Loopback Interface Function in Routing Protocols

ciscoloopbackospfpacket-tracerrouting

So I've got this task where I have to setup a virtual network with an OSPF routing protocol. I firstly ignored this loopback interface I had to configure on the routers since it didn't do any difference in this virtualization software called Cisco Packet Tracer (I thought). Then I've built the network in real life with some Cisco routers and it didn't do anything either. With or without this loopback the network worked (pinging from one host to another). Now my question is: Why is this loopback interface needed or what function does it do? In the picture below is the network I had to build (if it is any help).Network configuration

Best Answer

The loopback interface is useful because it is an interface with an IP address which never goes down. OSPF, without a specifically defined Router ID, will pick a Router ID on its own. It chooses the Router ID from the IP addresses of the configured and enabled interfaces. A loopback is a good choice since the loopback interface is always up unless someone specifically shuts it down. Other interfaces may go down if there is a problem on the link.

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