[Day 8] 3. Deployment of OSPF

Scenario:

• Router 1 (R1): IP 172.16.20.1
• Router 2 (R2): IP 172.16.20.2
• Directly connected via a single link.

Steps:

1. Activate OSPF: R1 and R2 enable OSPF on their interfaces.

2. Send Hello Packets: Both routers send Hello packets to the multicast address 224.0.0.5 to discover each other.

3. Form Neighbor Relationship: They exchange Hello packets and become OSPF neighbors if OSPF settings match (e.g., area ID).

4. Exchange LSAs: Routers exchange Link-State Advertisements (LSAs) to share information about their networks.

5. Build Link-State Database (LSDB): Both routers create an identical database containing network topology information.

6. Calculate Best Routes: Using the Dijkstra algorithm, each router calculates the shortest path to each network.

7. Update Routing Tables: Routers update their routing tables with the best routes and start forwarding packets.

R.ID (Router ID): it is the highest of the physical interfaces.

Stages:

Copy
Down
|
Init / Attempt (sending hellos)
|
Two Way (reply hellos)
|
Exchange-State (Master/Slave)
|
Exchange (DD)
|
Loading (Query)
|
Full

1. Down:

   • This is the initial state where no OSPF packets have been exchanged between routers.
   • The router is unaware of its neighbors.

2. Init / Attempt:

   • Init: The router has sent a Hello packet to its neighbors but has not yet received any Hello in response.
   • Attempt (used only in NBMA networks): The router actively tries to establish communication with a neighbor by sending Hello packets to a specific neighbor.

3. Two-Way:

   • The router has received a Hello packet from a neighbor and recognized its own router ID in the packet.
   • A bidirectional communication is established.
   • The OSPF DR/BDR election occurs in this state for broadcast or multi-access networks.

4. Exchange-State:

   • The router and its neighbor establish a master-slave relationship to synchronize their databases.
   • The router with the higher Router ID becomes the master and starts the database synchronization.

5. Exchange:

   • The routers exchange Database Description (DD) packets, which contain summaries of the link-state database (LSDB).
   • This allows each router to compare its database with its neighbor's.

6. Loading:

   • The routers send Link State Request (LSR) packets to request more detailed information about any missing or outdated LSAs (Link-State Advertisements) found during the exchange phase.
   • The neighbor responds with Link State Update (LSU) packets, which contain the requested LSAs.

7. Full:

   • The OSPF routers have synchronized their LSDBs and are now fully adjacent.
   • The neighbor relationship is established, and the routers can exchange routing information.

Explanation in Short:

• Down: No communication.
• Init: Sending Hello packets.
• Two-Way: Hello reply received; bidirectional communication established.
• Ex-Start: Master/Slave roles determined for database synchronization.
• Exchange: Summary of LSDBs (DD packets) exchanged.
• Loading: Missing information requested and updated.
• Full: LSDBs fully synchronized; neighbor relationship established.