Link-state Database (LSDB) and route updates

OSPF is based on links. The links between adjacent neighbor routers allow updates to be passed along the network. Network links allow the DR to flood the area with Link-state database (LSDB) updates. External links allow the OSPF area to connect to destinations outside the OSPF autonomous system. Information about these links is passed throughout the OSPF network as link-state updates.

The LSDB contains the information that defines the complete OSPF area, but the LSDB is not the routing table. It contains the information from all the link-state updates passed along the network. When there are no more changes required, and the network is stable then the LSDB on each router in the network will be the same. The DR will flood the LSDB to the area to ensure each router has the same LSDB.

To calculate the best route (shortest path) to a destination, the FortiGate unit applies the Shortest Path First (SPF) algorithm, based on Dijkstra’s algorithm, to the accumulated link-state information. OSPF uses relative path cost metric for choosing the best route. The path cost can be any metric, but is typically the bandwidth of the path, how fast traffic will get from one point to another.

The path cost, similar to “distance” for RIP, imposes a penalty on the outgoing direction of a FortiGate unit interface. The path cost of a route is calculated by adding together all of the costs associated with the outgoing interfaces along the path to the destination. The lowest overall path cost indicates the best route, and generally the fastest route. Some brands of OSPF routers, such as Cisco, implement cost as a direct result of bandwidth between the routers. Generally this is a good cost metric because larger bandwidth means more traffic can travel without slowing down. To achieve this type of cost metric on FortiGate units, you need to set the cost for each interface manually in the CLI.


	The inter-area routes may not be calculated when a Cisco type ABR has no fully adjacent neighbor in the backbone area. In this situation, the router considers summary‑LSAs from all Actively summary-LSAs from all Actively Attached areas (RFC 3509).

The FortiGate unit dynamically updates its routing table based on the results of the SPF calculation to ensure that an OSPF packet will be routed using the shortest path to its destination. Depending on the network topology, the entries in the FortiGate unit routing table may include:

• if the network contains OSPF areas and non-OSPF domains, routes to area boundary routers, which reside on the OSPF network backbone and are configured to forward packets to destinations outside the OSPF AS.

Once the OSPF domain is established, there should be few updates required on a stable network. When updates occur and a decision is required concerning a new route, this is the general procedure.

2. The router has an up to date LSDB of the entire area, containing information about each router, the next hop to it, and most importantly the cost to get there.

3. Our router, turns the LSDB into a shortest path first (SPF) tree using Dijkstra’s algorithm. It doesn’t matter if there is more than one path to a router on the network, the SPF tree only cares about the shortest path to that router.

4. Once the SPF tree has been created, and shows the shortest paths to all the OSPF routers on the network, the work is done. If the new route is the best route, it will be part of that tree. If it is not the shortest route, it will not be included in the LSDB.

5. If there has been a change from the initial LSDB to the new SPF tree, a link state update will be sent out to let the other routers know about the change so they can update their LSDBs as well. This is vital since all routers on the OSPF area must have the same LSDB.