All right.
Before we start troubleshooting virtual links
or configuring them, we first ought
to take a nice solid look at what exactly they are
and why we might need them.
And let's use this chalkboard to do exactly that.
And let's discuss the rules for OSPF, the two layer hierarchy.
So we're going to have a backbone always.
That's going to be area 0 in our OSPF network.
And then let me go ahead and use green over here.
So let's say this is area 1.
And let's create another area. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And let's create another area.
And let's make it a purply color like that,
and make that area 2.
All right.
So we're going to have a backbone.
Now, what is in common between any area that
is not the backbone?
The commonality is it needs to have an ABR that's
connected to the backbone.
So let me bring up some routers.
I'll use the white color for them.
So let's imagine we have a router here that's connected.
It has an interface in area 0.
And it has an interface in the other non backbone area.
That device-- ba da bing-- is going
to be an area border router, just WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to be an area border router, just
because it has those two interfaces as part
of area 0 and area 1.
And we have another one over here
that's connected with at least one interface in area 2,
and another interface connected to area 0.
So that would also be an ABR.
And so in our topology let's imagine that this is R3
and this is R4.
And we could have other routers that
are only in the backbone, maybe R1 and R2.
And we could have multiple other routers there.
And then in area 1 we could have additional routers as well. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And then in area 1 we could have additional routers as well.
Maybe we add a router here, R5, which is only in area 1.
It has no interfaces outside of area 1.
And maybe we have another router, R7.
I'll put it in right here that has at least one
or more interfaces in area 1.
And the same thing over here in area 2.
We could have routers like R6 that all of its interfaces
are in area 2 and router 8, with one or more interfaces
in area 2 as well. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
in area 2 as well.
And the rule is, if we want to add additional areas,
it's cool.
We can do that, but every area that's not the backbone
has to have an ABR connecting that area,
like area 5 or area 51 to the backbone.
So we can't have this.
Let's go ahead and use a different color like this.
We couldn't take another area like this.
Let's call this area 78.
And let's say R7 is connected to it and R8 is connected to it.
We couldn't have area 78 right here, because there's
no area border router.
There's no area border router that is a router in area 78 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
There's no area border router that is a router in area 78
that is also connected to the backbone.
So this would not fly.
And we could configure this.
We could configure R7 and R8 with area 78.
However, any routes in that area would not
be known by the rest of the OSPF domain.
So there's a few solutions to this.
Let's talk about those.
One of the solutions would be, well, we'll just go ahead,
and we'll connect R7 to the backbone area.
And that would make R7 an ABR.
Because R7's connected the backbone, and also area 78. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Because R7's connected the backbone, and also area 78.
And as a result, he's an ABR.
No problem.
That works.
And that is a possible solution.
Let me go ahead and back that off a few strokes here.
So another solution to this would be just
don't call this area 78.
Call it area 1, or call it area 2.
And that will just be an extension of the non backbone
area.
But if it's a different area that
has no ABR, short of having R7 connect directly
to the backbone, we could use the concept of a virtual link.
Think of it like a Band-Aid.
That's really what it is.
It's a temporary path that R7 can use to logically-- WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It's a temporary path that R7 can use to logically--
not physically, but logically connect to the backbone
and become an ABR even though it doesn't
have a real interface that's in a network that's
in the backbone.
And so here's how we would do it.
Let me go ahead and grab a different color
for the virtual link.
Let me go ahead and let's use red.
So to create a virtual link, we need
to find an ABR that's willing to help us out, says R7.
R7's closest ABR right here is R3.
So R3 has a router ID of 3.3.3.3. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So R3 has a router ID of 3.3.3.3.
And R7's router ID is 7.7.7.7.
Now, do they know how to reach each other?
The answer is, yes.
They're both in area 1.
And what we could do is we could create a virtual link that
goes from the router ID on R3 down to the router ID on R7.
And I'll just put an arrow here.
That's literally our virtual link.
Once we have that in place, R7 believes, logically,
that he is connected to the backbone
courtesy of this virtual link.
In fact, let me draw this a slightly different way. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
In fact, let me draw this a slightly different way.
Between R7, logically, and R3, again, using the router IDs.
That's probably one of the biggest mistakes when people
are implanting virtual link for the first time is
they point to the IP address on the interface instead
of the router ID of the router they're
going to logically connect with.
So R7 would say, I'm creating a virtual link over area 1.
That would now be what's called a transit area to R3.
And on R3, we'd go to router configuration mode
and say we're creating a virtual link down to R7.
And once we've done that on both sides, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And once we've done that on both sides,
R7, if we do a show IP OSPF it believes,
hey, I've got a connection, this logical, virtual link
connection into the backbone, which makes him
an ABR, which then allows these routes in area 78
to be propagated and advertised as they normally
would be if we had an area that had a normal ABR.
So it's simply a way to get these routes
in area 78 advertised in our OSPF domain,
even if we don't have a directly connected router to area 0,
using this concept called a virtual link.
Another good point to just be aware of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Another good point to just be aware of
is that when we're using virtual links,
it's only because we're solving a temporary problem.
We would never design a new network,
unless we're building a troubleshooting lab.
We'd never design a brand new network
that would require virtual links if we're designing it
from scratch.
So if we are using virtual links,
it's very possible that we're going
to redesign the network over time
to make sure that we don't have to have
those virtual links in place for an indefinite period of time.
All right.
So in the next video, here's what we're going to do.
I'm going to put us on a lab in the next video
where we'll have the opportunity to both implement
and verify a virtual link. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and verify a virtual link.
And that's a good idea.
It's a good idea to practice with it
before we're put in the throes of having to troubleshoot it.
So I'll see you next in that configuration and verification
lab in the very next video.
Meanwhile, I hope this has been informative for you.
And I'd like to thank you for viewing.