Hi,
I am also a university teacher and to be honest, I do not see much of a point in focusing on PPTP. It is a relatively simple protocol, that's for sure, but it has drawbacks and especially when considering a site-to-site VPN, I do not see a compelling reason to prefer PPTP to other solutions, such as L2TP, IPsec, or even a plain GRE. It is not a typical solution especially between Cisco routers (as can also be seen by the difficulty you have encountered yourself when trying to find out how a PPTP client can be configured), and the PPTP is long considered obsolete. I am afraid that if the thesis focuses primarily or only on PPTP, it will not be representative. What are the particular reasons of your professor to stick with PPTP?
In any case, let's see how the configuration can be done. The PPTP Remote Access Server is configured as follows:
ip local pool PPTP_Clients 192.168.1.11 192.168.1.200
!
interface Loopback0
ip address 10.255.255.5 255.255.255.255
!
interface Loopback1
ip address 192.168.1.1 255.255.255.255
!
interface Virtual-Template1
ip unnumbered Loopback1
peer default ip address pool PPTP_Clients
!
vpdn enable
!
vpdn-group PPTP_RAS
accept-dialin
protocol pptp
virtual-template 1
source-ip 10.255.255.5
Here, the PPTP_Clients pool holds a pool of IP addresses to be assigned to PPTP clients, starting from 192.168.1.11 and going up to 192.168.1.200. The Loopback0 interface represents the IP address of the Remote Access Server. It does not have to be a loopback address, though, it can (and probably would) be the Remote Access Server's uplink interface to internet, carrying a public IP address. The Loopback1 interface holds the IP address of the RAS used for every incoming PPP session tunneled across PPTP - it is the IP address that every PPTP client will see over its own PPP virtual interface. This Loopback1 is then referred to in the Virtual-Template1 interface configuration that shares the IP address of Lo1 using the IP Unnumbered feature. The configuration of the VPDN and the corresponding VPDN group then specifies the tunneling protocol in use, the Virtual-Template interface to be used for incoming PPTP clients, and the source IP address the RAS should use - the IP address from Lo0 is used here.
The PPTP Client is configured as follows:
service internal
!
vpdn enable
!
vpdn-group PPTP_CLIENT
request-dialin
protocol pptp
pool-member 1
initiate-to ip 10.255.255.5
!
interface Dialer0
ip address negotiated
encapsulation ppp
dialer pool 1
dialer string 1234
dialer persistent
dialer vpdn
First and foremost, you need to enter the service internal command. This command is used to enable some hidden IOS features and commands including the PPTP client functionality. Obviously, Cisco itself does not consider the PPTP client functionality on routers to be interesting to most people and therefore hides the feature. Without the service internal command, the VPDN group will not allow you to specify the PPTP as the tunneling protocol and will pretend no such protocol is supported.
The VPDN group specifies the IP address of the PPTP RAS (I am again referring to its Lo0 address 10.255.255.5) and declares the VPDN group as a possible "interface" to be used by a Dialer interface when making calls.
Next, the Dialer0 interface is configured. This is the client PPP interface. It is configured for PPP encapsulation and to obtain the IP address from the RAS. The specific configuration lies in the dialer commands:
- dialer pool 1 tells the Dialer0 interface to use any interface to do the actual "dialing" that is assigned to pool 1 - in this case, it is the VPDN PPTP client group (which is not exactly an "interface" but certainly provides a communication service similar to an interface)
- dialer string 1234 is only a phony command used to satisfy the obligatory requirements of a dialer interface to have a number to dial. This string is never used with PPTP session but the IOS will refuse to activate the interface without a dialer string configured
- dialer persistent tells IOS to "place a call", that is, initiate the PPTP tunnel immediately as soon as the Dialer0 interface is activated. Otherwise, the interface would need to wait for a specific traffic to trigger the call. This approach was used with dialed lines such as POTS or ISDN but has no meaning with PPTP sessions
- dialer vpdn allows the Dialer0 interface to also use VPDN groups as interfaces to do the calls. Without this command, the Dialer0 interface would only be looking for physical interfaces capable of truly placing a phone call (or for PPPoE clients but let's not complicate things here too much).
After configuring your Dialer0 interface this way, you may need to shutdown and no shutdown it to force the IOS to start the interface using all configuration. I've seen situations where the PPTP session did not come up right after creating the Dialer0 configuration, and shutting/unshutting it was necessary to kick IOS into action.
Please be sure that you can ping the RAS server from the client before actually trying to start the PPTP session.
Please let me know if this worked for you!
Best regards,
Peter
