VPDN Configuration Guide, Cisco IOS Release 12.4T
VPDN Technology Overview
VPDN Technology Overview
Last Updated: April 01, 2011
Virtual private dial-up networks (VPDNs) securely carry private data over a public network, allowing remote users to access a private network over a shared infrastructure such as the Internet. VPDNs maintain the same security and management policies as a private network, while providing a cost-effective method for point-to-point connections between remote users and a central network.
Information About VPDNs
Overview of VPDN Technology
VPDNs extend private network dial-in services to remote users. VPDNs use Layer 2 tunneling technologies to create virtual point-to-point connections between remote clients and a private network. VPDNs maintain the same security and management policies as a private network, while providing a cost-effective method for point-to-point connections between remote users and a central network.
Instead of connecting directly to the remote private network, VPDN users connect to a nearby access server, which is often located at an Internet service provider (ISP) local point of presence (POP). Data is securely forwarded from the access server to the private network over the Internet, providing a cost-effective method of communication between remote clients and the private network.
A benefit of VPDNs is the way they delegate responsibilities for the network. The customer can outsource responsibility for the information technology (IT) infrastructure to an ISP that maintains the modems that the remote users dial in to, the access servers, and the internetworking expertise. The customer is then responsible only for authenticating users and maintaining the private network.
The figure below shows a basic VPDN network deployment.
A PPP client dials in to an ISP access server, called the Network Access Server (NAS). The NAS determines whether it should forward that PPP session on to the router or access server that serves as the point of contact for the private network, the tunnel server. The tunnel server authenticates the user and initiates PPP negotiations. Once PPP setup is complete, all frames that are sent between the client and the tunnel server pass through the NAS.
VPDNs can use these tunneling protocols to tunnel link-level frames:
Using one of these protocols, a tunnel is established between the NAS or client and the tunnel server, providing secure data transport over a shared infrastructure such as the Internet.
VPDN Hardware Devices
Generally three devices are involved in VPDN tunneling. Two of these devices function as tunnel endpoints--one device initiates the VPDN tunnel, and the other device terminates the VPDN tunnel. Depending on the tunneling architecture, different types of devices can act as the local tunnel endpoint.
As new tunneling protocols have been developed for VPDNs, protocol-specific terminology has been created to describe some of the devices that participate in VPDN tunneling. However, these devices perform the same basic functions no matter what tunneling protocol is being used. For the sake of clarity we will use this generic terminology to refer to VPDN devices throughout this documentation:
Depending on the tunneling architecture, the NAS will function as follows:
Although technically a tunnel switch is a tunnel endpoint for both the incoming tunnel and the outgoing tunnel, for the sake of simplicity the tunnel endpoints in a multihop deployment are considered to be the device that initiates the first tunnel and the device that terminates the final tunnel of the multihop path.
The table below lists the generic terms and the corresponding technology-specific terms that are sometimes used to describe the NAS and the tunnel server.
A VPDN tunnel exists between the two tunnel endpoints. The tunnel consists of a control connection and zero or more Layer 2 sessions. The tunnel carries encapsulated PPP datagrams and control messages between the tunnel endpoints. Multiple VPDN sessions can use the same VPDN tunnel.
A VPDN session is created between the tunnel endpoints when an end-to-end PPP connection is established between a client and the tunnel server. Datagrams related to the PPP connection are sent over the tunnel. There is a one-to-one relationship between an established session and the associated call. Multiple VPDN sessions can use the same VPDN tunnel.
Client-Initiated Dial-In VPDN Tunneling
Client-initiated dial-in VPDN tunneling is also known as voluntary tunneling. In a client-initiated dial-in VPDN tunneling scenario, the client device initiates a Layer 2 tunnel to the tunnel server, and the NAS does not participate in tunnel negotiation or establishment. In this scenario, the NAS is not a tunnel endpoint; it simply provides Internet connectivity. The client device must be configured to initiate the tunnel.
The main advantage of client-initiated VPDN tunneling is that it secures the connection between the client and the ISP NAS. However, client-initiated VPDNs are not as scalable and are more complex than NAS-initiated VPDNs.
Client-initiated VPDN tunneling can use the L2TP protocol or the L2TPv3 protocol if the client device is a router. If the client device is a PC, only the PPTP protocol is supported.
The figure below shows a client-initiated VPDN tunneling scenario.
For further information about client-initiated tunneling deployments, see the “Configuring Client-Initiated Dial-In VPDN Tunneling” module.
Before configuring a client-initiated dial-in VPDN tunneling deployment, you must complete the required tasks in the “Configuring AAA for VPDNs” module.
