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.
|