EIGRP and OSPF - Are We Connected?

For both OSPF and EIGRP routers to become neighbors, their interface’s primary IP address must be on the same subnet. That statement is true. There is a difference in the definition of “same subnet”, though.

In OSPF, both routers have to be configured to be on the same subnet with the same mask or else they won’t neighbor up.  When an hello packet is sent, the subnet mask is sent embedded in there.  The router does a quick look to be sure the subnets are defined the same way on both ends.  If everything doesn’t match, they don’t neighbor. Here’s a Wireshark screenshot to show you the OSPF hello.  Note: See edit below.