NAS-Initiated Dial-In VPDN Tunneling
NAS-initiated dial-in VPDN tunneling is also known as compulsory tunneling. In a NAS-initiated dial-in VPDN tunneling scenario, the client dials in to the NAS through a medium that supports PPP. If the connection from the client to the ISP NAS is over a medium that is considered secure, such as digital subscriber line (DSL), ISDN, or the public switched telephone network (PSTN), the client can choose not to provide additional security. The PPP session is securely tunneled from the NAS to the tunnel server without any special knowledge or interaction required from the client.
NAS-initiated VPDN tunneling can be configured with the L2TP or L2F protocol.
The figure below shows a NAS-initiated dial-in tunneling scenario.
For further information about NAS-initiated tunneling deployments, see the Configuring NAS-Initiated Dial-In VPDN Tunneling module.
Before configuring a NAS-initiated dial-in VPDN tunneling deployment, you must complete the required tasks in the Configuring AAA for VPDNs module.
Dial-Out VPDN Tunneling
Dial-out VPDN deployments allow the tunnel server to tunnel outbound calls to the NAS. Dial-out VPDNs allow a centralized network to efficiently and inexpensively establish virtual point-to-point connections with any number of remote offices.
Dial-out VPDN tunneling can be configured only with the L2TP protocol.
A dial-out VPDN tunneling scenario is shown in the figure below.
For further information about dial-out VPDN tunneling deployments, see the Configuring Additional VPDN Features module.
Before configuring a dial-out VPDN tunneling deployment, you must complete the required tasks in the Configuring AAA for VPDNs module.
Multihop VPDN Tunneling
Multihop VPDN is a specialized VPDN configuration that allows packets to pass through multiple tunnels. Ordinarily, packets are not allowed to pass through more than one tunnel. In a multihop tunneling deployment, the VPDN tunnel is terminated after each hop and a new tunnel is initiated to the next hop destination. A maximum of four hops is supported.
Multihop VPDN is required for the scenarios described in these sections:
VPDN Tunneling to an MMP Stack Group
Multihop VPDN is required when the private network uses Mutlichassis Multilink PPP (MMP) with multiple tunnel servers in a stack group. Stack group configurations require the ability to establish Layer 2 tunnels between participating hardware devices. If the incoming data is delivered to the stack group over a VPDN tunnel, multihop VPDN is required for the stack group to function.
Multihop VPDN tunneling with MMP can be configured using the L2TP or L2F protocol.
The figure below shows a network scenario using a multihop VPDN with an MMP deployment.
For further information about configuring multihop VPDN for MMP deployments, see the Configuring Multihop VPDN module.
Before configuring a multihop VPDN for MMP deployment, you must configure MMP and you must complete the required tasks in the Configuring AAA for VPDNs module.
Tunnel Switching VPDNs
Multihop VPDN can be used to configure a router as a tunnel switch. A tunnel switch is a device that is configured as both a NAS and a tunnel server. A tunnel switch is able to receive packets from an incoming VPDN tunnel and send them out over an outgoing VPDN tunnel. Tunnel switch configurations can be used between ISPs to provide wholesale VPDN services.
Multihop tunnel switching can be configured using the L2TP, L2F, or PPTP protocol.
The figure below shows a network scenario using a tunnel switching deployment.
For further information about multihop tunnel switching deployments, see the Configuring Multihop VPDN module.
Before configuring a multihop tunnel switching deployment, you must complete the required tasks in the Configuring AAA for VPDNs module.
VPDN Tunneling Protocols
VPDNs use Layer 2 protocols to tunnel the link layer of high-level protocols (for example, PPP frames or asynchronous High-Level Data Link Control (HDLC). ISPs configure their NAS to receive calls from users and to forward the calls to the customer tunnel server.
Usually, the ISP maintains only information about the customer tunnel server. The customer maintains the users’ IP addresses, routing, and other user database functions. Administration between the ISP and the tunnel server is reduced to IP connectivity.
This section contains information on these Layer 2 protocols that can be used for VPDN tunneling:
L2TP is an Internet Engineering Task Force (IETF) standard that combines the best features of the two older tunneling protocols: Cisco L2F and Microsoft PPTP.
L2TP offers the same full-range spectrum of features as L2F, but offers additional functionality. An L2TP-capable tunnel server will work with an existing L2F NAS and will concurrently support upgraded components running L2TP. Tunnel servers do not require reconfiguration each time an individual NAS is upgraded from L2F to L2TP. The table below compares L2F and L2TP feature components.
Traditional dialup networking services support only registered IP addresses, which limits the types of applications that are implemented over VPDNs. L2TP supports multiple protocols and unregistered and privately administered IP addresses. This allows the existing access infrastructure--such as the Internet, modems, access servers, and ISDN terminal adapters (TAs)--to be used. It also allows customers to outsource dial-out support, thus reducing overhead for hardware maintenance costs and 800 number fees, and allows them to concentrate corporate gateway resources.
