I have a topo like this:

SpokeRouter<-->ADSL/NAT_Router<--->ADSL_Network<---->HubRouter

I configure GRE tunnel between HubRouter and SpokeRouter, SpokeRouter is placed behind a ADSL/NAT Router.Before I configure GRE Tunnel, SpokeRouter and Hubrouter can ping each other. When I turn on the Tunnel, the HubRouter receive Registration Request pakets from SpokeRouter and reply Registration Reply packets, but the Spoke can't receive Registration Reply from HubRouter. I don't know what happened, I think the ADSL/NAT Router may drop Registration Reply packets from HubRouter. My ADSL/NAT Router is DrayTek Vigor 2500 and I turn off all firewall on it but the problem still happens. If the problem is my ADSL/NAT Router, please tell me what ADSL/NAT Router is supported GRE Tunnel. Thank everyone so much.