ZyXEL Communications ZyWALL USG 200 Series User Manual page 397

Each type of area is illustrated in the following figure.
Figure 304 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.
ZyWALL USG 100/200 Series User's Guide
Chapter 16 Routing Protocols
397