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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 Overview, History, Standards and Versions 1 2 3 OSPF Hierarchical Topology, Areas and Router Roles

OSPF Basic Topology and LSDB Example

For example, let's consider the same autonomous system we looked at in our examination of the RIP route determination algorithm. This internetwork has four individual networks, connected as follows:

• Router A (RA) connects Network 1 (N1) to Network 2 (N2).
  
  
• Routers B (RB) and C (RC) connect Network 2 to Network 3 (N3).
  
  
• Router D (RD) connects Network 3 to Network 4 (N4).

To make this example more interesting, I am also going to add a direct link between RB and RC; the resulting AS is shown in Figure 179. The LSDB for this autonomous system would look something like Table 123.

In practice, each of the bullets (“[“) in Table 123 would be replaced by a metric value indicating the cost to send a datagram from the particular router to another router or network. Note that the chart is symmetric, since if RB can reach RC, RC can reach RB. However, the costs do not have to be symmetric. It is possible for RB to have a metric that is higher for it to send to RC than for RC to send to RB.

Note also that there is no cost to reach a router from a network. This ensures that only one cost is “charged” for a router to send to another router over a network—the cost to reach the network from the router. This makes sense, since each router is a member of the network upon which it is connected.

OSPF Overview, History, Standards and Versions 1 2 3 OSPF Hierarchical Topology, Areas and Router Roles