FW1与RT2之间用Internet互联地址建立GRE Over IPSec VPN，实现Loopback4之间IPv4加密访问配置并用DCN的格式

好的，以下是根据DCN格式的FW1与RT2之间建立GRE Over IPSec VPN的配置示例，以实现Loopback4之间IPv4加密访问：

// FW1配置
interface Tunnel0
 description GRE to RT2
 ip address 10.1.1.1 255.255.255.0
 tunnel source GigabitEthernet0/0
 tunnel destination 203.0.113.2
 tunnel ttl 255

crypto isakmp policy 10
 encr aes
 authentication pre-share
 group 2
crypto isakmp key vpnkey address 203.0.113.2

crypto ipsec transform-set vpn-transform esp-aes esp-sha-hmac
 mode transport

crypto map vpn-map 10 ipsec-isakmp
 set peer 203.0.113.2
 set transform-set vpn-transform
 match address vpn-acl

access-list vpn-acl permit ip host 10.2.2.4 host 10.3.3.4

// RT2配置
interface Tunnel0
 description GRE to FW1
 ip address 10.1.1.2 255.255.255.0
 tunnel source GigabitEthernet0/0
 tunnel destination 203.0.113.1
 tunnel ttl 255

crypto isakmp policy 10
 encr aes
 authentication pre-share
 group 2
crypto isakmp key vpnkey address 203.0.113.1

crypto ipsec transform-set vpn-transform esp-aes esp-sha-hmac
 mode transport

crypto map vpn-map 10 ipsec-isakmp
 set peer 203.0.113.1
 set transform-set vpn-transform
 match address vpn-acl

access-list vpn-acl permit ip host 10.3.3.4 host 10.2.2.4

// Loopback4配置
interface Loopback4
 ip address 10.2.2.4 255.255.255.255

ip route 10.3.3.4 255.255.255.255 10.1.1.2

以上是根据DCN格式的示例配置，仅供参考。在实际配置过程中，还需要根据具体网络架构、设备型号和软件版本等因素进行调整和修改，以确保VPN的稳定性和安全性。