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Web OS 10.0 Application Guide
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 inter-
faces. 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 peri-
odic 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 data-
base 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 data-
base 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 infor-
mation to the other neighbors.
The 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.
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Chapter 4: OSPF
212777-A, February 2002
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