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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 Hierarchical Topology, Areas and Router Roles 1 2 3 4 OSPF General Operation and Message Types

OSPF Route Determination Example

I can almost see your eyes glazing over, so let's go back again to the example we have been using in this section. I am going to assume we are looking at the whole AS as a whole in basic topology, for simplicity. In Table 124, I have repeated the LSDB for this AS, and I have taken the liberty of replacing the “[“s with cost metrics; these are shown in Figure 181 as well. Again, remember there is no cost to reach a router from a network, so those links have a non-zero cost only going from the router to the network.

Now, let's construct the SPF tree for RC. We can do this in iterations, as follows (this is shown graphically in Figure 182 as well for your convenience):

To construct the first level of the tree, we look for all devices that RC can reach directly. We find:

• RB, with a cost of 5.
  
  
• N2, with a cost of 3.
  
  
• N3, with a cost of 6.

OSPF Hierarchical Topology, Areas and Router Roles 1 2 3 4 OSPF General Operation and Message Types