The Point-to-Point Tunneling Protocol (PPTP) is typically used for VPN solutions. (It is defined in RFC 2637.) Traditionally, the PPTP session negotiation is done over TCP port 1723 and the data traverses over the generic routing encapsulation (GRE) protocol (IP protocol 47). GRE does not have any Layer 4 port information. Consequently, it cannot be port address translated (PATed). PAT is performed for the modified version of GRE (RFC 2637) only when negotiated over the PPTP TCP control channel. PAT is not supported for the unmodified version of GRE (RFC 1701 and RFC 1702).
The Cisco ASA inspects PPTP protocol packets and dynamically creates the necessary translations to permit PPTP traffic.
Cisco ASA supports only PPTP version 1.
Use the inspect pptp command to enable PPTP inspection on the Cisco ASA.