In an OSPF Network, there are different router roles. In this lesson, we will focus on two of these special router roles that we use in OSPF networks. These special OSPF routers are Area Border Router (ABR) and Autonomous System Boundary Router (ASBR). We will learn what is ABR and what is ASBR, why we use them in an OSPF network or Autonomous System.
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Table of Contents
As you know, OSPF consist of one area or multiple areas. If we use one area in our OSPF network, this sis called Single Area OSPF. If we use multiple areas in our OSPF network, then this is Multi Area OSPF. And Area Border Router (ABR) are used here.
If we are working with Single area OSPF, there is no need to have an Area Border Router. But if we are using multiple areas in our OSPF network, then, we should use ABR. Because ABR connects different areas each other. In other words, it is the bridge between different OSPF areas. In such a network, OSPF ABR has interfaces in different areas.
Area Border Router (ABR) is responsible for building OSPF LSDB (Link State Database) for each area. ABRs use this information to calculate best path to a destination and then they share this information to the other routers in the Autonomous System.
In OSPF network, the routing inside an area is called Intra-Area routing and the routing between areas are Inter-Area routing. Area Border Router (ABR) is the border router for Inter-Area Routing. When a router in an area wants to send a packet to the other router in another area, it firstly sends the packet to ABR and then ABR routes this packet to the other router in another area. ABR is the bridge router here.
Another important router role in OSPF is Autonomous System Boundary Router (ASBR). ASBR is the door to the other autonomous network. With this router, OSPF network can communicate with the other autonomous systems which uses different routing protocol.
For example, if we would like to communicate with an EIGRP network or a BGP network, we can do this over Autonomous System Boundary Router (ASBR) in OSPF network. Here, external routes are redistributed on ASBR.
Like ABR, ASBR is also a bridge. But this time, it is the bridge to the outside network.
ABR and ASBR are two important routers for an OSPF network. But there is a common difference between these routers. Area Border Router (ABR) is the bridge between different areas in the OSPF network. They connect areas and provide routing between areas.
On the other hand, Autonomous System Boundary Router (ASBR) is the exit point of OSPF network. We can reach outside autonomous systems over ASBR. In other words, external routes are available over Autonomous System Boundary Router (ASBR).