Loopback interface I have a theoretical question about the loopback implementation on a router. I understand that the idea of a loopback interface is that it is a virtual interface which never goes down due to a physical line or network problem. Loopback interfaces can be used for testing. I also understand that Loopbacks are often used as the source address for various services (NTP, TFTP, FTP, SSH, TACACS, domain lookup, etc.). These services would, by default, use the IP address of the interface by which they exit the router. What happens if the routers in the networks are trunked and vlan tagged? Is it then still possible to use the loopback interface to perform domain lookup, ftp or curl?

> What happens if the routers in the networks are trunked and vlan tagged? Is it then still possible to use the loopback interface to perform domain lookup, ftp or curl?

VLAN tagging or LAG trunking are options _for physical interfaces_. You can't tag or LAG _a router_ itself (or any other device).

You need to use an interface address to send anything to a device. Since any interface can be down in case of failure, a virtual loopback interface can be created and used. The loopback is always up and can always be sent to as long as there's a path to the device. Also, it's address doesn't need to change when you renumber networks and interfaces, so it can be used as a constant reference to the device.