To connect GNS3 to an existing loopback interface, add a router and a cloud devices.
By itself the Cloud doesn’t do anything. We need to add an interface to it. To do that, right click on the Cloud and choose configure.
In the Cloud’s Node properties choose the previously configured loopback interface from the drop-down options in the Ethernet tab and click Add.
nio_gen_eth:MS Loopback interface is added.
Now lets connect Cloud’s nio_gen_eth:MS Loopback interface to Router’s f0/0 and power it on.
Now lets configure Router’s f0/0 interface and check connectivity to the loopback interface.