This is one of multiple OSPF videos.
I was asked to explain OSPF multiple areas and this is one of multiple videos helping explain how OSPF
works when you have multiple areas configured.
So we've now configured rather one and rather two.
Rather one is once again only running your copy.
As shown here.
Right.
It too is running multiple routing protocols.
So we are running both IGP.
And OSPF.
So let me just do that as show run.
It's running both IGP as well as OSPF.
This command redistributes OSPF into IGP or translates, if you like, OSPF into air copy in the same
way that you would translate French into English.
And it sets what's called the seed metric or beginning metric to try and set an initial value for the
route when it's redistributed.
That's because we have to translate or convert OSPF, which only supports bandwidths into IGP, which
supports bandwidth and delay as part of its calculation.
Okay.
So let's configure router three now router three in this topology is a area, a border router.
It has an interface in area one and an interface in area zero.
So hostname router three interface gigabit is zero zero.
No shut IP address ten 122.
Slash 24 mosque interface gigabit zero one no shut IP Address ten 131.
Slash 24.
Interface loopback.
Zero.
IP Address.
Quadruple three.
Now, here's another question for you.
Can you run OSPF without a backbone area?
So I'm going to enable OSPF.
I'm only going to enable OSPF on gigabit zero zero in area one.
Will that work?
Notice neighbor relationship has gone to full.
So show ip ospf neighbor router three has a neighbor relationship with router two and we are receiving
OSPF routes.
What are called E2 rots or external rots.
From rather one and we can ping the loopback of rata one on rata one debug ip icmp.
Ping that loop back.
Notice we see the echo replies sent from writer one back to writer three.
So the traffic is definitely getting to write a one.
So this is the thing to remember.
You can and this is not an exam answer.
This is a real world answer.
You can not that you typically want to do it, but technically you can run OSPF in an area that's not
area zero.
If and this is the big if.
If you only have one area.
So at the moment it works fine because we only have area one configured.
But as soon as we want to configure multiple areas, we need to configure the backbone area or area
zero.
So what we'll do now?
Is enable ospf on the loopback in area zero and on gigabit is zero one.
We'll put both of those interfaces into area zero.
So now show ip ospf interface brief the loopback and gigabit is zero one in area zero gigabit is zero,
zero is an area one.
So that's the configuration of router three.
What you'll notice as well is router one running IGP is learning about the loopback of router three
because router three is advertising the loop back to router two using OSPF and that's redistributed
into IGP and advertised to router one.
Okay.
So we've configured we're out of three now.
Let's configure router four, which is a backbone rather.
So post the initial configuration dialog.
We don't want to configure the router that way.
Host Right.
Of full interface gigabit.
Zero zero no shut IP address.
Ten.
One, two, three, two.
So that's the IP address.
Once again, interface gigabyte.
Is there one IP address ten one for one.
No, shut that.
Create a loopback.
This is going to have a slash 32 mosque so do show IP interface brief.
They are the IP addresses configured.
This route is fairly simple to configure.
All we're going to do is configure OSPF on all interfaces using this command.
So I'm simply going to enable OSPF everywhere.
So show IP OSPF interface brief.
Notice OSPF is running on all interfaces of the router.
Notice a neighbor relationship was formed to router three.
So show ip ospf neighbor.
We have a neighbor relationship to router three show ip route.
We are learning multiple types of OSPF routes.
We have got what are called intra area routes.
Those are routes that exist in the local area.
In this case, area zero.
We have what are called inter area routes.
Those are routes that exist in a different area.
And then we have external routes.
Those are routes that originated from a different routing protocol.
So show IP OSPF database will show us more details.
We have two routers at the moment configured in area zero, router three and router four have interfaces
in area zero.
We have one designated router that's router three for this link.
We have type three routes.
These are summary links, date advertisements or summary losses.
So type one LSA is a router.
LSA Type two losses are advertised by designated routers and advertise segments such as the segment
ten 130.
They are type two losses.
These are type three losses or summary losses.
They advertise routes from different areas into OSPF.
We also have a type for Alyssa.
This is an Alyssa identifying the autonomous system border router, which is rather two.
We don't see this in area one.
So show IP OSPF database because that router exists in area one.
But for other areas to get to the Aspire, a summary Aspire advertisement is pumped in by the ABR,
so essentially rather three is telling right of four how to get to the autonomous system border router.
And then we have what are called type five Lisas.
These are Alice's that come from outside of OSPF.
So those routes come from IGP and we can see them in the routing table as this and this.
The local writers should be able to ping the loopback router one which it can.
We can see the debug is shown on router one.
So that's right.
A full configured lets configure out of five.
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