[AUDIO LOGO]
Moving on to Trouble Ticket 5, we're back on IS-IS Topology 1.
And this time, the ticket is that the IS-IS neighborship
between router 2 and 9 is currently down.
So now we'll be looking up here between routers 2 and 9.
And one thing to note as we head into this that router 2
is in area 10.
Router 9 is in area 30.
So this is going to have to be a level 2 adjacency.
Just keep that in mind, as we head into this ticket,
these are some of the things we'll be looking for.
So let's jump into the command line
and see what's going on with this neighborship.
So as always, let's begin with verifying the ticket.
We'll do a show isis neighbor.
And we see that, in fact, router 9 has no neighbors.
Not a big surprise.
That is what the ticket said.
So let's do a show clns interface.
Specifically, of course, we care about the Gigabit 2 interface
here.
We see that it is up, up.
We see that the MTU is 1497.
We know that's the default. We see that the routing protocol
is IS-IS.
And we see that the circuit type is level-1.
It's also showing us that the number of level 1 adjacencies
is zero.
Now, clearly, we can see the problem here already.
We see that this is a level 1 connection.
And we even discussed during the introduction
that this needs to be level 2.
So we do sort of see the problem already.
But before we just jump in and fix it,
let's explore some other things that
would show us this problem.
That way, if you're not just looking right
at a topology diagram and are, well,
led right into this problem, so to speak,
what are some other things or other places
that we can see this to realize that it's a problem?
So let's explore this just a little bit more
before we just go ahead and fix it.
So let's begin by turning on terminal
monitor here on router 9.
And let's do a debug isis adjacency packets
and see if these are showing us anything.
So what are we really seeing here
on the output of the debug?
Well, we see that we're sending a serial hello on Gigabit 2,
that it's in a three-way state, and it's currently INIT.
So again, the neighborship is not forming.
Then we see that we're actually receiving a hello from router
2 on our Gigabit 2 interface.
And there's two things we can see about this hello.
First, we can see that the circuit
type is L2, which we already know that our interface is L1.
So that's a problem.
And we also see further down here
that it's saying that there's no matching areas.
Remember, it's trying to form a level 2.
We're only configured for level 1.
And we're in different areas.
So we can't form a level 1 adjacency.
Therefore, we have no matching areas
we can use to bring up an adjacency.
We can also see what's going on in Wireshark.
So here we have a hello from router 2.
And if we open up the ISIS HELLO header, of course,
there's going to be two main things here we're looking for.
The first is that this is level 2 only.
The second is the area address.
So if we look at our area address,
we can see that it's area 10.
And let's look at the one from router 9.
We see here on the hello from router 9
that, again, it's level 1 only, which
we know is incorrect, particularly when the area
address we can see is area 30.
So again, we can't form a level 1 adjacency
when we have no areas in common.
This must be a level 2 adjacency.
So this one's actually going to be a fairly easy fix.
All we have to do is go into router 9
and change our connection to be level 2.
So let's go ahead and do that and then verify
that it's working.
So we'll go into configuration mode into interface Gigabit 2.
We'll set the isis circuit-type to be level-2-only.
We'll go ahead and exit out of configuration.
We can see here already that our neighborship with router 2
has come back online.
We, of course, still want to verify it
with a show isis neighbor.
And we can see that it is in fact up, and it's a level 2.
And if everything's working completely correctly,
if we do a show ip route isis, we
should actually have some IS-IS routes in our routing table.
And we can see here that we do.
So in this video, we sort of saw two things
related to this ticket.
The first is that in order for a neighborship to form at all,
they have to be the same level.
So we can have one router configured for level 1 only
and the other router configured for level 2 only.
The other thing is we have to use the correct level type,
particularly between areas.
Within an area, we could theoretically
use level 1 or level 2.
Although, if it's not a joining router, meaning it's
not an edge router to another area,
we would generally use a level 1 adjacency to that router.
But the point is, between areas, it has to be level 2.
We have no choice.
I hope this has been informative for you,
and I'd like to thank you for viewing.