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

OSPF Hierarchical Topology Example

I'm sure this all made perfect sense the first time you read it. Uh-huh. J Let's take an example to help make things more concrete. We can use the autonomous system in the preceding topic. This AS is really small enough that it's unlikely one would use hierarchical topology, but it will suffice for sake of illustration. Let's divide this AS into two areas, as follows (see Figure 180):

• Area 1: This area would contain N1, RA, N2, RB and RC.
  
  
• Area 2: This area would contain RB, RC, N3, RD and N4.
  

  Figure 180: Example OSPF Hierarchical Topology Autonomous System

  This is the same AS we saw in Figure 179 but arranged into OSPF hierarchical topology. The AS has been split evenly into Area 1 and Area 2. Area 0 contains RB and RC, which are area border routers for both Area 1 and Area 2 in this very simple example AS.

  
  

In this example, Router A and Router D are internal routers. Router B and Router C are area border routers, and comprise the backbone (Area 0) of the internetwork. Routers A, B and C will maintain an LSDB describing Area 1, while Routers B, C and D will maintain an LSDB describing Area 2. Routers B and C maintain a separate LSDB for the backbone. There is no backbone router other than the area border routers B and C. However, suppose we had a router E that had only direct connections to RB and RC. This would be a backbone router only.

You have probably already discovered the chief drawback to hierarchical topology: complexity. For large autonomous systems, however, it has significant advantages over making every router a peer. At the same time, the conceptual complexity is made worse by the need for very careful design, especially of the backbone. If the hierarchy is not set up properly, a single failure of a link between routers could disrupt the backbone and isolate one or more of the areas (including all the devices on all networks within the area!)

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