A network administrator is setting up a secure remote access VPN that uses Layer 2 Tunneling Protocol over IPsec (L2TP/IPsec). Place the steps required to establish a fully functional L2TP/IPsec VPN connection in the correct chronological order from first to last.
- 1The VPN client and gateway negotiate IPsec Phase 1 (ISAKMP Security Association) over UDP port 500 to establish a secure management channel.
- 2IPsec Phase 2 negotiates Quick Mode SAs to encapsulate and encrypt all subsequent traffic destined for UDP port 1701 using ESP.
- 3An L2TP control connection and session are established inside the encrypted IPsec SA channel.
- 4PPP Link Control Protocol (LCP) and user authentication (such as MS-CHAPv2 or EAP) take place over the L2TP tunnel.
- 5PPP IP Control Protocol (IPCP) completes layer 3 parameter negotiation, assigning an internal IP address and DNS settings to the client adapter.
Answer
The correct order for establishing an L2TP/IPsec remote access VPN session is: 1) Negotiating IPsec Phase 1 (ISAKMP SA) over UDP port 500, 2) Negotiating IPsec Phase 2 Quick Mode SAs to encrypt UDP port 1701 via ESP, 3) Establishing the L2TP control connection and session inside the IPsec tunnel, 4) Performing PPP LCP negotiation and user authentication (MS-CHAPv2/EAP), and 5) Completing PPP IPCP to assign an internal IP address and network configuration to the client.
L2TP/IPsec is a dual-protocol VPN solution where IPsec provides confidentiality/authentication and L2TP provides data link tunneling. The sequence must strictly begin with IPsec Phase 1 (UDP 500) to build the ISAKMP SA, followed by IPsec Phase 2 to secure UDP 1701 via ESP. Once IPsec encryption is active, the L2TP tunnel is established, followed by PPP user authentication (MS-CHAPv2/EAP), and finally PPP IPCP address assignment to complete remote network integration.
Step-by-Step Solution
Key Concept
L2TP/IPsec Encapsulation and Session Establishment Sequence