IPSec VPN is a widely-acknowledged solution for enterprise network. What are the four steps tosetup an IPsec VPN？（）

A.The cypto isakmp keepalive command is used to configure the Stateful Switchover （SSO）protocol
B.Reverse Route Injection （RRI） is configured on at the remote site to inject the central sitenetworks
C.Each Hot Standby Routing Protocol （HSRP） standby group has two well-known MACaddresses and a virtual IP address
D.The cypto isakmp keepalive command is used to configure stateless failover

A.The crypto ACL number
B.The IPSEC mode （tunnel or transport）
C.The GRE tunnel interface IP address
D.The GRE tunnel source interface or IP address， and tunnel destination IP address
E.The MTU size of the GRE tunnel interface