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.
Here's a question for you.
In this topology, which OSPF router types are you able to identify as an example?
Which routers in this topology are area border routers or neighbors which rather or routers are backbone
routers, which router or routers are internal routers, which router or routers are Espers or autonomous
system border routers.
So in this topology, we have IGP running on the left hand side.
Then we have OSPF Area one, OSPF Area zero, and OSPF area to an ABR or area border router is a router
that has interfaces in multiple areas.
So as an example, router three is an area border router because it has an interface in area one and
an interface and area zero.
Router five is an area border rider because it has an interface in Area Zero and an interface an area
two.
Router two is an autonomous system border rider because it has an interface configured in a different
routing protocol in this case IGP, and it has an interface configured in OSPF.
So router two is an ASPIRE or autonomous system?
Border Rider.
Rather three and rather five are area border riders or ABS.
Rather four is a backbone.
Rather or backbone area.
Rather it is a router with interfaces in the backbone area.
Now riders can take on multiple roles.
So rather three and rather five are also backbone routers in that they have interfaces in the backbone
or area zero.
But if you were asked to this question, I would answer that rather three and rather five.
Our area, border riders.
Area border riders always touch the backbone area, a virtual link, so out of the scope of the CCNA.
But I will discuss how to configure virtual links which allow you to pull or extend the backbone area
across another area.
So Area Zero would be extended across another area such as area one to allow an area border router to
have an interface in the backbone area and meet the criteria that an area border rider must touch.
The backbone area router six is an internal router.
It's a router with interfaces only in an internal area.
In other words, area two.
So now that we know the right of types, let's configure these routers.
These routers have just booted up and have no configuration.
So I'll bypass the initial configuration of the routers so that we can configure them manually.
So on router one, this is the router on the left hand side.
I'm going to configure interfaces in these subnets as per the diagram.
As well as configure a loopback on the routers with a IP address of the router name.
So this is rather one.
So we'll configure the loopback as follows.
So do show IP interface brief shows us the IP address as configured on the router.
Now this router will only run IGP, so we'll configure.
IGP on this rota, in this example in Autonomous System 100 and I'm going to configure IGP on all interfaces
of the router.
I'll also turn off automatic summarization.
So do show IP copy interface shows us that IP is enabled on the gigabit zero zero interface and the
loopback interface.
He has brought a to configure a hostname of rather to go on to the gigabyte zero interface.
No, shut it and give it an IP address of ten 112.
Slash 24 mosque.
Interface gigabit.
Zero one IP Address.
Ten 121.
Slash 24 mask and I'll create a loopback of quadruple to.
Now this Rada needs to run two writing protocols.
Firstly, it needs to run GP and I want to enable GP only on gigabit zero zero.
So notice the eye dropping neighbor relationship has come up.
So do show IP copy neighbor shows us the neighbor relationship.
Now do show IP interface brief shows us that the IP address configured on gigabit is zero zero is this
ten 112 and do show.
Run pipe section IGP shows us that we've only configured IP on that interface.
I'll enable OSPF in this example on ten one, two, one.
This is the gigabit zero one interface and I'll configure it in area one.
So do show IP OSPF interface brief shows us that OSPF is enabled on gigabit zero one in area one and
process ID one.
Process ID was this number area number we configured over there.
After a while we'll see the state go to DDR if we don't configure router three quickly.
But I'm not going to do that right now.
Rather two is an autonomous system border rider.
We need to do what's called redistribution from one routing protocol to another.
So basically we need to translate, if you like, or import routes from one routing protocol into another.
This is very similar to translating a language like French into English or English into Spanish.
We are going to translate routes from one routing protocol into another.
Often as it is in the real world, when you translate from one language to another, you can lose meaning.
So sometimes words in one language.
Don't quite mean the same thing in another language.
And that's true when you do Redistribution II GOP supports different metrics or different ways of determining
the best route when compared to OSPF.
So when we redistribute IGP into OSPF, we can change what's called the seed metric.
OSPF will set it by default.
We can also change the default cost of a route that's redistributed.
ERP uses bandwidth and delay to determine the best route.
OSPF doesn't understand that it only understands bandwidth, so you typically need to specify a seed
metric when you redistribute routes into a routing protocol.
OSPF has a default, but notice when we redistribute OSPF into IGP, we're going to want to specify
a metric and you'll often see the metric.
Written something like this.
So we are taking rats from OSPF and advertising them into air drop and we are also advertising routes
from air drop into OSPF.
That's what an area border router does.
Now at the moment I've only enabled OSPF on gigabit zero one and ergo P on gigabit zero zero.
What I'll do with a loopback is advertise that into OSPF.
So advertise this network into OSPF.
So now when we use the command show IP OSPF database, you can see different LSA types in the database.
We've got what's called LSA type one, or rather LSA.
At the moment we only have one router in the database.
We have LSA Type two or summary Alice's.
Now, I did make a mistake here and as I put that interface into Area Zero.
Actually want to put that into area one hence.
US seeing Area zero and Area one in the link state database.
So I'm going to remove that from Area Zero and put that into Area one.
So show IP OSPF database.
We only see Area one in the database for type one and type two.
Liz's type one is once again router aliases, type two or summary aliases, and we have what are called
type five lists.
These are routes that came from a different routing protocol.
In other words, they were redistributed by an SVR or autonomous system border router into OSPF, and
that's what Rather two is doing in the topology.
So at the moment, if we type show IP route, we will see the loopback of router one in the routing
table.
As a parent and on router one we see the loopback of router two advertised into IGP as an external route.
It was redistributed from OSPF into IGP and that's also true for this route.
So those are called external routes.
They came from a different routing protocol.
I hope you found this video useful.
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I wish you all the very best.