AI, ML, Development + Cisco Learning Blog Learning about Machine Learning, Artificial Intelligence, related devlopment topics and formerly Routing and Switching, Datacenter, Security and other topics, CCIE #23664, Frank Wagner

Each area has to be connected to area 0. If this is not the case, there is the possibility to set up an virtual link.

Then you could transit one area with „virtual-link“. This is done on router with ID 1.1.1.1 under „router ospf“ like that:

area 1 virtual-link 3.3.3.3

And on router with area 3 like this:

area 1 virtual-link 1.1.1.1

Make sure that you use the router ID and not an IP address. Lookup the ID with „show ip protocols“.

Source:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfospf.htm#wp1001317