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 This is the initial state of the neighbor relationship.
Init 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.
2-Way In this state the communication between the router and the neighbor is
bidirectional.
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.
ExStart Preparing to build adjacency.
Exchange Routers are exchanging Data Descriptors.
Loading Routers are exchanging LSAs.
Full This is the normal state of an adjacency between a router and the DR/BDR.
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 4.6.3.5, “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
Chapter 4: Routing
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