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[VIDEO LOGO]

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Next up is trouble ticket 4.

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For this one now, we're going to
be using IS-IS IPv6 topology 1.

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That would be because
this ticket involves IPv6.

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So let's take a
look at our ticket.

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It says that router 5 is missing
the IPv6 routes to R2 and R4's

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loopback interfaces.

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So R2 and R4, of
course, have loopbacks.

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Each of these have
IPv6 addresses on them.

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And according to the
ticket right now,

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router 5 does not
have these routes.

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So let's start, as always,
by jumping in and verifying

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the ticket, and then let's
see what the problem is.

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Let's go ahead and jump
into the CLI on router 5.

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So we'll start off on
router 5 by just looking

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at the show IPv6 routing table.

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And what we see here
is we're actually

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getting an IPv6 default
from router 2 and router 4.

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Now, this is actually
fairly significant from

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a troubleshooting standpoint.

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The reason I say that is we can
actually skip over things like

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are the neighborships up?

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Clearly, the
neighborships are up

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because we're getting
routes from them.

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Are we getting the
routes that the ticket

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says we should be getting?

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No, but we are getting routes.

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So this doesn't appear to
be a neighborship issue.

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Honestly, it looks more like
a filtering issue possibly,

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or maybe those routers aren't
bringing their loopback

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interfaces into IS-IS.

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So this looks like
some other, sort of,

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an issue other
than neighborship.

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So let's skip over the
neighborship troubleshooting

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at least for now, and start
going after other things.

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My first guess
would be filtering.

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However, I will say that before
we jump right into filtering,

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we probably should at least make
sure that router 2 and router 4

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do, in fact, have the
interfaces in IS-IS.

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So before we go
right for filtering,

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let's jump throughout or
R2 and do a little bit

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of troubleshooting there.

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So on router 2, let's do a
show IS-IS interface brief.

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And we see that, in fact,
loopback 0 is in IS-IS.

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And just for curiosity,
let's see if router 2

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has router 4's loopback.

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So let's do a show
route IPv6 IS-IS,

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and we can see that it does.

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Right here is a route
to router 4's loopback.

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So this indicates
that at least router 4

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is bringing its loopback in.

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We already saw that router 2's
interface is enabled for IS-IS.

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So it really seems like 2 and
4 are advertising properly.

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Of course, just to
be completely sure,

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we could look at router 4.

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Just to show route, IPv6 IS-IS.

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And here, we see
router 2's loopback.

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So yes, both sides are
advertising their loopbacks

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into IS-IS.

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So this would bring
up the question

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that maybe it's filtering.

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Well, let's take a look here.

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Let's do a show
run router IS-IS.

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So if we look at
this output, there's

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no filtering or no configuration
at all for the address family

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IPv6.

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Now, it is enabled
under our interfaces.

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However, there's no
configuration where

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we're doing any filtering.

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Let's check router 2 as
well just to make sure.

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And we see the same thing here.

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We do have some
propagation in such setup,

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but again, these are
under address family IPv4.

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Nothing for IPv6
aside from enabling it

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under the interfaces.

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Well, maybe the filtering
is happening on router 5.

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So let's go look there.

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On router 5, we'll do a show
run section router IS-IS,

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and we see there's absolutely
nothing configured here either.

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We simply have
address family IPv6

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and then exit address family.

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There's no IPv6 configuration
here on router 5.

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So this doesn't look like
it's a filtering issue.

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So maybe we should just go right
for looking at the database.

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Let's do a show
IS-IS IPv6 topology,

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and we see that there's
actually no information

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in the IPv6 topology.

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So this is interesting.

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Let's maybe take a
look at some debugs.

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So let's turn on terminal
monitor and let's debug IS-IS.

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Let's go for adjacency
packets since, again, this

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seems to possibly be an
issue with the databases.

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So we've got a lot of
debugs there fairly quickly.

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So we turn them back off.

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We have an interface level
mismatch on gigabit 2.

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But is that really our problem?

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The link between
router 2, 4, and 5

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is a Multi-Access segment.

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Now, we have level 1 and
level 2 adjacencies configured

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between routers 2 and 4.

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Router 5 is actually
only configured

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for level 1 adjacencies.

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This is why we're
getting these messages.

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Now, that may or may
not be a good design.

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We won't get into that
argument right now,

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whether we should have
these type of HELLOs

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going on a network where
not all the neighbors are

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participating.

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That's another discussion.

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It's probably not
the best design.

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However, it's also
not our problem.

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So let's go ahead
and keep digging

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a little further because this
debug, although it's giving us

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some information, isn't anything
that's actually causing us

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a problem for this
particular ticket.

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So maybe, in this case, we need
to go to Wireshark for help

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once again.

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So let's start by looking
at this HELLO coming

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from router 4, and let's
expand the IS-IS HELLO header.

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So looking over
the HELLO, we see

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that it's set to circuit
type level 1 and level 2.

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That we saw in the debug.

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And we already
discussed the fact

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that 2 and 4 are
both set that way.

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So that's not really a concern.

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Our System ID is correct.

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Our PDU is 1497, which
we know is the default.

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If we look at our
protocols supported,

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we can see that we're
supporting both IP and IPv6.

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If we scroll down,
we see something here

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that may be of interest.

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We see that this is
set to Multi-Topology.

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So this has a TLV type
229 for Multi-Topology.

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But we just looked at the
configuration on router 5.

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Router 5 had nothing in
its configuration for IPv6.

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It was empty.

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So we know that IOS XE actually
defaults to single topology.

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But here, we have router 4
sending a Multi-Topology packet

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onto the wire.

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Let's check router
2 and router 5,

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and I bet we're going to
find a difference here.

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Now, router 2 here, we can see
does have the Multi-Topology

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TLV.

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However, if we look
here at router 5,

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we can see that as we
suspected, it does not

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have the Multi-Topology TLV.

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So even though we have the
correct protocols supported

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here, IP and IPv6, we have
to match on Multi-Topology.

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So what we need to do
here is change router 5

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to Multi-Topology.

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We could, of course,
change router 2 and 4

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to Single-Topology.

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But in this particular case,
it would make more sense

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to change router 5
to Multi-Topology.

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So let's jump into
Configuration on router 5,

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do that, and see if
it solves our problem.

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Now, again, this
is a reminder here.

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If we do show run
section IS-IS, we

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can see that there is no
configuration under address

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family IPv6.

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Again, we saw this
earlier, but I just

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wanted to bring it up
again as a reminder.

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So let's go to Configuration
mode, into router IS-IS 1,

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into address family
IPv6, and simply

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change it to Multi-Topology.

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If this solves our
problem, we should

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be able to verify the original
ticket with a show IPv6 route.

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And we can see that we do,
in fact, now have router 2

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and router 4's loopback
in our routing table.

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So Multi-Topology was,
in fact, the issue.

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So in this video, we
took a look at a ticket

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involving a
Single-Topology versus

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a Multi-Topology mismatch.

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Keep in mind that we
can support both IPv4

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and IPv6 in either
Single-Topology

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or Multi-Topology.

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However, the devices
within the area

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do have to agree on whether it
is Single or Multi-Topology.

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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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