Dynamic GRE tunnel creation

When a dynamic generic routing encapsulation (GRE) tunnel is configured on a device, a corresponding logical tunnel interface is created. This is called an interface-based dynamic tunnel and is the default mode for creating dynamic tunnels. 

Dynamic GRE tunnels support the creation of a tunnel composite next hop for every GRE tunnel configured, where the GRE tunnel encapsulation is implemented as a next-hop instruction without creating a logical tunnel interface. This is called a next-hop-based dynamic GRE tunnel, and because it removes the dependency on physical interfaces, it provides a scaling advantage for the number of dynamic GRE tunnels that can be created on a device. 

The next-hop-based dynamic GRE tunnel feature should be explicitly configured on a device. The tunnel composite next hop created for every dynamic GRE tunnel includes the dynamic tunnel's source and destination IP address, encapsulation data, and a VPN label (when chained composite next hop is not enabled). At a given point in time, for the same tunnel destination, the next-hop-based dynamic tunnel encapsulation can either be GRE or UDP.