D-Link DFL-1660 User Manual page 204

4.5.2. OSPF Concepts
Router. The routers use OSPF Hello messages to elect the DR and BDR for the network based on
the priorities advertised by all the routers. If there is already a DR on the network, the router will
accept that one, regardless of its own router priority.
With NetDefendOS, the DR and the BDR are automatically assigned.
Neighbors
Routers that are in the same area become neighbors in that area. Neighbors are elected by the use of
Hello messages. These are sent out periodically on each interface using IP multicast. Routers
become neighbors as soon as they see themselves listed in a neighbor's Hello message. In 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
in the routing table. This is commonly used to minimize the routing table.
To set this feature up in NetDefendOS, see Section 4.5.3.5, "OSPF Aggregates".
Virtual Links
Virtual links are used for the following scenarios:
A. Linking an area that does not have a direct connection to the backbone area.
B. Linking backbone areas when the backbone is partitioned.
The two uses are discussed next.
A. Linking areas without direct connection to the backbone
The backbone area always needs to be the center of all other areas. In some rare cases where it is
impossible to have an area physically connected to the backbone, a Virtual Link is used. Virtual
This is the initial state of the neighbor relationship.
When a Hello message is received from a neighbor, but does NOT include the Router
ID of the firewall in it, the neighbor will be placed in the Init state.
As soon as the neighbor in question receives a Hello message it will know the sending
router's Router ID and will send a Hello message with that included. The state of the
neighbors will change to the 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 OSPF interfaces, the state will be changed
to Full. On Broadcast interfaces, only the DR/BDR will advance to the Full state 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