ZyXEL Communications USG40 User Manual page 249

• A Not So Stubby Area (NSSA, RFC 1587) has routing information about the OSPF AS and
networks outside the OSPF AS to which the NSSA is directly connected. It does not have any
routing information about other networks outside the OSPF AS.
Each type of area is illustrated in the following figure.
Figure 166 OSPF: Types of Areas
This OSPF AS consists of four areas, areas 0-3. Area 0 is always the backbone. In this example,
areas 1, 2, and 3 are all connected to it. Area 1 is a normal area. It has routing information about
the OSPF AS and networks X and Y. Area 2 is a stub area. It has routing information about the OSPF
AS, but it depends on a default route to send information to networks X and Y. Area 3 is a NSSA. It
has routing information about the OSPF AS and network Y but not about network X.
OSPF Routers
Every router in the same area has the same routing information. They do this by exchanging Hello
messages to confirm which neighbor (layer-3) devices exist, and then they exchange database
descriptions (DDs) to create a synchronized link-state database. The link-state database contains
records of router IDs, their associated links and path costs. The link-state database is then
constantly updated through Link State Advertisements (LSA). Each router uses the link state
database and the Dijkstra algorithm to compute the least cost paths to network destinations.
Like areas, each router has a unique 32-bit ID in the OSPF AS, and there are several types of
routers. Each type is really just a different role, and it is possible for one router to play multiple
roles at one time.
• An internal router (IR) only exchanges routing information with other routers in the same area.
• An Area Border Router (ABR) connects two or more areas. It is a member of all the areas to
which it is connected, and it filters, summarizes, and exchanges routing information between
them.
• An Autonomous System Boundary Router (ASBR) exchanges routing information with routers in
networks outside the OSPF AS. This is called redistribution in OSPF.
Table 106 OSPF: Redistribution from Other Sources to Each Type of Area
SOURCE \ TYPE OF AREA
Static routes
RIP
Chapter 9 Routing
NORMAL NSSA
Yes Yes
Yes Yes
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STUB
No
Yes