[AUDIO LOGO]
So in this video, let's jump in and take a deeper look at IS-IS
adjacencies or neighborships.
Now there's really sort of two major attributes
here we need to look at.
The first is the network type.
And this is very similar to the network types
that we have in OSPF as well.
But IS-IS has two network types that we need to look at,
so we'll look at both of those.
The other thing that affects adjacency
is the adjacency level.
Remember, we have Level 1 and Level 2 in IS-IS,
and we form different adjacencies for each level.
Now depending exactly what level each router is in
and how we configure our interfaces will determine which
level or levels of adjacency that will form.
So let's go ahead and jump in and look at those details.
We'll start off by looking at the different network types.
So as we mentioned during our introduction,
IS-IS does, in fact, support two different network types
for our different media types.
It really depends on what type of neighborships
we want to form.
These two types are first broadcast.
And this is going to be generally used
for multi-access networks.
This is when there's going to be more than one device
on the same segment or broadcast domain.
The other choice is point-to-point.
And as the name pretty much implies,
this is going to be for almost all other topologies, where
we're just going to have two routers, 1
on each end of a circuit.
Each of these network types have different attributes.
The broadcast network type, as we mentioned,
is really for multi-access networks.
Of course, the biggest example of this would be Ethernet.
But also, full-mesh WANs.
Remember that although we don't use it very often anymore,
things like Frame Relay was originally
designed to be a full mesh.
And therefore, the broadcast network type
could be appropriate in that environment.
On the other hand, if the Frame Relay was not
running as a full mesh, then point-to-point
would probably be a better choice
in that type of an environment.
Now broadcast network type uses something
called a Designated Intermediate System, or a D-I-S or a DIS.
This is very similar to the OSPF, DR, or designated router.
Now, one sort of fundamental difference
about the DIS-- and we will talk about this in more detail
later.
But one fundamental difference here
is where OSPF has something called
the BDR, which, of course, is to back up the DR should it fail.
IS-IS does not have any similar concept.
If the DIS fails, then another one is simply elected.
Also, similar to other routing protocols,
this does use multicast for its communication.
The hello timer actually defaults to 3.3 seconds.
Again, the idea here being that a multi-access network
is generally considered faster than what
a point-to-point system would be.
Also, keep in mind that part of the point of this
is this does create a full mesh of adjacencies.
That's sort of why it had to be a full-mesh WAN for this
to make any sense so that we can form all those neighborships.
And the same thing with Ethernet.
Again, the assumption is that all the devices
can talk to all of the devices on that segment.
Therefore, we create a full mesh of adjacencies.
Now, this, of course, does create the potential
for a large number of adjacencies.
We have the formula here.
It's just going to be n times n minus 1 divided by 2 because we
don't create two neighborships between each router.
But this can still create a large number of adjacencies,
so just keep this in mind if we have a lot of routers
on a single segment.
It's generally not a problem but always something
good to keep in mind.
Broadcast networks also use either Level 1 or Level 2 IS-IS
hellos.
And these are referred to as LAN hellos,
and we will see this in just a little bit.
Now, of course, the point-to-point networks
have slightly different attributes.
First, these are meant, of course,
for use on point-to-point networks, just some examples
being PPP, which, of course, is point-to-point.
That's obviously going to be a point-to-point network type.
HDLC; partial-mesh WANs, like Frame Relay,
like we mentioned earlier.
Now this, of course, is not going
to use a DIS because it's only two routers.
Remember that the point of the DIS much like the DR in OSPF
is to reduce the flooding of our link state database updates
on that LAN segment.
Now, if there's only two neighbors
like in a point-to-point connection,
I have to send you my updates, and you
have to send me your updates.
There's really no way to reduce that.
So there's no point in a DR in OSPF or, of course,
a DIS in IS-IS on a point-to-point link.
It just wouldn't make any sense.
Also, since there's only the two devices,
we can simply do our communication with unicast.
The hello timer is actually going to be slower here.
Notice the hello timer is 10 seconds.
Now these, of course, can be adjusted.
However, it defaults to 10 seconds
because traditionally, things like PPP, HDLC, WAN links,
these were generally considered to be slower.
Since these are generally considered
to be slower than the LAN, we slowed down the timers.
Now, of course, in today's environment,
that may not always be the case.
Therefore, we can, of course speed this up.
Also note that these do use their own special type
of hello.
They're still called IIHs because they're IS-IS hellos,
but it is a point-to-point hello rather than a LAN hello.
And we'll see this difference as we move into our demonstration.
The other thing we mentioned during our introduction
was that we're going to form different adjacencies based
on the area ID and the router type.
And again, what we mean here is whether it's a Level
1 or a Level 2 or both device.
And even more specifically, the link type.
So let's take a look at the different combinations
and what will or will not form a neighborship.
So our first example here, we have two routers.
They're in the same area.
That's denoted by the single box.
And they're both Level 1.
So the first question is, will this form an adjacency?
And the answer to that is yes, it will.
And, in fact, what it will form is a Level 1 adjacency.
What about Level 1, Level 2 in the same area?
