The crypto isakmp identity hostname command is required to define the identity used by the router when participating in the Internet Key Exchange protocol. This command is needed when you specify preshared keys for authentication. The ip name-server <DNS Server IP Address> command is required to specify the IP address of the DNS server that can resolve the FQDN of the remote peer. This command is needed when you use the crypto isakmp key command with the hostname option instead of the address option. The other commands are not required or correct for this scenario. References: 1, 2, 3