I have a GRE tunnel on a 1811 and it doesn't seem to work with the same configuration as the 2811 I was using previously. The 1811 has an ISDN interface and a switch but I don't see how this would make a difference. The tunnel forms properly, as it should, and I'm able to NAT to the outside world but the nets on the other side of the tunnel all I'm able to do is ping which has higher than usual latency or send clear text in a telnet session - beyond that it's too slow to do anything.
I'm using (C181X-ADVIPSERVICESK9-M), Version 12.4(15)T1 release software - should I be using a different IOS version?
Any help would be greatly appreciated!