The figure below shows the basic L2TP architecture in a typical dial-in environment.
Using L2TP tunneling, an ISP or other access service can create a virtual tunnel to link remote sites or remote users with corporate home networks. The NAS located at the POP of the ISP exchanges PPP messages with remote users and communicates by way of L2TP requests and responses with the private network tunnel server to set up tunnels. L2TP passes protocol-level packets through the virtual tunnel between endpoints of a point-to-point connection. Frames from remote users are accepted by the ISP NAS, stripped of any linked framing or transparency bytes, encapsulated in L2TP, and forwarded over the appropriate tunnel. The private network tunnel server accepts these L2TP frames, strips the L2TP encapsulation, and processes the incoming frames for the appropriate interface.
The figure below depicts the events that occur during establishment of a NAS-initiated dial-in L2TP connection.
The following describes the sequence of events shown in the figure above and is keyed to the figure:
Subsequent PPP incoming sessions (designated for the same tunnel server) do not repeat the L2TP tunnel negotiation because the L2TP tunnel is already open.
L2TPv3 is an enhanced version of L2TP with the capability to tunnel any Layer 2 payload. L2TPv3 defines the L2TP protocol for tunneling Layer 2 payloads over an IP core network using Layer 2 Virtual Private Networks (VPNs).
In VPDN deployments, L2TPv3 can be used to establish a client-initiated tunnel from a local router to the remote customer network over an emulated circuit known as a pseudowire. There is one pseudowire associated with each L2TPv3 session.
Rather than using a VPDN group configuration, L2TPv3 uses an L2TP class configuration that is associated with the pseudowire. L2TPv3 pseudowires can also be used to establish L2TP tunnels by configuring an L2TP class on the local device and an accept-dialin VPDN group on the customer network.
For detailed information about the L2TPv3 protocol, see the Additional References section.
L2F is an older tunneling protocol, but still offers a wide range of useful features. L2F offers a comparison of L2F and L2TP feature components.
The figure below shows the basic L2F architecture in a typical dial-in environment.
Using L2F tunneling, an ISP or other access service can create a virtual tunnel to link remote sites or remote users with the corporate home network. The NAS located at the POP of the ISP exchanges PPP messages with remote users and communicates by way of L2F requests and responses with the private network tunnel server to set up tunnels. L2F passes protocol-level packets through the virtual tunnel between endpoints of a point-to-point connection. Frames from remote users are accepted by the ISP NAS, stripped of any linked framing or transparency bytes, encapsulated in L2F, and forwarded over the appropriate tunnel. The private network tunnel server accepts these L2F frames, strips the L2F encapsulation, and processes the incoming frames for the appropriate interface.
The figure below depicts the events that occur during establishment of a NAS-initiated dial-in L2F connection.
The following describes the sequence of events shown in the figure above and is keyed to the figure:
Subsequent PPP incoming sessions (designated for the same tunnel server) do not repeat the L2F tunnel negotiation because the L2F tunnel is already open.
PPTP is a network protocol that enables the secure transfer of data from a remote client to a private enterprise server by creating a VPDN across TCP/IP-based data networks. PPTP supports on-demand, multiprotocol, virtual private networking over public networks, such as the Internet.
Cisco supports only client-initiated VPDNs using PPTP. Only the client and the tunnel server need to be configured for VPDN. The client first establishes basic connectivity by dialing in to an ISP NAS. Once the PPP session has been established, the client initiates a PPTP tunnel to the tunnel server.
Microsoft Point-to-Point Encryption (MPPE), an encryption technology developed by Microsoft to encrypt point-to-point links, can be used to encrypt PPTP VPDNs. It encrypts the entire session from the client to the tunnel server.
The following describes the protocol negotiation events that establish a client-initiated PPTP tunnel:
VPDN Group Configuration Modes
Many VPDN configuration tasks are performed within a VPDN group. A VPDN group can be configured to function either as a NAS VPDN group or as a tunnel server VPDN group, but not as both. However, an individual router can be configured with both a NAS VPDN group and a tunnel server VPDN group.
You can configure a VPDN group as a specific type of VPDN group by issuing at least one of the commands listed in the table below:
Many of the commands required to properly configure VPDN tunneling are issued in one of the VPDN subgroup configuration modes shown in the table above. Removing the VPDN subgroup command configuration will remove all subordinate VPDN subgroup configuration commands as well.
Where to Go Next
Once you have identified the VPDN architecture that you want to configure and the tunneling protocol that you will use, you should perform the required tasks in the Configuring AAA for VPDNs module.