Some drawbacks to static routers are that they must be manually configured and updated when the network topology changes. Some drawbacks to dynamic routing protocols are that they use network bandwidth and router resources. In a hub-and-spoke network with hundreds of spokes, both the configuration needed for static routes and the resource usage of dynamic routing could be considerable.
There is a third option－ODR. ODR uses the Cisco Discovery Protocol(CDP) to carry network information between spoke (stub) routers and the hub. ODR provides IP routing information with minimal overhead compared to a dynamic routing protocol. ODR requires less manual configuration than static routes.
ODR is applicable in a hub-and-spoke topology, each spoke router is adjacent only to the hub. Another term for this is stub router. The stub router may have some LAN networks connected to it and typically has a WAN connection to the hub router. The hub router neds to recognize the networks connected to each spoke but the spokes need only a default route pointing to the hub.
When ODR is configured, the stub routers use CDP to send IP prefix information to the hub router. Stub routers send prefix information for all their directly connected networks. ODR reports the subnet mask, so it supports variable-length subnet masking(VLSM).
the hub router, in turn, sends a default route to the spokes that point back to itself. It installs the stub networks reported by ODR in its routeing table, and the hub router can be configured to redistribute them into a dynamic routing protocol. For a next-hop address, the hub router uses the IP address of the spoke routers as reported to it by CDP.
ODR is not a true routing protocol because the information exchanged is limited to IP prefixes and a default route. There is no metric information reported by ODR; the hub router uses a hop count of one as the metric for all routes reported via ODR. However , by using ODR, routing information for stub networks can be obtained dynamically without the overhead of a dynamic routing protocol, and default routes can be provided to the stub routers without manual configuration.