An Analysis of the Seven States of OSPF

Open Shortest Path First (OSPF) is an Interior Gateway Protocol (IGP), which is used to make routing decisions in a single autonomous system (AS). It is an implementation of the link state routing protocol and belongs to the IGP, so it operates inside the autonomous system. When OSPF is establishing a neighbor relationship, it will go through seven states. Today, SPOTO will analyze the seven states of OSPF.

What is the neighbor relationship? As mentioned earlier, OSPF will go through seven states when establishing the neighbor relationship. So spoto will introduce the neighbor relationship to you first. It refers to the interaction between two parties. When the Hello time, Dead time, Area ID, verification information and Stub Flag information in the Hello message are consistent, two direct broadcast networks will select DR on one port, and the other port will be selected as BDR, and then it will be in the 2-way state. As long as it can enter the 2-way state normally, the neighbor relationship is established.

An analysis of the seven states: the seven states that OSPF routers go through before full adjacency are Down, Init, Two-way, ExStart, Exchange, Loading and Full Adjacency.


In this state CISCO has not exchanged information with other routers. First, send out hello packets from its CISCO interface without knowing DR (in case of broadcast network) and any other routers. To send hello packets, use multicast address 224.0 0.5. In the Down state, the CISCO process has not exchanged information with any neighbors. CISCO is waiting to enter the Init state.


The CISCO router sends type 1 (Hello) packets at a fixed time interval (default 10s) in order to establish a special relationship with the neighbor router. The Hello packet is received in the DeadInterval, and the 2-way communication has not been established.


Each CISCO router uses packets to try to establish two-way status or two-way communication with all neighbor routers in the same IP network. The Hello packet contains a list of CISCO neighbors known to the sender. When the router sees itself in the Hello packet of a neighbor router, it enters a two-way state. A two-way session is established, and RIDs appear in each other’s neighbor list.


Initial state of information exchange. When the router and its neighbors enter the ExStart state, the session between them is characterized as an adjacent relationship, but the router has not yet been into a fully adjacent state. The ExStart status is established using the DataBase Description (DBD) packet of type 2. The two routers use the Hello packet to negotiate the relationship between them about who is the “superior” and who is the “subordinate”.


Information exchange status. The local router exchanges one or more DBD packets (also known as DDP) with its neighbors. The DBD contains summary information about LSA entries in the LSDB.


Information loading status. After receiving the DBD, use LSACK packet to confirm that the DBD has been received. Compare the received information with the information in the LSDB. If there is an updated link status entry in the DBD, send the other party an LSR to request a new LSA.

Full Adjacency

Full adjacency state. After the loading state is completed, the router enters the full adjacency state. Each router keeps a list of adjacent routers, which is called adjacent database.

The above is the analysis of the seven states of CISCO. Hope it will be helpful to you.

Leave a Reply

Your email address will not be published. Required fields are marked *