- [Instructor] In this video, we want to talk about
the concept of an OSPF Virtual Link.
Now, normally we're not going to have these Virtual Link,
but we might be in a situation where we have a couple
of companies that merge
and we connect to their OSPF network with our OSPF network.
And we end up with an area that's not adjacent
to Area 0, and that's a requirement.
All OSPF areas need to be adjacent to the backbone area,
which could be numbered Area 0,
or it could be numbered areas 0.0.0.0.
If we want to use that dotted decimal notation for our areas,
most people just use Area 0 though.
And this adjacency yet could be physically adjacent.
That's the way it's normally done
is we're designing an OSPF network.
But if we do have something like that
company merger example that I was giving you,
we might have to create a Virtual Link.
A Virtual Link will allow us to logically span
a transit area to make us logically adjacent
to the backbone area.
That's what's going on here with area to Area 2,
Area 2 is not physically adjacent to Areas 0.
But we go into the ABR as we leave Area 2
the discontiguous area.
And we go into the ABR as we leave the backbone area,
Area 0, in other words, routers R2 and R3,
and we create a Virtual Link between them
that spans this transit area.
And the transit area in this topology is Area 1.
And over that Virtual Link,
we're going to be sending our OSPF updates.
However, it's interesting that the data flows
directly through Area 1.
It is not logically go through the Virtual Link.
OSPF messages again, they are encapsulated
and they are sent over the Virtual Link,
but this is different than something like a GRE Tunnel,
where we have to worry about the Tunnel Overhead.
We're sticking on our data
and we end up oftentimes reducing our MTU size.
We don't have to worry about that here,
because here the data is still going through Area 1
without doing any sort of encapsulation
to go through the Virtual Link.
It's only the OSPF messages
that are using the Virtual Link,
but it makes Area 2 look, like gets adjacent logically
to the backbone area.
So we're only tunneling the routing updates,
not the data itself.
And there's a requirement for this transit area.
It cannot be a stub area.
So Area 1 in this case could not be a stub area.
Now let's go out to a live interface
and take a look at how to configure an OSPF Virtual Link.
Now currently in this topology,
we've got OSPF configured on all three routers,
but notice that network 172.16.1.0/24.
It lives in Area 2, and that area is breaking the rules,
specifically Area 2 is not adjacent,
to the backbone area, Area 0.
That means if we go into R1,
we probably would not be able to see a 172.16.1.0/24
and the same holds for that loop back interface on R3
3.3.3.3/32, let's confirm that.
On R1, I'm going to do a show ip route command,
and there is no sign of 172.16.1.0 or 3.3.3.3/32.
Now to fix this issue,
what we can do is create a Virtual Link,
To do that, we're going to go into the ABR,
leaving the backbone area,
which is Areas 0, in this case, that ABR is R2.
And we're going to go into the ABR
leaving the discontinuous area, which is Area 2.
And this topology that ABR leaving Area 2, that's our R3.
And we're going to go to each of those routers R2 and R3,
and we're going to create a Virtual Link
that spans the transit area.
We need to specify that transit area that we're spanning.
In this case, we're spanning Area 1,
that's our transit area.
And we're going to point to the far side of the Virtual Link.
So R2 is going to point to R3 and vice versa.
Let's begin on route R2.
On route R2, let's go into Global Configuration mode
and I'll say, router, OSPF 1,
I've already got that OSPF writing process started.
I just need to add on the Virtual Link configuration,
I'm going to say area, and I specify the transit area here.
I don't specify my area.
I don't specify the discontinuous area.
I specify the transit area, which is Area 1.
And I say, I want to create a Virtual Link.
So virtual-link, and I give the IP address.
I'm going to use the loop back interface of R3.
That's going to be at the far side of this link.
I'll say 3.3.3.3, and we'll press enter.
And we're done with our Virtual Link Configuration on R2.
Let's go over to R3 and do sort of a mirrored configuration.
On R3 we'll say router ospf process-id 1, area 1
same transit area, virtual-link.
And I'm going to point to the loopback IP address on R2
that lives at the other side of this virtual link.
And that's going to be 2.2.2.2.
And in just a few moments,
we should have a virtual link form.
Let's see if we get an adjacency pop up here in a moment.
All right, after a few seconds,
we saw an adjacency be established.
Let's take a look at who our new adjacency
or who our new neighbor is.
I'll do a show ip ospf neighbor command.
Well, look at this.
I'm a neighbor with router R2, a couple different times.
I'm a neighbor with R2 over that ethernet link,
which is electing a DR and a BDR,
but I'm also a neighbor over this interface.
This OSPF Virtual Link one interface.
Notice that we're not electing a DR or BDR there.
Now let's go over to R1 and see if we've learned about
that 172.16.1.0/24 network, and the 3.3.3.3/32
loopback IP address on R3.
We didn't see that before.
Let's go back over to R1 and once again,
we'll do a show ip route command.
And look at this,
we now know about the 172.16.1.0/24 network.
And we also know about the 3.3.3.3 loopback IP address
over on route R3.
Now let's check out the OSPF link state database on R1
and see if anything is different.
Now that we've got this Virtual Link in our topology,
let's do a show ip ospf, database command.
And some of these entries are using a Virtual Link.
For example, there are a couple of ways
to get to 203.0.113.0 and one is using a Virtual Link.
And anytime we have learned about a network
via a Virtual Link,
we've got this DNA tag that shows up.
What is up with DNA?
Well, DNA stands for DoNotAge,
and that helps reduce the amount of traffic
flowing over our Virtual Link.
Specifically, a Virtual Link
is treated like an OSPF Demand Circuit,
which was traditionally used on lease lines,
but a Demand Circuit, tells OSPF not to send
any periodic Hello messages
or any periodic LSA Refreshes.
Now if there's a change in the network,
we'll send an update,
but there's not going to be a periodic refresh.
It helps reduce the amount of traffic flowing
over a Demand Circuit.
And a Virtual Link is a Demand Circuit.
Now, finally, let's go to router R2
and take a look at the OSPF characteristics
of this virtual interface that we now have.
Let's go over to R2
and we'll do a show ip ospf, interface command.
Notice the name of the interface is OSPF_VL1
for Virtual Link one.
And there's a network type just for these Virtual Link.
However, it's acting as a point-to-point link.
Notice it's got the same at timers
as we would have for a point-to-point link,
and because there are only two devices
that can be on this Virtual Link.
There's no need to let the DR or BDR,
and also notice that we're configured as a Demand Circuit.
And that's a look at the theory and configuration
of an OSPF Virtual Link.