What exactly is a sub interface? What is a PVC? And, what is a loopback and why set one on a routed configuration?
A sub interface allows you to use one physical interface and divide it into multiples logical interfaces. This is commonly used in Frame Relay networks to route multiple protocols and prevent split horizon.
A PVC is a permanent virtual circuit. Telco?s use PVC?s in Frame Relay networks. If you have multiple offices and want to directly connect them you would use a PVC through your Telco to connect them. For example, if you have an office in Boston and Chicago and want to directly connect them through routers without going through 6 ISP networks you would have your ISP set up a PVC to directly connect the two networks.
A loopback is used on routers to troubleshoot problems between the router and the service provider. If you set a loopback you can send out traffic and it will come back to the router to help debug Wan problems you may be having.