If two router are connected through a serial link, they form neighborship even if they belong from different network.

I configured 1st router with IP and 2nd with and they formed neighborship. Why it happened so??

  • this is not an enterprise networking question therefore off-topic
    – marctxk
    Nov 24 '16 at 10:17
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 15 '17 at 5:29
3 is a subnet of

Both of your routers are in the network so it is normal that they form a neighbor relationship.

You should fix your addressing.


OSPF hellos do not use the subnet address, they use multicast so they will be received by each router. I guess this should be off-topic since it's not an entreprise networking question.


OSPF does not require neighbors to be on the same IP network if the network type is point-to-point. An adjacency will form even if the interfaces are unnumbered. In this case, they are both on the 10.1/16 network though.

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