OSPF, LSR, LSA & LS-DB Everything you must know !!

Link State Routing Protocol:

• Link state protocols are based on Shortest Path First (SPF) algorithm to find the best path to a destination.
• Shortest Path First (SPF) algorithm is also known as Dijkstra algorithm, since it is conceptualized by Dijkstra.

LS-DB [Link-State Data Base] Tables:

• OSPF is link state protocol and when configured it will collect available data to develop a map of all paths available in its network, this data is later saved in its db which is known as LS-DB.
• LS-DB is collection of all OSPF LSA's received.
• The information stored in this LS-DB is used to calculate the shortest path using SPF algorithms.
• Each OSPF router has identical LS-DB.
• LS-DB are exchanged between neighboring routers soon after they are discovered as neighbors.



The information is propagated in DB tables of single OSPF Area.

LS-DB includes below tables :

Neighbor Table → Which contains details of discovered OSPF neighbor.

Topology Table → Which contains details of entire topology map within OSPF area and is also use to calculate the alternate & best paths.

Routing Table → Which contains details of presently being used the best working path which can be forwarded to neighboring routers.

• All routers in same Area share single topology table, but different routing tables.
• This is only because OSPF calculates separate best paths for each router on basis of its location in the topology, while all router share the same Link State Topology Table.
• This helps the topology table to be in sync with each and every available router in network.

What is LSA and how does it helps router to communicate ?

• LSA stands for Link State Advertisement is basic communication means of OSPF routing protocol for the internet protocol (IP).
• OSPF uses LS-DB and fills this with LSA.

OSPF uses LSR potocols via LSA.

• In Shortest Path First (SPF) algorithm, whenever a link's state changes, a routing update called a Link-State Advertisement (LSA) is exchanged between routers.
• When a router receives an LSA routing update, the link-state algorithm is used to recalculate the shortest path to affected destinations.
• Each router constructs a map of the complete network. An example of Link State protocol is OSPF.