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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

Second Level

To construct the second level, we look for all devices that the devices on the first level can reach directly. We then add the cost to reach each device on the first level to the cost of each device at the second level:

• RB: RB has a cost of 5 and can reach:
  • RC, with a cost of 5, total cost of 10.
    
    
  • N2, with a cost of 4, total cost of 9.
    
    
  • N3, with a cost of 5, total cost of 10.
  
  
  
• N2: N2 has a cost of 3 and can reach:
  • RA, with a cost of 0, total cost of 3.
    
    
  • RB, with a cost of 0, total cost of 3.
    
    
  • RC, with a cost of 0, total cost of 3.
  
  
  
• N3: N3 has a cost of 6 and can reach:
  • RB, with a cost of 0, total cost of 6.
    
    
  • RC, with a cost of 0, total cost of 6.
    
    
  • RD, with a cost of 0, total cost of 6.

You probably can see immediately that we ended up with a number of different paths to the same devices or networks, some of which make no sense. For example, we don't really care about any path that goes to RC, since we are RC! Similarly, we can weed out certain paths immediately because we already have a shorter path to them; taking a path through RB to N3 with a cost of 10 makes no sense when we can go directly at the first level for a cost of 6. So, after separating out the chaff, we end up with the following “wheat” at the second level:

• N2 to RA, with a cost of 3.
  
  
• N3 to RD, with a cost of 6.

We continue the process by looking for devices that connect to the “weeded-out” devices we found on the second level (this time I am only showing the meaningful ones):

• RA: RA connects to N1, with a cost of 2, total cost of 5.
  
  
• RD: RD connects to N4, with a cost of 4, total cost of 10.

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