In chapter 2, I have explained why you need to have multiple areas in OSPF. To remind you, multiple areas are needed whenever you have an OSPF network of more than 80 routers, then you can create a new area and put the additional routers in that area. Of course, you can create more than 1 area in case you have a lot of routers.
I also spoke about the ABR and his function. In this chapter, I will apply a LAB to show you how you can configure OSPF having routers in different areas and if they will be able to reach the destination networks even if they are in different areas. Let’s start with the LAB.
LAB: OSPF Multi Area Configuration
In this LAB, I do have R1and R3 in area 0 connected to the same switch, while R2 is acting as an ABR connected to area 0 and area 1, and R4 which is completely in area 1. Every router has a loopback interface acting like a remote network as the following:
- R1: 1.1.1.1
- R2: 2.2.2.2
- R3: 3.3.3.3
- R4: 4.4.4.4
The goal is to configure OSPF on all routers, advertise the connected interfaces including the loopback interfaces, and check if all routers can reach each other’s loopback interfaces.
I have already set the IP addresses and created the loopback interfaces. Let’s start directly by configuring OSPF. I will go from R1 all the way to R4.
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R1 has OSPF fully configured. Now I need to configure OSPF on R2. If you check the graph above, you see that R2 is an ABR and connected to area 0 and area 1.
For this, I have to configure R2 having the following interfaces in the following areas:
Area 0:
- G0/0/0
- Loopback 0
Area 1:
- G0/0/1
Done with R2. R3 has his networks in area 0, so let’s publish them in OSPF.
Finally, R4 has all his interfaces in area 1, so let’s publish them into OSPF.
Excellent!!!! OSPF has been configured on all routers. Let’s see if each router in this network can see all networks of all loopback interfaces of all routers.
In the routing table of R1, he can see the remote networks via OSPF, and of course his own network (which is 1.1.1.0/24) he will see it “Direct” because it is directly connected to him.
Let’s check R2 now.
R2 is can also see all destination networks.
I will check R3 then finally R4.
Excellent!!! All routers can reach to the destination networks of each other. To test, I will go to R1 (which is in area 0) and ping the loopback interfaces of all other routers. You will see that R1 will be able to ping the loopback interface of R4 which is inside area 1 without any problem. Let’s do that.
As you can see, I have issued a ping from R1 interface loopback 0 to all other router’s loopback interfaces, and I could successfully reach them. You can see that I could also reach to the loopback interface of R4 which is in area 1.
Before I finish this LAB, I just would like to show you that R2 (as being the ABR) will have the LSBD tables of both areas because he is participating in the SPF algorithm of both areas.
Let me show you that.
You can see clearly that R2 has the LSDB for both area 0 and area 1. You can see also in the 1st column the “Type” – this is the LSA types that I am going to explain it to you in the upcoming chapter.
Now let’s check what is the LSDB in R1.
You can see that R1 has the LSDB of only area 0. That’s the reason why I said that in case you need to have a router as an ABR then you should have a bigger router because he will have to participate in the OSPF process of 2 areas.
This is all what I needed to show you in this chapter, so I hope you enjoyed it. Please leave the LAB open because we will need to use it for the upcoming chapter. See you in a while.
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