OSPF Operation

OSPF Operation
OSPF operation can be divided into three categories:

Neighbor and adjacency initialization

LSA flooding

SPF tree calculation
We will discuss each in the following sections. Before we discuss these three categories in more
detail, let’s first take a look at the basic step-by-step operation of OSPF:

OSPF routers send Hello packets out all interfaces participating in the OSPF process. If the
router and the router on the other side of the connection agree on the parameters set forth
in the Hello packet, the routers will form neighbor relationships.

Some of the neighbors will form adjacencies. Forming adjacencies is dependent upon the
type of network the Hello packet is being sent across and the types of routers exchanging
the Hello packets.

The routers will send link-state advertisements (LSAs), which contain descriptions of the
router’s links and the state of each link to the adjacent router.

The routers that receive the LSAs will then record the information into their link-state database
and forward the LSAs on to their respective neighbors. This allows all routers participating in
the OSPF process to have the same view of the network, although from their own perspective.

After learning all LSAs, each router will run the Dijkstra SPF algorithm to learn the shortest
path to all the known destinations. Each router uses this information to create its SPF tree.
The information contained in the SPF tree is then used to populate the routing table.
Now that you have a basic understanding of how OSPF operates, let’s take a more in-depth
look at each of the three categories previously mentioned.