Sometimes in OSPF networks, we have to use a Band-Aid.
And when we have to use a Band-Aid called a virtual link,
sometimes authentication bites us.
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So let me give you a sneak peek at the world of a virtual link,
why we need it.
We have a whole separate skill coming up
on troubleshooting OSPF with virtual links.
But let me explain it, basically, like this.
In OSPF, we have some rules in place.
So you have area 0.
And any other areas have to have an area border router.
That means you cannot do something like this. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
That means you cannot do something like this.
You can't take router 8 and have a link on R8.
You put it into area 50, and as a result, expect that network--
let's call it the 10.50.50 network--
to be advertising the rest of OSPF.
Because this network in area 50 doesn't have--
area 50 does not have an ABR.
The backbone's up here, area 0.
This is area 2.
And then there's this network hanging off here in area 50.
So R8 knows about that network in that area, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So R8 knows about that network in that area,
but nobody else does.
So to solve that, what we can do is
we can do something called a virtual link.
And it goes something like this.
We just simply tell R8 and R4, which
is the area border router, who is connected to the backbone
area.
We simply create this logical neighborship
between R8, who has this extra area, and R4, the area border
router.
And that is called a virtual link.
And the way you set it up-- and again, we'll will
have some hands-on practice in a separate skill.
But the way we set it up is we tell R4, hey, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But the way we set it up is we tell R4, hey,
I need you to use area 2 as a transit area
for this virtual link between you and R9.
And so basically, you say, virtual link, my neighbor
is going to be R8, and his router ID.
And R8, we say, hey, my neighbor over this virtual link
is going to be R4, the area border router.
And we're going to point to its router ID.
And what happens is R8 now logically
shows up as being connected to the backbone.
And as a result, also, this area 50,
because it has R8 as an area border router, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
because it has R8 as an area border router,
all those routes can now be advertised in the area
because they were not breaking the rules.
All right, so Band-Aid, Band-Aid, Band-Aid.
We would not want to design a network that
required a virtual link.
It's meant to be a temporary solution.
However, this is where it can bite you with authentication.
If area 0 is requiring MD5 authentication
like we configured in the previous video, and all
of a sudden, R8 is now connected to area 0,
we need to tell R8 that for area 0, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
we need to tell R8 that for area 0,
we're using MD5 authentication.
See, before, R8 wasn't in the backbone area.
But now it is.
And so if we forget to do that authentication to tell it
that area 0 on R8 is using MD5, it won't form that adjacency.
It won't neighbor up, and it won't work.
All right, so having said that, what I'd like to do
is give you two troubleshooting labs
that are going to focus on troubleshooting
with an eye towards authentication,
something going wrong with authentication.
And I thought to myself, also, for these troubleshooting labs,
if I just do authentication, it might be too obvious. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
if I just do authentication, it might be too obvious.
Like, I bet it's authentication-based.
So what we're going to do in these next two labs coming up
right now, there will be at least
one authentication-related problem in the network,
and something else that we will also have to troubleshoot.
And that way, we're going for full reachability.
Everything functioning.
And you can have practice and troubleshooting that
includes authentication issues.
All right.
So if you're ready, I'll see you in the next video
for our troubleshooting lab number 1.
Meanwhile, I hope this has been informative for you,
and I'd like to thank you for viewing. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and I'd like to thank you for viewing.