D-Link DFL-1660 User Manual page 156

4.5.2. OSPF
Link-state Routing
OSPF is a form of link-state routing protocol that defines the sending of link-state advertisements
(LSAs) to all other routers within the same area. Each router maintains a database, known as a
link-state database, describing the AS topology. From the information in this database, each router
constructs a tree of shortest paths with itself as root. This shortest-path tree gives the best route to
each destination in the AS.
All OSPF protocol exchanges can be authenticated. This means that only routers with the correct
authentication can join the AS. Different authentication schemes can be used, like none, passphrase
or MD5 digest. It is possible to configure separate authentication methods for each AS.
OSPF Areas
OSPF allows sets of networks to be grouped together and this is called an OSPF Area. The topology
of an area is hidden from the rest of the AS. This information hiding reduces the amount of routing
traffic exchanged. Also, routing within the area is determined only by the area's own topology,
lending the area protection from bad routing data. An area is a generalization of an IP sub netted
network.
Components of OSPF
A more detailed description of OSPF area components is given below:
Areas
ABRs
ASBRs
Backbone Areas
Stub Areas
Transit Areas
The Designated Router
Each OSPF broadcast network has a designated router and a backup designated router. The routers
uses OSPF hello protocol to elect the DR and BDR for the network based on the priorities
advertised by all the routers. If there already are a DR on the network, the router will accept that
one, regardless of its own router priority.
An area consists of networks and hosts within an AS that have been grouped
together. Routers that are only within an area are called internal routers, all
interfaces on internal routers are directly connected to networks within the
area. The topology of an area is hidden from the rest of the AS.
Routers that have interfaces in more than one area are called Area Border
Routers (ABRs), these maintain a separate topological database for each area
to which they have an interface.
Routers that exchange routing information with routers in other Autonomous
Systems are called Autonomous System Boundary Router (ASBRs). They
advertise externally learned routes throughout the Autonomous System.
All OSPF networks need to have at least the backbone area, which is the area
with ID 0. This is the area that all other areas should be connected to, and the
backbone make sure to distribute routing information between the connected
areas. When an area is not directly connected to the backbone it needs a
virtual link to it.
Stub areas are areas through which or into which AS external advertisements
are not flooded. When an area is configured as a stub area, the router will
automatically advertises a default route so that routers in the stub area can
reach destinations outside the area.
Transit areas are used to pass traffic from an area that is not directly
connected to the backbone area.
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Chapter 4. Routing