ZyXEL Communications ZyWall USG 50-H Series User Manual page 240

Chapter 11 Routing Protocols
• A normal area is a group of adjacent networks. A normal area has routing information
about the OSPF AS, any networks outside the OSPF AS to which it is directly connected,
and any networks outside the OSPF AS that provide routing information to any area in the
OSPF AS.
• A stub area has routing information about the OSPF AS. It does not have any routing
information about any networks outside the OSPF AS, including networks to which it is
directly connected. It relies on a default route to send information outside the OSPF AS.
• 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 178 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.
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ZyWALL USG 50-H User's Guide