Small Intermediate System-to-Intermediate System (IS-IS) networks are
built as a single area that includes all the devices in the network. As the
network increases in size, all Level 2 devices from all areas are connected
through a backbone. This network backbone is, in turn, connected to local
areas. Within a local area, devices reach all system IDs. Between areas,
devices reach the backbone, and the backbone devices reach other areas.
Devices establish Level 1 adjacencies to perform routing within a local
area (intra-area routing). Devices establish Level 2 adjacencies to perform
routing between Level 1 areas (interarea routing).
Some networks use legacy equipment that supports only Level 1 routing.
These devices are typically organized into many small areas that cannot be
aggregated due to performance limitations. Cisco devices are used to
interconnect each area to the Level 2 backbone.
IP routes from the Level 1 device are advertised by default on the
Level 2 device. Even when multiple Level 1 routing processes are configured on
the same unit, they are advertised on the Level 2 device. No additional
configuration is required to redistribute all Level 1 IP routes into the Level
The figure below represents a Telco network used to monitor the status
of the switching equipment in multiple remote central offices (where the telco
equipment resides) from a central monitoring point. In this example, the CLNS
network of the Telco is used to monitor the status of the switching equipment.
Figure 1. Telco IS-IS Network for Monitoring Remote Equipment
The figure below shows the same network reconfigured using multiarea
IS-IS. The number of local access devices has been reduced. Each device
continues to provide access to the backbone, but also participates in multiple
Level 1 areas. In this example a 3:1 reduction in the number of devices
required is shown.