OSPF Neighbor States
OSPF neighbors go through states before they can begin exchanging LSAs. You can examine the state of an OSPF neighbor with the help of following command:-
A#show ip ospf neighbor
a. DOWN STATE
This state occurs when configuration is mismatched on both sides e.g. If STUB is configured on one side only. STUB is one of the fields in HELLO packet. HELLO packet contains following information/fields:-
The Router-ID of the originating router.
The Area-ID of the originating router interface.
The address mask of the originating interface.
Authentication type and authentication information for the originating interface.
The Hello Interval of the originating interface.
The Router Dead Interval of the originating interface.
The router priority.
The Designated Router (DR) and Backup Designated Router (BDR) information.
Five flags for options.
The Router-ID of the originating router's neighbors.
This is the initial state of the neighbor which also indicates that no hellos were heard from this neighbor within the last dead time interval.
b. INIT OR ONE WAY STATE
One router (A) sent Hello packet to other router (B) and the information is matched on the other router (B). In response other router (B) sent acknowledgment, but it is in the pipeline. This state indicates that a router has seen its own Router-ID in the neighbor field of the HELLO packet that it received from the neighbor. It also means that bidirectional conversation was established and that DR and BDR election can occur.
c. TWO WAY STATE
Router-A sent HELLO Packet and Router-B sent acknowledgment, after that, Router-B sent Hello packet and Router-A sent acknowledgment in response. Then neighbor table is completed and DR/BDR Election process begins.
When an OSPF interface first becomes active, it begins to send hello packets. When two routers receive each other's hello, they place the neighbor in INIT STATUS. When a neighbor is in init status, it places its own Router-ID into the hello packet. When a router receives one of the new hellos with the Router-ID of its neighbor, it places the neighbor in a new state of 2-WAY. The 2-Way state ensures that here is a two-way communication between the routers. The routers must be in this state before they can negotiate a DR/BDR and exchange LSAs. After the routers have achieved the 2-Way state, OSPF enters into its final states:
d. EXSTART (MASTER/SLAVE)
A simple meaning of this state is "who will initialize the communication first". At this state, another election for the selection of Master and Slave Router begins. The neighbor with the highest IP address becomes the Master and other becomes the Slave to exchange LSA.
e. EXCHANGE (LSA REQUEST)
The router sends its database description packets to neighbors in the exchange state. The database description packets describe the entire link state database. The link-state database is synchronized after this phase. After synchronization, the router enters one of two final states. Every network in the routing table of a router has a corresponding sequence number. At this state, a router sends the sequence numbers to the neighbor router. In response, as acknowledgment, the neighbor router also sends its sequence numbers.
The router also sends link state request packets to all neighbors that are in the loading state. The loading phase requests that more recent LSAs be sent. At this state the database table is completed.
At this state, SPF algorithm is applied and best path is calculated. Neighbors in this state are fully adjacent and routing table of neighbors are completed.
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