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This is one of multiple OSPF videos.

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I was asked to explain OSPF multiple areas and this is one of multiple videos helping explain how OSPF

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works when you have multiple areas configured.

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Here's a question for you.

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In this topology, which OSPF router types are you able to identify as an example?

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Which routers in this topology are area border routers or neighbors which rather or routers are backbone

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routers, which router or routers are internal routers, which router or routers are Espers or autonomous

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system border routers.

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So in this topology, we have IGP running on the left hand side.

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Then we have OSPF Area one, OSPF Area zero, and OSPF area to an ABR or area border router is a router

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that has interfaces in multiple areas.

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So as an example, router three is an area border router because it has an interface in area one and

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an interface and area zero.

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Router five is an area border rider because it has an interface in Area Zero and an interface an area

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two.

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Router two is an autonomous system border rider because it has an interface configured in a different

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routing protocol in this case IGP, and it has an interface configured in OSPF.

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So router two is an ASPIRE or autonomous system?

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Border Rider.

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Rather three and rather five are area border riders or ABS.

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Rather four is a backbone.

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Rather or backbone area.

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Rather it is a router with interfaces in the backbone area.

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Now riders can take on multiple roles.

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So rather three and rather five are also backbone routers in that they have interfaces in the backbone

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or area zero.

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But if you were asked to this question, I would answer that rather three and rather five.

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Our area, border riders.

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Area border riders always touch the backbone area, a virtual link, so out of the scope of the CCNA.

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But I will discuss how to configure virtual links which allow you to pull or extend the backbone area

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across another area.

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So Area Zero would be extended across another area such as area one to allow an area border router to

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have an interface in the backbone area and meet the criteria that an area border rider must touch.

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The backbone area router six is an internal router.

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It's a router with interfaces only in an internal area.

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In other words, area two.

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So now that we know the right of types, let's configure these routers.

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These routers have just booted up and have no configuration.

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So I'll bypass the initial configuration of the routers so that we can configure them manually.

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So on router one, this is the router on the left hand side.

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I'm going to configure interfaces in these subnets as per the diagram.

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As well as configure a loopback on the routers with a IP address of the router name.

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So this is rather one.

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So we'll configure the loopback as follows.

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So do show IP interface brief shows us the IP address as configured on the router.

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Now this router will only run IGP, so we'll configure.

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IGP on this rota, in this example in Autonomous System 100 and I'm going to configure IGP on all interfaces

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of the router.

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I'll also turn off automatic summarization.

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So do show IP copy interface shows us that IP is enabled on the gigabit zero zero interface and the

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loopback interface.

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He has brought a to configure a hostname of rather to go on to the gigabyte zero interface.

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No, shut it and give it an IP address of ten 112.

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Slash 24 mosque.

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Interface gigabit.

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Zero one IP Address.

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Ten 121.

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Slash 24 mask and I'll create a loopback of quadruple to.

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Now this Rada needs to run two writing protocols.

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Firstly, it needs to run GP and I want to enable GP only on gigabit zero zero.

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So notice the eye dropping neighbor relationship has come up.

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So do show IP copy neighbor shows us the neighbor relationship.

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Now do show IP interface brief shows us that the IP address configured on gigabit is zero zero is this

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ten 112 and do show.

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Run pipe section IGP shows us that we've only configured IP on that interface.

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I'll enable OSPF in this example on ten one, two, one.

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This is the gigabit zero one interface and I'll configure it in area one.

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So do show IP OSPF interface brief shows us that OSPF is enabled on gigabit zero one in area one and

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process ID one.

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Process ID was this number area number we configured over there.

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After a while we'll see the state go to DDR if we don't configure router three quickly.

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But I'm not going to do that right now.

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Rather two is an autonomous system border rider.

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We need to do what's called redistribution from one routing protocol to another.

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So basically we need to translate, if you like, or import routes from one routing protocol into another.

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This is very similar to translating a language like French into English or English into Spanish.

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We are going to translate routes from one routing protocol into another.

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Often as it is in the real world, when you translate from one language to another, you can lose meaning.

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So sometimes words in one language.

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Don't quite mean the same thing in another language.

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And that's true when you do Redistribution II GOP supports different metrics or different ways of determining

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the best route when compared to OSPF.

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So when we redistribute IGP into OSPF, we can change what's called the seed metric.

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OSPF will set it by default.

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We can also change the default cost of a route that's redistributed.

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ERP uses bandwidth and delay to determine the best route.

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OSPF doesn't understand that it only understands bandwidth, so you typically need to specify a seed

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metric when you redistribute routes into a routing protocol.

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OSPF has a default, but notice when we redistribute OSPF into IGP, we're going to want to specify

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a metric and you'll often see the metric.

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Written something like this.

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So we are taking rats from OSPF and advertising them into air drop and we are also advertising routes

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from air drop into OSPF.

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That's what an area border router does.

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Now at the moment I've only enabled OSPF on gigabit zero one and ergo P on gigabit zero zero.

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What I'll do with a loopback is advertise that into OSPF.

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So advertise this network into OSPF.

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So now when we use the command show IP OSPF database, you can see different LSA types in the database.

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We've got what's called LSA type one, or rather LSA.

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At the moment we only have one router in the database.

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We have LSA Type two or summary Alice's.

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Now, I did make a mistake here and as I put that interface into Area Zero.

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Actually want to put that into area one hence.

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US seeing Area zero and Area one in the link state database.

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So I'm going to remove that from Area Zero and put that into Area one.

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So show IP OSPF database.

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We only see Area one in the database for type one and type two.

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Liz's type one is once again router aliases, type two or summary aliases, and we have what are called

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type five lists.

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These are routes that came from a different routing protocol.

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In other words, they were redistributed by an SVR or autonomous system border router into OSPF, and

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that's what Rather two is doing in the topology.

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So at the moment, if we type show IP route, we will see the loopback of router one in the routing

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table.

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As a parent and on router one we see the loopback of router two advertised into IGP as an external route.

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It was redistributed from OSPF into IGP and that's also true for this route.

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So those are called external routes.

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They came from a different routing protocol.

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I hope you found this video useful.

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If it's been of benefit to you, please like it and please subscribe to my YouTube channel.

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I wish you all the very best.