D-Link NetDefend DFL-210 User Manual page 105

4.4.2. OSPF
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.
Neighbors
Routers that are in the same area become neighbors in that area. Neighbors are elected via the Hello
protocol. Hello packets are sent periodically out of each interface using IP multicast. Routers
become neighbors as soon as they see themselves listed in the neighbor's Hello packet. This way, a
two way communication is guaranteed.
The following Neighbor States are defined:
Down
Init
2-Way
ExStart
Exchange
Loading
Full
Aggregates
OSPF Aggregation is used to combine groups of routes with common addresses into a single entry
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, that 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 a area that is not directly connect
to the backbone area.
This is the initial stat of the neighbor relationship.
When a HELLO packet is received from a neighbor, but does NOT include the Router
ID of the firewall in it, the neighbor will be placed in Init state. As soon as the
neighbor in question receives a HELLO packet it will know the sending routers
Router ID and will send a HELLO packet with that included. The state of the
neighbors will change to 2-way state.
In this state the communication between the router and the neighbor is bi-directional.
On Point-to-Point and Point-to-Multipoint interfaces, the state will be changed to
Full. On Broadcast interfaces, only the DR/BDR will advance to Fullstate with their
neighbors, all the remaining neighbors will remain in the 2-Way state.
Preparing to build adjacency.
Routers are exchanging Data Descriptors.
Routers are exchanging LSAs.
This is the normal state of an adjacency between a router and the DR/BDR.
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Chapter 4. Routing