[OSPF neighbor status has 7 types]
Down--Init--2-Way--Exstart--Exchange--Loading--Full
OSPF neighbor establishment process: A---B establishes a connection.
Here it is assumed that the router ID of A is 172.16.1.1, and that of B is 172.16.1.2
1. First, when the two parties have not started to establish a relationship, the status of both parties is DOWN
2. As shown in the figure below, router A wants to establish a relationship with B. First, A will send a Hello message (the message contains its own route-id, where the route-id is just the name of the router). When router B receives The message from A will set the state of the neighbor to Init . B will include A's Router ID in the Hello message sent by B.
3. Router B sends a Hello message to A. A receives the hello message from B and finds that it carries its own Router ID. At this time, it sets the status of the neighbor to 2-way (if the message does not carry its own Router ID , since the hello packet of route B contains its own route-id, the state of the neighbor will first change to the Init state, and then to the 2-way state)
4. When the state of the neighbors has changed to 2-way state, the two parties have only established a neighbor relationship (not established a connection relationship). In the neighbor state, routing entries cannot be passed.
5. After the neighbor relationship is established, in the NBMA network ( Non-Broadcast Multi-Access non-broadcast network ), the routers on both sides will compare the size of the route-id to elect the DR/BDR. Once the master-slave relationship is determined, the state directly changes to the Exstart state. ( If it is not elected, a timer will be generated when OSPF is started and the waiting time is 40s. After the 40s is over, the DR/BDR election has not yet determined the master-slave relationship. If the state of both parties will always stay in the 2-way state, It is because the routers on both sides have the same route-id, and there are many situations that cause the status of the 2-way, you can directly go to the Internet to search )
6. In the Exstart state, router A and router B will send empty DD packets to each other to obtain the approval of both parties, and A will send an empty DD packet (where I, M, and MS are set to one ) , Once the master-slave is officially confirmed, the neighbor status changes from Exstart to Exchange status. Afterwards, the DD message sent will carry the LSA database summary information. At this time, the I and MS bits will be set to zero.
7. When router B receives the DD empty message from A, it will also change the state of the neighbor from Exstart to Exchange state. When both parties start to send data packets with database descriptions, they need to reply with confirmation packets. When the received request contains a packet with its own routing entry, it will change from the Exchange-->Loading state
8. When there is no own routing entry in the data request message or there is no LSA entry in the link state request list, the state is changed to the Full state at this time, and the connection between the two neighbors is successfully established.