The answer to this one is no, it will not.
There has to be a level in common in order for these
to form a neighborship.
So we can have one Level 1 and one Level 2
form a neighborship.
How about a Level 1 and a Level 1-Level 2 router?
The answer here is once again yes,
they will form a neighborship.
And, of course, they're going to form that neighborship
over the only level they have in common.
So this again will be a Level 1 adjacency.
What about an L2 and an L1 L2 in the same area?
Now, in the same area, again, we can do just Level 1
like we did in our first example.
But guess what, Level 2 will work as well.
So first, yes, this will form an adjacency.
And, of course, what it's going to do
is it's going to form a Level 2 adjacency.
Our next choice is a Level 1 and a Level 2 for both devices.
The answer to this one is first off, yes,
they will definitely form an adjacency.
But what they'll do is they'll actually
form two separate adjacencies, one for Level 1 and one
for Level 2.
So in this case, you're going to end up
with two adjacencies between the routers.
So now let's talk about what happens if the routers are
in different areas.
So here we have two Level 1 routers in two different areas.
Now, in order to join areas, this has to be Level 2.
So our simple answer here is this simply will not work.
We will get no adjacency Level 1 to Level 1 between areas.
What about Level 2?
Well, as we just stated, this is sort of exactly the purpose
of Level 2.
So this is definitely going to form an adjacency.
And since they're both Level 2 and Level 2
is the only adjacency that will work here,
the adjacency will, of course, be Level 2.
Next option, Level 2 to Level 1-Level 2.
Well, once again, this is a good combination
because we have both devices that
are Level 2, which means they will form a Level 2 adjacency.
So this again will work fine.
This one is a little bit different.
What about an L1-L2 to an L1-L2?
Well, again, they will form a neighbor relationship
or an adjacency because we have that Level 2.
But keep in mind that only the Level 2 will work here.
We will not get a Level 1 adjacency
because they're between areas.
So now let's do a quick demonstration
of the different network types and a little bit
about the adjacencies by taking a look at the command line,
and we'll jump in and take a look at the hello packets
in Wireshark.
Let's begin by taking a look at the topology.
Just so we have a rough idea of what's going on.
And then we'll jump into the CLI.
So for this demonstration, we'll just
be using the IS-IS basic topology.
We're going to be mostly focusing
on router 1 and the link between R1 and R2,
which as we can see here is in fact Ethernet.
It's out both of these routers gigabit 0000 interface.
It is currently set to a LAN network type, which we'll see.
And then we'll convert this to point-to-point.
So let's get started by just jumping into the command
line on router 1.
So first, let's go ahead and look at that gigabit 0000
interface.
So let's just show IS-IS interface gig 0000.
What we're looking for specifically is right here.
It's currently set to media type, LAN.
This is what we're going to be changing.
However, before we do, we're going
to hop over into Wireshark and take
a look at these hellos that are being sent between router 1
and router 2.
Also, note that this is also a Level 1 circuit.
We'll see that in Wireshark as well.
So here we have a capture of router 1's gigabit 0000
interface, and the capture has been filtered for IS-IS.
Now, if we look here, the very first packet
is actually an IS-IS hello.
We can actually see in the info that it's
an L1 hello specifically from system ID 1, which is router 1.
If we open the ISO header, we can see right here
that this is a Level 1 hello.
It's type 15.
We can see the same thing if we look at a hello coming
from router 2.
In this example, packet 5 happens
to be a hello coming from router 2,
and we see that just like router 1, this is an L1 hello type 15.
So now let's go ahead and change that gigabit 0000 link over
to point-to-point.
So simply go into configuration mode
then into the router IS-IS process,
ours is called SP1 then into our interface.
And all we have to say here is point-to-point.
Now pretty much as soon as we commit this,
we're going to start getting console messages
where we were receiving a type 15 Hello when
what we're expecting is a point-to-point hello.
So it's telling us that we're receiving
the incorrect hello type.
Now we have to change this on router 2 as well.
We see we have the message on router 2 also.
Once again, all we have to do is go into our interface,
change it to point-to-point.
So we'll verify this.
Let's go back over to router 1 just to do the verification.
Let's start with a show IS-IS neighbor.
We see that the connection to router 2
is in fact now set to point-to-point.
We can also verify this with the show IS-IS interface again
for gig 0000.
And we see that now the media type is set to point-to-point.
Let's go look at this on Wireshark.
So once again, we've captured the hellos.
Notice, however, under info, it now
specifically says point-to-point hello.
And if we look down here at our headers,
specifically, if we open up the ISO,
we now see that this is a point-to-point hello type 17.
So we can see that it's using a completely different hello type
on the point-to-point connection.
So in this video, we saw a little bit more information
about the adjacencies that have to be formed with IS-IS.
We started off by taking a look at the different network
types, broadcast versus point-to-point.
And then we also talked a bit about the neighborships that'll
be formed between Level 1 and Level 2 devices, depending
if they're in the same area, different areas, and all
the different combinations.
I hope this has been informative for you,
and I'd like to thank you for viewing.