Penetration Testing: VPN protocols

["Dan Tesch" <dan.tesch () comcast net>]

I am not very up on VPN protocols, I am testing a network that
has an edge router which has several site-to-site VPN's set up
and several clients connecting to it via MS PPTP client.

I know that the router has gre and esp and tcp 1723 open

A standard Nessus scan shows 1723 but doesn't pick up gre or esp

What setting might I look for in Nessus to allow it to see gre and esp?

What kinds of tests should be run against these next?

Thanks