some problems with GRE Tunnel
i can`t understanding these configurations
1- if we are using ip address 184.108.40.206 under tunnel interface , then why we need tunnel source s0/0 configuration?
by default source ip address of the packet go threw must be source 220.127.116.11 address , so why we need to change this address by using tunnel source command?
2- if new packet is created into the original ip packet before it sent out of the tunnel interface , then it`s P2P interface , why we need Destinatio0n ip address command?
3- ccna certification book told us that tunnel interfaces must be into same ip address range, then why we need this if these two logical interfaces are P2P interfaces like serial
if anyone have good link about GRE that explaining these commands , please share it