Need to understand Loopback interface, specifically in OSPF

[sheryarqazi4]

Need to understand Loopback interface, specifically in OSPF, so please elaborate ?

[balaji.bandi · Accepted solution]

Loopback interface is useful because it is an interface with an IP address which never goes down and always stay up.(until admin shutdown)

In case of OSPF, without a specifically defined Router ID, OSPF will pick a Router ID on its own. It chooses the Router ID from the IP addresses of the configured and enabled interfaces.

If you have multiple path still Loopback is reachable to nework.

[Martin L · Accepted solution]

see https://community.cisco.com/t5/routing/loopback-interface-influence-in-ospf/m-p/3053102

https://community.cisco.com/t5/routing/loopback-on-ospf/m-p/1677821

as mentioned by BB, loopback is important in OSPF (and BGP as well);  OSPF picks its Router ID among highest IPs that are UP UP state BUT it prefers Loopbacks.  OSPF will pick IP of a Loopback interface even if that IP is lower then IP from regular interfaces.  Still, Router ID should be set manually - recommended practice. 

Regards, ML
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[Richard Burts · Accepted solution]

Previous responses have focused on OSPF use of loopback interfaces for Router ID. There is another aspect of loopback interfaces use in OSPF that we should point out. You might configure something like this

interface loopback1

ip address 192.168.1.1 255.255.255.0

If you configure an OSPF network statement for this interface/network you might expect that OSPF would advertise a /24. But OSPF recognizes that the loopback is not a normal interface and will advertise the route as /32. If you want to have OSPF advertise the /24 under the interface configuration you need to specify an OSPF network type.

[Richard Burts]

I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.