In OSPF, the routers will have different name. Some of the names:
- Backbone Router
- Area Border Router
- Autonomous System Border Router
A backbone router is a router that has all its connected network in area 0. Let’s see this scenario:
In our scenario, all 5 routers are called backbone routers because they have all their connected networks only in area 0. They are also called Internal Routers because an Internal Router is an OSPF router having all its connected networks in the same area, and in our case they are all in area 0.
An Area Border Router (ABR) is an OSPF router which has 1 interface (or directly connected network) in an area and the 2nd interface (or directly connected network) in another area. Let me show it to you by an illustration:
Which of the 6 routers is the ABR? ……. Yes correct!!!! That’s R2. Why so? Because R2 has 1 interface in area 0 and another interface in area 1. The function of the ABR is to allow routers in one area to know about networks in the other area (and vice versa) without the routers in the 2 different areas participate in the OSPF process of each other. That means, R2 in our case will participate in the OSPF process of area 0 and of area 1. That means what? It means that we should be careful to choose a big router when we have to put it as an ABR because he will be doing double job.
Another job that the ABR can do is summarization (in some books they call it aggregation). For example, in case you have 20 networks in area 1, and those network have similarity in their number such as:
- Network 1: 172.16.0.0/24
- Network 2: 172.16.1.0/24
- Network 3: 172.16.2.0/24
And so forth……
Then what you can do, you can create a summary address for all those similar addresses which represent all of them, and you allow the ABR to advertise this summary address to all routers in area 0. This will reduce the size of the routing table of the routers in area 0 which can help the router to function faster and reduce the resources on the router such as RAM utilization (we will speak more about route summarization in this book).
The 3rd type of routers in OSPF is Autonomous System Border Router (ASBR). The ASBR is a router that have one interface in an OSPF area and another interface in a different network. Let me make it easy and show it to you here:
You see that R5 is having one connected network on OSPF area 0 and another connected network on another routing protocol which is RIP. In this case, R5 is called an ASBR. To allow all OSPF routers know about the networks in RIP, we should redistribute the RIP networks to OSPF and this is done on R5. Same we do to allow all routers on RIP to know about the networks of OSPF, we redistribute the OSPF networks into RIP on R5.
Bottom line, once you have 1 router who has an interface in an OSPF area and another interface in another routing protocol (or the internet), then this router is called an ASBR.