The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)                      9  Open Shortest Path First (OSPF)

OSPF Basic Topology and the Link State Database (LSDB) 1 2 3 OSPF Route Determination Using SPF Trees

Router Roles in OSPF Hierarchical Topology

The topology described above is hierarchical because the routers in the AS are no longer all peers in a single group. The two-level hierarchy consists of the lower level containing individual areas, and the higher level that connects them together, which is called the backbone and is designated as “Area 0”. The routers are no longer all peers, but in fact play different roles depending on where they are located and how they are connected. There are three different labels applied to routers in this configuration:

• Internal Routers: These are routers that are only connected to other routers or networks within a single area. They maintain an LSDB for only that area, and really have no knowledge of the topology of other areas.
  
  
• Area Border Routers: These are routers that connect to routers or networks in more than one area. They maintain an LSDB for each area of which they are a part. They also participate in the backbone.
  
  
• Backbone Routers: These are routers that are part of the OSPF backbone. By definition, this includes all area border routers, since those routers pass routing information between areas. However, a backbone router may also be a router that connects only to other backbone (or area border) routers, and is therefore not part of any area (other than Area 0).
  
  To summarize: an area border router is always also a backbone router, but a backbone router is not necessarily an area border router.

Okay, I bet you are now wondering… what is the point of all this? Well, the point is exactly the same as the point of using autonomous system architecture in the first place. The topology of each area matters only to the devices in that area. This means that changes in that topology only need to be propagated within the area. It also means that internal routers within Area 1 don't need to know about anything that goes on within Area 2, and don't need to maintain information about any area other than their own. Only the backbone routers (which include at least one area border router within each area) need to know the details of the entire autonomous system. These backbone routers condense information about the areas so that only a “summary” of each area's topology needs to be advertised on the backbone.

Routing in a hierarchical topology AS is performed in one of two ways, depending on the location of the devices. If the source and destination are in the same area, then routing occurs only over networks and routers in that area. If they are in a different area, then the datagram is routed from the source to an area border router in the source's area, over the backbone to an area border router in the destination's area, and then finally, delivered to the destination. Again, this is analogous to how routing works between ASes in the big-picture internetwork.

OSPF Basic Topology and the Link State Database (LSDB) 1 2 3 OSPF Route Determination Using SPF Trees