This is one of multiple videos discussing multi area OSPF.
This is both a troubleshooting as well as an explanation of video.
We've been told in this topology that router one on the left is not able to ping router six on the right,
router one is running IGP.
So the command show IP protocols.
Shows us that the only routing protocol enabled on this rudder is air GOP.
En route of six show IP protocols shows us that OSPF is enabled on router six.
We can also see that the router has a loopback address of 6.6.6.6, so quadruple six can write a one
ping that address and the answer is no pings or failing.
All disable IP domain, look up to speed things up and let's do a trace to that loopback after the moment
of the trace is going nowhere.
I'll do a debug IP packet to see what happens and I'll ping with only one ping to that address.
So ping 6262626.
Repeat one.
And let's do that again.
And I'll turn off debugging.
Notice here.
Source 1.1.1.1.
Pinging 6262626.
We're told that it's unreadable.
These are the messages are air copy messages.
The ping is unsuccessful.
So show IP rot shows us that that network doesn't exist in the routing table.
If we try and ping ten .1.5.1, that also fails and so does 10.15 to 2.
So we can't ping this network or the loopback of router six.
What about the loopback of router five?
So we able to ping the loopback or out of five, but not the loopback of router six.
And if we trace to ten .1.5.1, the trace fails immediately because the route doesn't exist in the routing
table.
Ten .1.4.0 does exist in the routing table, so we can ping 10.1 to 4 to one as well as ten .1.422.
In other words, we can get to this network.
Now again, we are running choppy on this router.
So these routes have been redistributed from OSPF into IGP.
Let's look at rather two and see if rather two can ping the loopback of router six.
It's not able to do that, so show IP route.
We are not seeing that loopback in the routing table.
So let's have a look at out of five.
Ken Rotter five paying the loot back of Rider six.
The answer is it can so show IP route.
It's able to ping the loopback or write a six.
The loopback exists as an OSPF rat in the routing table of right of five.
What about router four?
Paying loopback over out of six that ping fails show IP route that two loopback doesn't exist in the
routing table.
So let's have a look at router of five by using the command show IP protocols and tell me if you can
see the problem by looking at this output.
What do you think's the problem when you look at this output?
And to give you a hint, I'll do the same command on router four to show IP protocols on router four.
There's the output for router four.
He has the output four out of five.
What's the problem with this configuration?
Okay.
So notice the following.
It's routing for areas one and two.
The strata has interfaces in multiple areas but doesn't have area zero configured.
So show run section OSPF.
Shows us that the router is configured for Area one and Area two, but is not configured for Area zero.
Waterfall is configured for Area one.
It's a requirement in OSPF that area zero be connected to other areas.
In other words, this is configured as area one.
This is configured as area two, but area two is not adjacent or bordering area zero.
That's a problem.
If we look at router three over here, show IP protocols, we can see that this router is configured
as an area border router.
It has interfaces in area zero and interfaces in area one.
So show IP OSPF interface brief shows us that gigabit zero zero is configured in area zero gigabit zero
one is configured in area one.
So that looks good.
But rather five isn't configured in that way.
So show IP OSPF interface brief notice.
It has interfaces in area one and area two, but not in area zero.
That's why rather one can ping the loopback of router four and the loopback of router five, but not
the loopback of router six.
Because.
OSPF is correctly configured with Area zero.
And Area one over here.
It works.
But here we have a problem.
We need to create what's called a virtual link.
Between rather three and rather five.
So we have to essentially pull or extend area zero from router three to right of five to make root of
five and area border router.
So we will be essentially creating a tunnel where Area Zero traverses across area one to router five.
Area zero is going to be tunneled across area one.
So that root of five becomes an area border rider, which means that Area two is adjacent to area zero.
That is required in OSPF.
All areas have to be adjacent to area zero when you have multiple OSPF areas.
So show run section OSPF.
We can see the configuration of router three.
Now when you create your virtual links, you have to configure the other side using the router ID,
not the IP address of the router but the router ID.
So the command show ip ospf interface will show us the local routers router id.
So the rata ID is 3 to 3 to 3 to 3.
We can also use the command show ip ospf and we'll be able to see that router id.
So notice router ids 3.32323.
En route of five show IP OSPF.
The right ID is 5.5 to 5.5.
So back on route a three show run section OSPF.
That's the configuration of OSPF on router three.
And that's the configuration of rather of five.
So.
Rather three.
Rather.
OSPF one.
Area one.
This is the area that we are traversing.
The Virtual link and the neighbors router ID is 5.5 to 5.5.
Now there are options where you can set authentication and other options.
I'm not going to do that.
I'm simply going to create the virtual link.
So Radha ospf one.
Area.
One virtual link and the right ID that we'll connect to on this side is 3.3 to 3.3.
So show ip ospf neighbor.
You can see that a neighbor relationship was established.
Using Virtual Link zero.
We can see that over here.
No election takes place on a virtual link.
Show ip ospf neighbor.
On this side we can see something similar.
A neighbor relationship has been formed from router three directly to router five through the OSPF virtual
link.
So logically it's as if these two routers rather three and router five are connected to one another.
So Ken wrote a three ping the loopback of router six.
Notice the router is in the routing table, so that looks better.
And notice router three can ping the loopback of router six.
And if we go back to router one, right, a one can now ping the loopback of router six and when we
do a trace notice it goes to router two.
Rather three out of four.
We're out of five and rather six.
This tunnel is not like a jerry tunnel.
It's an OSPF mechanism.
So traffic still goes via the physical links and you see every hop in the network which you wouldn't
see if you were using Jerry.
The virtual link is simply used for OSPF link advertisements.
So in other words, to advertise routes.
So we've now solved the problem.
We've been able to configure the network to allow right of one to ping router six.
Let's have a look at some more information on router three.
So show IP route.
We are learning about that network as an inter area network.
It's coming from a separate area.
So IP OSPF database.
For Area zero notice we have rather two, rather three and root of five in area zero.
Rather five is shown his DNA.
The neighbor relationship will not age.
So do not age this neighbor relationship because it's set through a virtual link.
So we see rather link states or LSA type ones, net link states or LSA type tos.
We see summary losses or LSA type threes.
We see something similar for Area one and we see external routes in the OSPF database.
So that was an example of how to configure OSPF virtual links to solve a problem where you've got an
area separated by another area which is not area zero.
In OSPF, all areas have to touch the backbone area.
In other words, area zero.
So here we are extending area zero across area one to meet the OSPF design criteria.
I hope you enjoyed this video.
If you did, please like it and please subscribe to my YouTube channel.
I wish you all the very best.