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Sometimes in OSPF networks,
we have to use a Band-Aid.

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And when we have to use a
Band-Aid called a virtual link,

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sometimes
authentication bites us.

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[MUSIC PLAYING]

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So let me give you a sneak peek
at the world of a virtual link,

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why we need it.

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We have a whole
separate skill coming up

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on troubleshooting OSPF
with virtual links.

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But let me explain it,
basically, like this.

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In OSPF, we have
some rules in place.

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So you have area 0.

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And any other areas have to
have an area border router.

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That means you cannot
do something like this.
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That means you cannot
do something like this.

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You can't take router 8
and have a link on R8.

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You put it into area 50, and as
a result, expect that network--

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let's call it the
10.50.50 network--

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to be advertising
the rest of OSPF.

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Because this network in
area 50 doesn't have--

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area 50 does not have an ABR.

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The backbone's up here, area 0.

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This is area 2.

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And then there's this network
hanging off here in area 50.

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So R8 knows about that
network in that area,
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So R8 knows about that
network in that area,

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but nobody else does.

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So to solve that,
what we can do is

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we can do something
called a virtual link.

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And it goes something like this.

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We just simply tell
R8 and R4, which

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is the area border router, who
is connected to the backbone

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area.

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We simply create this
logical neighborship

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between R8, who has this extra
area, and R4, the area border

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router.

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And that is called
a virtual link.

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And the way you set it
up-- and again, we'll will

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have some hands-on practice
in a separate skill.

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But the way we set it
up is we tell R4, hey,
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But the way we set it
up is we tell R4, hey,

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I need you to use area
2 as a transit area

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for this virtual link
between you and R9.

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And so basically, you say,
virtual link, my neighbor

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is going to be R8,
and his router ID.

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And R8, we say, hey, my
neighbor over this virtual link

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is going to be R4, the
area border router.

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And we're going to
point to its router ID.

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And what happens
is R8 now logically

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shows up as being
connected to the backbone.

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And as a result,
also, this area 50,

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because it has R8 as
an area border router,
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because it has R8 as
an area border router,

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all those routes can now
be advertised in the area

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because they were not
breaking the rules.

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All right, so Band-Aid,
Band-Aid, Band-Aid.

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We would not want to
design a network that

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required a virtual link.

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It's meant to be a
temporary solution.

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However, this is where it can
bite you with authentication.

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If area 0 is requiring
MD5 authentication

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like we configured in the
previous video, and all

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of a sudden, R8 is now
connected to area 0,

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we need to tell R8
that for area 0,
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we need to tell R8
that for area 0,

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we're using MD5 authentication.

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See, before, R8 wasn't
in the backbone area.

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But now it is.

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And so if we forget to do
that authentication to tell it

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that area 0 on R8 is using MD5,
it won't form that adjacency.

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It won't neighbor up,
and it won't work.

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All right, so having said
that, what I'd like to do

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is give you two
troubleshooting labs

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that are going to focus
on troubleshooting

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with an eye towards
authentication,

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something going wrong
with authentication.

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And I thought to myself, also,
for these troubleshooting labs,

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if I just do authentication,
it might be too obvious.
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if I just do authentication,
it might be too obvious.

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Like, I bet it's
authentication-based.

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So what we're going to do in
these next two labs coming up

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right now, there
will be at least

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one authentication-related
problem in the network,

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and something else that we
will also have to troubleshoot.

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And that way, we're going
for full reachability.

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Everything functioning.

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And you can have practice
and troubleshooting that

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includes authentication issues.

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All right.

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So if you're ready, I'll
see you in the next video

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for our troubleshooting
lab number 1.

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Meanwhile, 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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and I'd like to thank
you for viewing.