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OSPF Overview Configuring OSPF
page 1-10 OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007
Stub Areas
OSPF allows certain areas to be configured as stub areas. A stub area is an area with routers that have no
AS external Link State Advertisements (LSAs).
In order to take advantage of the OSPF stub area support, default routing must be used in the stub area.
This is accomplished by configuring only one of the stub area’s border routers to advertise a default route
into the stub area. The default routes will match any destination that is not explicitly reachable by an intra-
area or inter-area path (i.e., AS external destinations).
OSPF Stub Area
Area 1 and Area 3 could be configured as stub areas. Stub areas are configured using the OSPF ip ospf area
command, described in “Creating an Area” on page 1-17. For more overview information on areas, see “OSPF
Areas” on page 1-8.
The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been config-
ured as a stub. This guarantees that no confusion will arise in the flooding of AS external advertisements.
Two restrictions on the use of stub areas are:
• Virtual links cannot be configured through stub areas.
• AS boundary routers cannot be placed internal to stub areas.
Area 3
(stub)
Area 2
Area 1
(stub)
Backbone
Backbone