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An area can be qualified as a stub or totally stubby when it
meets the following criteria:
- There is a single exit point from that area, as shown in the main figure. If there are multiple
exits (ABRs), routing to destinations outside of the area does not
have to travel an optimal path. If the area has multiple exits,
one or more ABRs will inject a default into the stub area. In this
situation, routing to other areas or autonomous systems could take
a suboptimal path in reaching the destination by going out of the
area via an exit point that is farther from the destination than
alternate exit points.
- All OSPF routers inside the stub area (ABRs and internal
routers) are configured as stub routers so they will become
neighbors and exchange routing information. The configuration
commands for creating stub networks are covered later in this
chapter.
- The area is not needed as a transit area for virtual links.
(Virtual links are discussed at the end of this chapter.)
- No ASBR is internal to the stub area.
- The area is not the backbone area (area 0).
These restrictions are important because a stub/totally stubby
area is configured primarily to exclude external routes. As such,
all of the situations described cause external links to be injected
into that area, invalidating their "stubby" nature.
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