Configuration restrictions and guidelines
When you configure a GRE/IPv4 tunnel, follow these restrictions and guidelines:
You must configure the tunnel source address and destination address at both ends of a tunnel. The tunnel source or destination address at one end must be the tunnel destination or source address at the other end.
As a best practice, do not configure the same tunnel source and destination addresses for local tunnel interfaces that use the same tunnel mode.
You can enable or disable GRE checksum at each end of a tunnel. If GRE checksum is enabled at a tunnel end, the tunnel end sends packets carrying the checksum to the peer end. A tunnel end checks the GRE checksum of a received packet if the packet carries a GRE checksum, whether or not the tunnel end is enabled with GRE checksum.
To ensure correct packet forwarding, identify whether the destination network of packets and the IP address of the local tunnel interface are on the same subnet. If they are not, configure a route reaching the destination network through the tunnel interface. You can configure the route by using one of the following methods:
Configure a static route, using the local tunnel interface as the outgoing interface of the route.
Enable a dynamic routing protocol on both the tunnel interface and the interface connecting the private network. This allows the dynamic routing protocol to establish a routing entry with the tunnel interface as the outgoing interface.
The IP address of the tunnel interface and the tunnel destination address configured on the tunnel interface must be in different subnets.