As we all know, one of the conditions of OSPFv2 neighborship is for
two routers to become adjacent in OSPF and be neighbors that the link
connecting them together to be on the same subnet
anyway, this was the status until OSPFv3 came into the scene and this issue was resolved, for example you are an engineer working on two companies merging into one network and you got OSPFv3 in use and the problem is the IP subnets in use is totally different and let's say one is in class A and the second in class C, and you got two routers (R1, R2) that will be used as merge point as in the figure below:
so now we will activate OSPFv3 on R1 and R2 interface (E0/0) and first thing you will notice that they will form adjacency and become neighbors as below:
anyway, this was the status until OSPFv3 came into the scene and this issue was resolved, for example you are an engineer working on two companies merging into one network and you got OSPFv3 in use and the problem is the IP subnets in use is totally different and let's say one is in class A and the second in class C, and you got two routers (R1, R2) that will be used as merge point as in the figure below:
so now we will activate OSPFv3 on R1 and R2 interface (E0/0) and first thing you will notice that they will form adjacency and become neighbors as below:
use the command # show ospfv3 neighbors you can see from the output that the two routers formed the adjacency.
Here this can be seen in chapter 17: IPv6 Routing Protocols and Redistribution from the CCNP Route OCG. shows comparison between OSPF version 2 and 3 as you can see in the table below in the highlighted part:
When there is newer version of the same protocol, this means that something new added to enhance the existing features, and this applies to OSPF or RIP versions and BGP.
I hope this was useful !
Samer R. Saleem
