IP Routing Fundamentals – On Demand Routing (ODR)

In addition to understanding how to implement routing protocols in Cisco IOS software, as a Cisco network engineer, it is also important to understand how to leverage one of the most common Cisco protocols, which is the Cisco Discovery Protocol, when designing and implementing large, scalable networks. This section describes On Demand Routing (ODR).

The Cisco Discovery Protocol (CDP) is a Cisco proprietary protocol that, amongst other things, is used to discover other Cisco devices on either broadcast or non-broadcast media. CDP provides administrators with information that includes the IP address, software version, as well as the capabilities of the neighbor device. On-Demand Routing is an enhancement to Cisco Discovery Protocol that advertises the connected IP prefix or prefixes of a stub router via CDP. ODR also supports VLSM, which means that it can be used in just about any network.

It is important to know that ODR is not a routing protocol. Instead, it is simply an enhancement to CDP that is used to dynamically propagate routing information at Layer 2. The primary reasons ODR is often incorrectly referred to as a routing protocol is because it allows routers to dynamically exchange routing information. The second reason is because ODR is enabled using the router odr global configuration command.

The primary benefits of using ODR is that it is not CPU intensive and it consumes very little bandwidth. Consider a network using the topology illustrated in Figure 1-6, for example:

Fig. 1-6. A Hub-and-Spoke Network

Figure 1-6 illustrates a typical hub-and-spoke network. The branch office routers (spokes) are connected to the hub (headquarters) using low-speed WAN links. While a dynamic routing protocol such as EIGRP or OSPF could be used to dynamically exchange routing information between the hub and the spoke routers, the amount of bandwidth consumed by the routing protocol updates becomes a great concern, especially on the low-speed WAN links.

Another alternative would be to use static routing. However, the administrative overhead that is required to manually configure static routing, especially as the network grows, quickly becomes a cumbersome and negative factor despite the low overhead afforded by static routing. On Demand Routing can be used in such cases due to its low bandwidth consumption and resource requirements. This is because ODR only requires an additional five bytes, which can contain the IP address of the connected subnet plus 1 byte for the subnet mask in comparison to OSPF which sends Hello packets that are comprised of 20 bytes of IP header, 24 bytes of OSPF header, 20 bytes of hello parameters, and 4 bytes for each neighbor seen, for example.

At the hub router, the prefixes received via ODR can then be redistributed into another routing protocol, such as OSPF, and propagated to the rest of the network. This allows the network to scale, while taking into consideration the bandwidth limitations at the spokes.

Configuring ODR

In order to use ODR, CDP must be enabled on the router. If disabled, CDP can be enabled on the router using thecdp enable global configuration command. There is no explicit configuration required to enable ODR on the spoke routers. However, it is important to ensure that there are no other routing protocols running on the spoke routers. If Cisco IOS detects that a dynamic routing protocol is configured, ODR will not be used to exchange routing information with the hub router(s). This is a common configuration mistake when using ODR.

On the hub router, ODR is enabled using the router odr global configuration command. Unlike the spoke router(s), a routing protocol can be enabled on the hub router. The ODR routes can then be redistributed into the dynamic routing protocol and propagated throughout the routing domain. The ODR configuration example in this section will be based on the network topology illustrated in Figure 1-7:

Fig. 1-7. Implementing ODR in a Hub-and-Spoke Network

It is assumed that in the network illustrated in Figure 1-7 the OSPF routing protocol is enabled between the Distribution and Core. The HQ networks are being advertised to the Distribution router via OSPF by the Core router. ODR will be enabled between the Distribution router and the spoke (Access). However, prior to the implementation of ODR, OSPF routing between the Core and Distribution routers is verified as illustrated in the following output:

In addition to verifying OSPF between the Core and Distribution routers, it is also important to verify and ensure that CDP is running between the Distribution and Access routers:

Finally, it is important to ensure that there are no routing protocols enabled on the spoke router. If any routing protocol is configured, it must be removed using the no router [protocol] global configuration command. This is mandatory on the spokes when implementing ODR. You can verify configured routing protocols using the show ip protocols summary command:

As previously stated, no explicit configuration is required on the spoke router. The only single configuration command required on the hub router is the router odr global configuration command. This is implemented on the Distribution router as illustrated in the following output:

When ODR is enabled on the hub router, the following two things automatically occur:

• The spoke router dynamically advertises connected prefixes to the hub router
• The hub router dynamically advertises a default route to the spoke router

Referencing the first point made above, the hub router receives the 192.168.1.0/24 prefix from the spoke router dynamically. This is present in the routing table as illustrated below:

Referencing the second point made above, the spoke router receives a default route from the hub router. This is performed because it is assumed that the spoke router has a single ingress and egress point, which is the hub router. Advertising a single default route to the spoke reduces the routing table size on the, router, which keep resource, e.g. CPU and memory, utilization to a minimum, while allowing the spoke router access to the rest of the networks.

A more detailed look at the default route received by the spoke reveals the following:

In order to allow the spoke routers to reach all other routers in the network, the received ODR routes must be redistributed into OSPF on the Distribution. When redistributing on the Distribution router, the subnet space for the links between the Distribution and Access routers. The route redistribution configuration is omitted in this section for brevity and also because redistribution is a core requirement for the ROUTE exam and as such is covered in detail later in this guide. Assuming the correct configuration, the spoke router and the Core router now have full IP reachability between their respective subnets as illustrated in the following output:

While ODR is not a routing protocol, the configuration logic of ODR parameters, such as timers, is similar to routing protocol configuration and is performed in router configuration mode following the issuing of the router odr global configuration command. On Demand Routing configuration options are illustrated in the following output:

NOTE: You are not required to go into any advanced ODR configuration in the ROUTE exam. The options presented above will therefore not be described in further detail in this guide.