why ospf neighborship is on loopback interface

[krahulbgp]

 i see, in most of organization   ospf  neighborship is  on loopback interface  ,i dont understand why we  require physical port & cable connectivity ,what is use of it. can any  one explian in deatils

[Flavio Miranda]

Hello

 The idea os use loopback interface is because the loopback interface will not went down for flap like happen with physical interface frequently. The physical ports and cabling is required because otherwise the device would be isolated from the rest of the network, unless it supports wireless.

 https://study-ccna.com/loopback-interface-loopback-address/

[krahulbgp]

but here loopback ip is in different subnet of ospf ,in this case how configuration is done 

[M02@rt37]

Hello @krahulbgp,

Loopback interfaces are not tied to a particular physical location or network segment. They can be assigned IP addresses that are independent of the underlying physical interfaces. This flexibility allows OSPF neighborship to be established over loopback interfaces across different network segments. 

@Flavio Miranda give you the answer! Loopback interfaces provide a stable and reliable source for OSPF neighborship. Unlike physical interfaces that can be affected by link flapping or hardware failures, loopback interfaces are always available, ensuring continuous OSPF neighborship.

[MHM Cisco World]

Freind 

The ospf as you mention must happen between same subnet in both opsf neighbor' but LO what you see is router-id.

Opsf select lo as router id and it appear in show opsf but that not meaning that the opsf is established via LO.

Hope this clear for you.

[Joseph W. Doherty]

"neighborship" on loopback?  Did you mean a loopback interface being advertised to OSPF?  If the latter, it's generally used as a stable management address.  Also used for OSPF's RID.

[balaji.bandi]

Adding to other comments, if you have P2P layer 3 interface then use that Physical interface for peering with OSPF neighbour. there is no advantage here using Loopback interface - since Physical interface go down, the Loopback can not be reached here.

Note in BGP different case.