I have two cisco routers connected with serial cables and they are being configured with a console cable. I set the ip address of both routers 192.168.1.1 255.255.255.0 192.168.2.1 255.255.255.0 and then run a no shut. I exit to exec mode and run a show interface and all I get is Serial is up, line protocol is down. How do I get the line protocol to also be up?