1. NE20 connects public network through default route. The public network interface IP of NE20 and client's device can be reachable.
2. Configure GRE tunnel. Source address is public network interface and destination address is opposite public network interface. Tunnel interface uses private network address.
3. After GRE tunnel has been set up, enable OSPF protocol on both sides of tunnel interface and import direct route.
4. GRE tunnel sometimes is unusable.
1. Solution: A. Use static route.
B. OSPF does not import direct route.
2. Users use static route and the problem is solved.
1. GRE tunnel sometimes is not reachable. It is possible that there is loop.
2. Remote land on NE20 and check OSPF routing table. There is one external route ro public network interface of opposite device. Obviously this route is learnt when opposite device imports direct route in OSPF.
3. NE20 will regard mistakenly that it can reach public network interface of opposite device through GRE tunnel. The route is better than default route. In fact, the tunnel is loaded on public network route. Public network route is out through default route. Then it forms a dead circulation and the tunnel is not reachable.
4. After the tunnel is not reachable, ospf neighbor dies through some period (40S). The public network route through OSPF loses. NE20 can access public network interface of opposite device through default route. The tunnel is reachable again. OSPF neighbor relation is established again.....
Notice routing prefernce and matching principle for OSPF configuration.