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OSPF States

OSPF (Open Shortest Path First) uses several states to describe the progression of its neighbor relationships. These states help in understanding how routers discover each other and form adjacencies. Here’s an overview of the OSPF states:

• Down:

  No Hello packets have been received from the neighbor. This is the initial state for all OSPF routers.

• Init:

  A router has received a Hello packet from a neighbor, but the receiving router's ID is not listed in the neighbor's Hello packet. This indicates that the router has seen the neighbor but the neighbor has not yet seen the router.

• Two-Way:

  Bi-directional communication is established. Both routers have received Hello packets from each other with their own router IDs listed. In broadcast and non-broadcast multi-access (NBMA) networks, DR and BDR elections occur at this state.

• ExStart:

  Routers start the process of establishing adjacency. The master-slave relationship is established for the exchange of Database Descriptor (DBD) packets. The router with the higher router ID becomes the master.

• Exchange:

  Routers exchange DBD packets, which contain summaries of the LSAs (Link-State Advertisements) in their link-state databases. Each router compares the received DBD packets with its own database to determine which LSAs need to be requested.

• Loading:

  Routers send Link State Request (LSR) packets to request more detailed information about LSAs that were discovered during the Exchange state but are not yet known or are outdated. Neighbors respond with Link State Update (LSU) packets.

• Full:

  The routers have synchronized their databases. They are now fully adjacent, meaning they have the same LSAs in their link-state databases. This is the normal operational state for OSPF routers.

Summary of OSPF Neighbor States

• Down: No Hello packets received.
• Init: Hello packet received, but not acknowledged.
• Two-Way: Bi-directional communication established.
• ExStart: Master-slave election for DBD exchange.
• Exchange: Exchange of DBD packets.
• Loading: Requesting and receiving detailed LSA information.
• Full: Full adjacency and synchronization achieved.