We have 3 site.
Site A: 2910
Site B: 2930
Site C: 2910

Previously site B was setup using a 2910, but an upgrade to ADSL2+ we moved to using a 2930 and a pair of Vigor 120 modems.

When site B was using a 2910, it was possible to setup an an inbound VPN from Site A, and an outbound VPN to site C.

Now since the move to the 2930 at site B, the VPN connection is made, but checking site C logs for example, it shows the connection made, and an ip assigned, but shows site B terminating the connection. Although I haven't checked the site B logs, the same thing happens when site A connects to site B. The connection is made, lasts approx 5 seconds, then Disconnects.

All connections between the router that dial out to another have "always on" set, hence the reconnect.

Site A 2910 firmware: Current Firmware Version: 3.1.3
Site B 2930 firmware: Current Firmware Version: v3.2.2
Site C 2910 firmware: Current Firmware Version: 3.2.1

The links between the two 2910 routers is rock solid 70+ hours each.
But the link from 2910 to 2930, or the other way round seems flaky.
Cheers

Have you tried with IPSec VPN's? The security should be better with them if you choose a good PSK, and it is worth seeing if it fixes your problems.

I'll give that a try

Tried Ipsec and l2tp and like wise the connection between site A and B drops after a few second.

As it happens, the link between site B and site C is stable now for over 30 minutes and remote desktopping into the remote site server works ok, but that is on PPTP still.

I have had one other thought.
I know the router at site B has been rebooted since I tried setting these up.
And I think that site C may have been.

So I'm going to try rebooting site A once I know the network is clear to do that.

Studying the pattern of the behaviour.
It looks like the 2930 can't hold more than one Lan to Lan VPN solidly.

I've tried pptp, ipsec, and l2tp.

In all possible variations, and in and outbound connections.

And while the 2910 can hold stable multiple connections, the 2930 can't.