8.5 On Demand Routing
8.5.1 What is on demand routing?
The configuration of static routes is a fairly simple process in a hub and spoke environment such as the one in Figure . However, many network administrators still view static routes as administratively undesirable. The difficulty is not so much adding routes as new stub networks are brought online, as it is remembering to remove routes when stub networks or stub routers are taken offline. Beginning with the release of IOS 11.2, Cisco offers a proprietary alternative for hub routers called On-Demand Routing (ODR).

With ODR, a hub router can automatically discover stub networks while the stub routers still use a default route to the hub. ODR conveys address prefixes - that is, only the network portion of the IP address as opposed to the entire address. The network portion does not have to be strictly classful; that is, VLSM is supported. Further, because only minimal route information is traversing the link between the stub and hub routers, bandwidth is conserved.

It is important to note that ODR is not a true routing protocol. It discovers information about stub networks, but does not provide any routing information to the stub routers. The link information is conveyed by a data link protocol and, therefore, does not go further than from the stub router to the hub router. However, ODR-discovered routes can be redistributed into dynamic routing protocols.

Figure shows a routing table containing ODR entries. The table shows that the administrative distance is 160 and the metric of the routes is 1. Because ODR routes are always from a hub router to a stub router, the metric (hop count) will never be more than 1. The routes also show that VLSM is supported.

The transport mechanism for ODR routes is the Cisco Discovery Protocol (CDP), a proprietary data link protocol that gathers information about neighboring network devices. CDP runs on any media that supports the subnetwork access protocol (SNAP), which means that ODR also depends on SNAP support. Although CDP is enabled by default on all interfaces of all Cisco devices running IOS 10.3 and later, ODR support does not begin until release 11.2. The configuration example on the next slide will show that ODR is configured on the hub router only. However, the stub routers must run IOS 11.2 or later for the hub router to discover their attached networks.