OSPF area inactive

In the output of the show ip ospf command, an area can be indicated as inactive, like so:

R2#sh ip ospf
 Routing Process "ospf 1" with ID 172.16.0.2
 Start time: 00:00:42.372, Time elapsed: 03:40:02.148
 !
 ! < -- output omitted -- >
 !
 Area BACKBONE(0) (Inactive)
 Number of interfaces in this area is 2
 !
 ! < -- output omitted -- >

The “inactive” indicates that there are no active OSPF interfaces in that area. In order to have an “active” area (backbone or otherwise), you must have at least one network command in OSPF referencing an IP address range that corresponds to an interface. That interface must be:

1. up
2. not in OSPF passive mode
3. assigned to the area in question

This error message will appear if you have enabled the passive-interface default command in the OSPF configuration. This will cause all interfaces to become passive by default, and if you don’t specify which interfaces will be active, OSPF remains passive on that area.