OSPF
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Neighbors and adjacencies
In areas with two or more routing devices, neighbors and adjacencies are formed.
Neighbors are routing devices that maintain information about each others’ health. To establish neighbor
relationships, routing devices periodically send hello packets on each of their interfaces. All routing
devices that share a common network segment, appear in the same area, and have the same health
parameters (hello and dead intervals) and authentication parameters respond to each other’s hello
packets and become neighbors. Neighbors continue to send periodic hello packets to advertise their
health to neighbors. In turn, they listen to hello packets to determine the health of their neighbors and to
establish contact with new neighbors.
The hello process is used for electing one of the neighbors as the area’s Designated Router (DR) and one
as the area’s Backup Designated Router (BDR). The DR is adjacent to all other neighbors and acts as the
central contact for database exchanges. Each neighbor sends its database information to the DR, which
relays the information to the other neighbors.
The BDR is adjacent to all other neighbors (including the DR). Each neighbor sends its database
information to the BDR just as with the DR, but the BDR merely stores this data and does not distribute it. If
the DR fails, the BDR will take over the task of distributing database information to the other neighbors.
Link-State Database
OSPF is a link-state routing protocol. A link represents an interface (or routable path) from the routing
device. By establishing an adjacency with the DR, each routing device in an OSPF area maintains an
identical Link-State Database (LSDB) describing the network topology for its area.
Each routing device transmits a Link-State Advertisement (LSA) on each of its interfaces. LSAs are entered
into the LSDB of each routing device. OSPF uses flooding to distribute LSAs between routing devices.
When LSAs result in changes to the routing device’s LSDB, the routing device forwards the changes to the
adjacent neighbors (the DR and BDR) for distribution to the other neighbors.
OSPF routing updates occur only when changes occur, instead of periodically. For each new route, if an
adjacency is interested in that route (for example, if configured to receive static routes and the new route
is indeed static), an update message containing the new route is sent to the adjacency. For each route
removed from the route table, if the route has already been sent to an adjacency, an update message
containing the route to withdraw is sent.
Shortest Path First Tree
The routing devices use a link-state algorithm (Dijkstra’s algorithm) to calculate the shortest path to all
known destinations, based on the cumulative cost required to reach the destination.
The cost of an individual interface in OSPF is an indication of the overhead required to send packets
across it. The cost is inversely proportional to the bandwidth of the interface. A lower cost indicates a
higher bandwidth.
