Router 1and Router 3 uplink to Router 2. Router 1 and Router 3 attach PC. Router 1 and Router 3 enable GRE tunnel.
On source router and destination router there must be route forwarded by Tunnel. The packet with GRE encapsulation can be forwarded correctly.
Confiugre dynamic routing protocol on two ports of Tunnel.
For configuring dynamic routing protocol, enable it on the router interface of Tunnel interface and private network. In order to choose correct route, avoid the route to physical address of Tunnel destination as Tunnel interface.
1. Check whether there is route forwarded by Tunnel on source and destination router.
2. Check configured route.
3. Check whether the dynamic routing protocol can be enabled on the router interface of Tunnel interface and private network.