Courtesy of Windows Tips and Tricks Update from Windows IT Pro,
PPTP
– PPTP tunnel maintenance traffic needs TCP 1723.
– PPTP tunneled data needs Protocol ID 47.
L2TP over IPSec
– Internet Key Exchange (IKE) needs UDP 500.
– IPsec Network Address Translation (NAT-T) needs UDP 5500.
– L2TP traffic needs UDP 1701.