Establishing GRE connection between 2 routers

Guest

Hi,

I try to setup office to office VPN tunnel ipsec over gre.

My first step is to setup GRE tunnel.

I do following:

On HQ router

configure terminal
interface tunnel 0
ip address 172.24.3.3 255.255.255.0
tunnel source FastEthernet 0
tunnel destination x.x.x.x (public IP of other router)
tunnel mode gre ip
no shut

ip route 172.24.3.6 255.255.255.255 tunnel 0

On remote router

configure terminal
interface tunnel 0
ip address 172.24.3.6 255.255.255.0
tunnel source FastEthernet 0
tunnel destination x.x.x.x (public IP of other router)
tunnel mode gre ip
no shut

ip route 172.24.3.3 255.255.255.255 tunnel 0

---------------------
Then I try to ping other interface and initially worked only from
remote router to HQ, but after 5 min there is no more connection.

I've tried
debug tunnel

(nothing appears)

the routers are 7206 (IOS 12.2) and 7300 (IOS 12.3)

Any suggestions how to troubleshoot?