Fundamentals of On-Demand Routing

Although the configuration of static routes is simple in a hub router such as the one in Figure 12.1, many network administrators still see 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 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 address ”rather than the entire address ”so VLSM is supported. And because only minimal route information is traversing the link between the stub and hub routers, bandwidth is conserved.

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, as a case study will show, ODR- discovered routes can be redistributed into dynamic routing protocols.

Figure 12.3 shows a routing table containing ODR entries. The table shows that the administrative distance is 160; 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.

Figure 12.3. This routing table shows several ODR entries.

Note

ODR and Cisco Discovery Protocol

The transport mechanism for ODR routes is Cisco Discovery Protocol (CDP), a proprietary data link protocol that gathers information about neighboring network devices. ^[3] Figure 12.4 shows the type of information collected by CDP.

^[3] CDP runs not only on routers but also on Cisco switches and access servers.

Figure 12.4. CDP collects information about neighboring Cisco 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 begins with IOS 11.2. The configuration case study 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.