We just started using IP Agent over a SecureRemote (checkpoint) VPN connection this week. We've had a lot of problems with the connection to the switch dying off. This has everything to do with the VPN connection, and hardware in between, however - IP Agent definitely appears to be more "sensitive" and less forgiving when it comes to packet loss. A couple of things I've learned this week:
[li] Stress test the connection that you'll be using for the ip transport to the switch.
[li] Make sure you turn on Event Logging in IP Agent (Tools, Program Options, Event Logging).
[li] Make sure you're on the latest rev's of everything.
[li] Be willing to wait days for Avaya Tier 3 to get back to you if you have problems.
[li] Make sure you have all the RTUs turned up: disp sys cust page 1 and disp cap pages 5, 8, & 9.
Avaya has firmware upgrades for the MedPro board to v34, and a QPPCN upgrade for the CLAN Board to 799c v3, plus there's also a QP out for IP Agent v2 that was rel'd on January 7, that provides support for WinXP and supposedly solves a reported dropped call problem, and offers some enhanced VPN configuration parms.
I don't doubt that in time we'll get this to work reliably with our VPN connection, but I have to say that it would have been a lot less problematic (but of course more costly) to be running over a frame port, or even a 56k circuit.