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[AUDIO LOGO]

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Moving on to Trouble Ticket 5,
we're back on IS-IS Topology 1.

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And this time, the ticket is
that the IS-IS neighborship

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between router 2 and
9 is currently down.

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So now we'll be looking up
here between routers 2 and 9.

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And one thing to note as we
head into this that router 2

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is in area 10.

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Router 9 is in area 30.

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So this is going to have
to be a level 2 adjacency.

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Just keep that in mind, as
we head into this ticket,

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these are some of the
things we'll be looking for.

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So let's jump into
the command line

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and see what's going on
with this neighborship.

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So as always, let's begin
with verifying the ticket.

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We'll do a show isis neighbor.

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And we see that, in fact,
router 9 has no neighbors.

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Not a big surprise.

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That is what the ticket said.

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So let's do a show
clns interface.

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Specifically, of course, we care
about the Gigabit 2 interface

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here.

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We see that it is up, up.

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We see that the MTU is 1497.

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We know that's the default. We
see that the routing protocol

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is IS-IS.

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And we see that the
circuit type is level-1.

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It's also showing us that the
number of level 1 adjacencies

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is zero.

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Now, clearly, we can see
the problem here already.

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We see that this is
a level 1 connection.

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And we even discussed
during the introduction

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that this needs to be level 2.

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So we do sort of see
the problem already.

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But before we just
jump in and fix it,

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let's explore some
other things that

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would show us this problem.

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That way, if you're
not just looking right

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at a topology diagram
and are, well,

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led right into this
problem, so to speak,

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what are some other
things or other places

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that we can see this to
realize that it's a problem?

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So let's explore this
just a little bit more

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before we just go
ahead and fix it.

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So let's begin by
turning on terminal

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monitor here on router 9.

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And let's do a debug
isis adjacency packets

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and see if these are
showing us anything.

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So what are we
really seeing here

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on the output of the debug?

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Well, we see that we're sending
a serial hello on Gigabit 2,

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that it's in a three-way
state, and it's currently INIT.

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So again, the neighborship
is not forming.

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Then we see that we're actually
receiving a hello from router

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2 on our Gigabit 2 interface.

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And there's two things we
can see about this hello.

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First, we can see
that the circuit

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type is L2, which we already
know that our interface is L1.

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So that's a problem.

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And we also see
further down here

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that it's saying that
there's no matching areas.

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Remember, it's trying
to form a level 2.

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We're only configured
for level 1.

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And we're in different areas.

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So we can't form a
level 1 adjacency.

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Therefore, we have
no matching areas

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we can use to bring
up an adjacency.

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We can also see what's
going on in Wireshark.

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So here we have a
hello from router 2.

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And if we open up the ISIS
HELLO header, of course,

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there's going to be two main
things here we're looking for.

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The first is that
this is level 2 only.

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The second is the area address.

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So if we look at
our area address,

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we can see that it's area 10.

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And let's look at the
one from router 9.

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We see here on the
hello from router 9

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that, again, it's
level 1 only, which

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we know is incorrect,
particularly when the area

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address we can see is area 30.

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So again, we can't form
a level 1 adjacency

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when we have no areas in common.

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This must be a
level 2 adjacency.

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So this one's actually going
to be a fairly easy fix.

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All we have to do
is go into router 9

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and change our
connection to be level 2.

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So let's go ahead and
do that and then verify

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that it's working.

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So we'll go into configuration
mode into interface Gigabit 2.

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We'll set the isis circuit-type
to be level-2-only.

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We'll go ahead and exit
out of configuration.

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We can see here already that
our neighborship with router 2

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has come back online.

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We, of course, still
want to verify it

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with a show isis neighbor.

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And we can see that it is in
fact up, and it's a level 2.

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And if everything's working
completely correctly,

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if we do a show
ip route isis, we

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should actually have some IS-IS
routes in our routing table.

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And we can see here that we do.

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So in this video, we
sort of saw two things

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related to this ticket.

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The first is that in order for
a neighborship to form at all,

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they have to be the same level.

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So we can have one router
configured for level 1 only

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and the other router
configured for level 2 only.

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The other thing is we have to
use the correct level type,

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particularly between areas.

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Within an area, we
could theoretically

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use level 1 or level 2.

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Although, if it's not a
joining router, meaning it's

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not an edge router
to another area,

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we would generally use a level
1 adjacency to that router.

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But the point is, between
areas, it has to be level 2.

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We have no choice.

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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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