Thought I'd share this tidbit that I ran across... It is possible to have two routers establish OSPF adjacency even it there on dissimilar networks. The key is that connection has to be using IP Unnumbered on the interfaces. Try this yourself. Once you get it working; change the interface that unnumbered points too. And you can even route across it too! This feature is part of the OSPF RFC
R1 - R2 - R3 R1: Int lo0 Ip add 1.1.1.1 255.255.255.0 Int fa0/0 Ip add 192.168.12.1 255.255.255.0 Router ospf 1 Network 192.168.12.0 0.0.0.255 area 0 Network 1.1.1.1 0.0.0.0 area 0 R2 Int lo0 Ip add 2.2.2.2 255.255.255.0 Int fa0/0 Ip add 192.168.12.2 255.255.255.0 Int s0/0 Ip unnumbered lo0 Router ospf 1 Network 192.168.12.0 0.0.0.255 area 0 Network 2.2.2.2 0.0.0.0 area 23 R3 Int lo0 Ip add 3.3.3.3 255.255.255.0 Int fa0/0 Ip add 192.168.3.3 255.255.255.0 Int s0/0 Ip unnumbered lo0 Router ospf 1 Network 192.168.3.0 0.0.0.255 area 23 Network 3.3.3.3 0.0.0.0 area 23
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