to all the other nodes.
So the first thing to note is that the pseudonode
will be logically connected to the other nodes.
And again, that's being shown by these neighborships
here, where each of the other nodes
is connecting to the pseudonode regardless
of the actual physical underlay.
The whole purpose of this pseudonode
is to represent the LAN Itself.
The idea here is, again, we have one device
acting on behalf of the network segment,
rather than all of these devices,
all having to exchange databases with each other.
It's just a much more organized system.
Some other things to note about this device DIS
is it can be different for the level 1 and the level 2
databases.
So just as an example, maybe this router is the DIS level 1,
and maybe this router is for level 2.
Again, they can be different for the two different databases.
So the next question might be, well,
how do each of these routers get elected as the DIS
to begin with?
Well, the DIS is elected based on three criteria.
The first criteria is that the router actually
has to have adjacencies.
You are not going to be a DIS if you
don't have any neighborships.
So in other words, if it were just
a single router on a segment, it's
not going to elect itself as a DIS.
Next, it will go to the highest interface priority.
The default is 64.
So if you wanted to force which router would be the DIS,
the easiest thing to do would be to raise
that router's interface priority to be above 64.
If all of the devices have the same interface priority,
it will then use the subnetwork point of attachment,
or the SNPA, as the tiebreaker.
Now, so far, this might sound a whole lot like an OSPF DR.
And while the fact of the matter is,
we've said it a few times now, it
is very similar, both in concept and in
how it actually operates.
However, it does have two fundamental differences, which
are our last two points here.
Let's take a look.
So the first of these is that the DIS
is a preemptive process.
That means that if a new router comes online
that would win based on these criteria,
it will actually take over and become the DIS.
Keep in mind that the DR in OSPF does not do preemption.
So this is not something that happens with OSPF.
If a new router comes on in OSPF, maybe it becomes the BDR,
maybe it becomes nothing, but it does not
take over just because based on the scenario it theoretically
should be the DR. It's not preemptive where the DIS is.
And, of course, the other big one
here is there is no backup DIS.
Of course, we know that in OSPF we
have the BDR, which is the Backup Designated Router.
And again, we're just saying no such equivalency here in IS-IS.
It's just the DIS.
And if the DIS fails, we simply elect another one.
So now that we've seen a bit about how this works.
Let's actually jump into the command line and Wireshark,
and take a look at this in a little bit more detail
in its actual operation.
Let's start by looking at our topology,
so we know what's connected to what, and well,
theoretically, which router should
be the DIS to begin with.
And again, we'll jump in and see how that's working.
Again, for this demonstration, we
will be using IS-IS Basic Topology,
and this is simply our five routers, 1 through 5,
all connected, as shown here.
For this particular demonstration,
we will be focusing on the LAN segment between R1, R3, and R5.
Now, given that nothing has been configured as far as priority,
all of these should right now have a 64 for their interface
priority, because, again, that's the default.
That means it's going to come to the SNPA for the tiebreaker.
That means that router 5 should be our current DIS
on this segment.
Let's jump into the command line and check it out.
And again, like we said, we'll take a look at Wireshark
as well.
So we'll start off here on router 1.
Let's start with a show isis adjacency detail now
router 2 here, of course, is a point-to-point link going over
to router 2.
That's not our concern right now.
We're concerned with the gigabit 0001 interface going to router
3 and router 5.
And, as we can see here, we see that the DIS priority
is currently 64 for both of these,
the local priority on each of them is also 64,
and the neighbor priority is 64.
So they're all the defaults.
However.
We can also see here that router 5 has won the DIS selection,
because it says right here, under the neighbor priority,
that this is, in fact, the DIS.
So let's jump into Wireshark real quick.
And see what this actually looks like on the wire in the hellos.
So we'll just capture on gigabit 0001 on router 1,
and let's see what we can see.
So here in our Wireshark capture,
well just look at packet 1 here, which is
a hello coming from router 3.
Now, if we open up the ISIS HELLO field on this,
we can see here under System ID that the designated
IS, or the DIS, is in fact router 5.
Let's actually look at a hello from router 5,
and see if that looks any different.
That would be packet 3.
Notice that with packet 3 highlighted, the fields update,
but it's still showing the system
ID of the designated IS as 5.
And, of course, we wouldn't expect
that to be any different.
If we look at a hello from router 1, which in this case
would be packet 7, we see this looks the same.
And, of course, that's absolutely critical
because, at the moment, everybody agrees
who the DIS is, router 5.
Now, of course, we could verify this on router 5, as well.
The commands a little bit different, since it's IOS XE.
So let's jump over and check there.
So if we do a show clns is neighbors detail,
we can get a little bit of a look here.
Notice for the gigabit 1 interface,
both from router 1 and router 3, they're listing the circuit ID
as router 5.01.
And, again, this is because the DIS is
responsible for this segment.
So this is another indicator that router 5
is, in fact, the DIS.
But let's take some control of this and let's make router 1
the DIS on this segment.
So on router 1, we'll go into configuration.
We're going to do our router is-is process
into the interface.
And let's set the priority.
You can see here that the range is 0 to 127.
And we know that the default is 64,
so let's just pick something like 120.
We'll commit our changes.
We'll exit configuration mode.
And let's do the show ISIS adjacency detail again.
Now notice under router 3 and router 5,
the DIS priority is listed as 120, which is what we just
set router 1 to.
And notice it's saying that our local priority
towards that neighbor is 120, and therefore we are the DIS.
Notice It's saying that the DIS is local.
Both of those neighbors have a priority of 64.
So this is an indicator that router 1 is, in fact, the DIS.
Now, let's go back to Wireshark and see if it agrees.
So packet 1 on this capture is actually a Hello from router 1.
Let's go ahead and expand the ISIS HELLO header.
And what we see here is that the system ID of the designated IS
is now router 1.
Of course, all the routers should agree.
Let's look at packet 5, which is from router 5.
We see that also agrees that the DIS is router 1.
And let's also verify router 3. router 3
also shows the system ID of the designated IS as router 1.
So we have successfully switched over our DIS to be router 1.
So in this video, we took a closer look
at the Designated Intermediate System or DIS operation
in IS-IS.
We actually saw how it was pretty similar to OSPF's DR
concept.
However, it does have some differences.
One of which being, if you notice during our little lab
demonstration here, we made a change to the priority
on router 1, and it immediately took over as the DIS.
So, clearly, it's a preemptive process.
Then, of course, we took a look at what
this looks like on the wire, and we
can see how all of the routers on that segment
agree who the DIS is.
I hope this has been informative for you.
And I'd like to thank you for viewing.