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Single-Area OSPFv2 Concepts, 1.3.2 OSPF Operating Statuses, image - Coggle…

- - - - Single-Area OSPF:Simplicity->Simple setup, Scalability ->Less scalable, Backbone requirement->All routers in Area 0, Area Border Routers->Not used
    - - Multi-Area OSPF:Simplicity->More complex, hierarchical, Scalability ->Better for large networks, Backbone requirement->All areas must link to Area 0, Area Border Routers->Required (ABR)
- - - - Discovers neighbors and builds adjacencies between them
    - - Checks for database synchronization between routers
    - - Requests specific link-state records from router to router
    - - Sends specifically requested link-state records
    - - Acknowledges the other packet types
  - - - he OSPF Type 1 packet is the Hello packet. Hello packets are used to do the following:
      - Discover OSPF neighbors and establish neighbor adjacencies.
      - Advertise parameters on which two routers must agree to become neighbors.
      - Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks like Ethernet. Point-to-point links do not require DR or BDR.
- - - - When OSPF is enabled on an interface, the router sends Hello packets with its router ID to the multicast address 224.0.0.5 (processed only by OSPFv2 routers). The router ID uniquely identifies each router. If a neighboring router receives a Hello packet with a new router ID not in its neighbor list, it attempts to establish an adjacency with that router.
  - - - Decide First Router (ExStart)
        
        Determines which router sends DBDs first.
        
        Router with the highest Router ID starts.
      - Exchange DBDs (Exchange)
        
        Routers exchange DBD packets with LSA headers.
        
        Contain: link-state type, advertising router address, cost, sequence number.
        
        Sequence number: defines freshness of info.
        
        Each DBD is acknowledged with an LSAck.
      - Send LSR (Loading)
        
        Router compares LSDB with received info.
        
        If newer info is found → sends an LSR.
        
        Response: LSU (Link State Update).
        
        Confirmed with LSAck.
        
        Router updates LSDB.
        
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  - - - Too many adjacencies → Every router forming adjacencies with all others is unnecessary and leads to excessive LSAs.
        
        Formula: n(n − 1) / 2 adjacencies.
        
        Example: 5 routers → 10 adjacencies; 20 routers → 190 adjacencies.
      - Excessive LSA flooding → Each topology change triggers LSA flooding to all neighbors.
        
        Solution: Elect a Designated Router (DR) and a Backup DR (BDR) to minimize adjacencies and control LSA flooding efficiently.
  - - - More routers → more LSAs exchanged, causing heavy flooding and acknowledgments → chaotic traffic.
        
        Problem: Each router flooding LSAs to all others creates inefficiency.
    - - DR: Collects and redistributes LSAs, reducing flooding.
      - BDR: Backup in case DR fails.
      - DROTHERs: All other routers (not DR/BDR).
      - Note: DR only manages LSAs; data forwarding still follows routing tables.