Many operators currently run level-1 IS-IS routing in their OSI-only SDH NEs, and then link up multiple areas using level-2 IS-IS routing in an external router network.
If an operator wishes to use a similar model for a dual network then they can run level-1 Integrated IS-IS in each area, and level-2 Integrated IS-IS in an external router network. This gives a very similar network to the previous one, as shown in Figure III-3
