
Nortel Switched Firewall 2.3.3 User’s Guide and Command Reference
Open Shortest Path First
99
213455-L, October 2005
Because of the overhead required for establishing a new DR in case of failure, the hello
process also elects a Backup Designated Router (BDR). 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.
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.
The 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.