IP Link-State Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 380

Stub Areas
Consider area 3 in Figure 8-4. Its only path to the external networks is through Router C. All
external routes are flooded to all areas in the OSPF AS. An area can be configured as a stub area
to prevent OSPF external LSAs (Type 5) from being flooded into that area. The ABR injects a
default route into the stub area rather than flooding the external routes into the area.

OSPF Network with Areas Figure 8-4

Area 3

Router C

Area 0

Router A

Area 1

Area 2

Router B

Consider area 1 in Figure 8-4. It has two ABRs connecting it to the backbone. Area 1 can also
be configured as a stub area. Both Router A and Router B inject default routes into the area.
Area 1 routers forward traffic destined to external routes (or any other unknown route) to the
closest (based on the metric of the default route) ABR.

Network summary LSAs (Type 3) from other areas are still flooded into area 1.

To configure area 1 as a stub area, all routers must be configured with the following OSPF
command:

area 1 stub