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router 876 with 3d party modem

Dear all,

i have a 876 cisco adsl router and need to configure a vpn with another router.

the problem is that the router cannot synchronize (due to bad line) so i will have to use a third party modem that can synchronize.

This is what i ve tried so far:

1) configure the modem in bridge mode and connect it via LAN and PPPoE with the router. The router would do the authentication and then i would create my tunnel with the other cisco router.

That didn't work (i couldn 't make the router authenticate the adsl line)

2) let the modem do the authentication and then create my tunnel interface. The router can go out in the internet but i don't know how to configure the vpn.

i have set a network in Fa 0 with the modem and a in Fa1 with my internal network.

The tunnel source IP in the 876 router is a local IP so i port forwarded UDP ports 500,4500 and 10000 in the modem, but i cannot make the vpn work. I am not sure if these are the ports used by the vpn.

Obviously i am doing something wrong but i don't know what!

Any help with either two ways would be much appreciated.

Thank you in advance.


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