OSPF Tables in Networking

OSPF Tables

• OSPF uses three tables to accomplish its goals:
  • Neighbor Table
  • Topology Table (also referred to as the Link State Database or LSDB)
  • Routing Table
• The Neighbor Table:
  • Maintained by every OSPF router
  • Lists all directly connected OSPF routers
  • Holds information such as the state of adjacency with each neighbor

The Topology Table (LSDB)

• Stores everything OSPF knows about the network topology
• A single table that includes all link state advertisements (LSAs)
• Each router has an identical LSDB, ensuring all routers have the same view of the network
• This table is used to calculate the shortest path first (SPF) algorithm
• It contains information about the network, including routers, links, and their respective states
• The LSDB is a crucial component of OSPF, as it enables routers to make informed decisions about routing traffic
• The table is updated dynamically as the network changes, ensuring OSPF routers always have an accurate view of the network topology
• Stores everything OSPF knows about the network topology
• A single table that includes all link state advertisements (LSAs)
• Each router has an identical LSDB, ensuring all routers have the same view of the network
• The Routing Table:
  • The router's actual routing table
  • Calculates best routes from the topology table and sends them to the routing table
  • Decides which routes are best and makes them available in the routing table

OSPF Packets

• 5 packets are involved in every OSPF conversation:
  • Hello Packets
  • Database Descriptor (DBD) Packets
  • Link State Request Packets
  • Link State Update (LSU) Packets
  • Link State Acknowledgement (LS ACK) Packets
• Hello Packets:
  • Sent periodically to the multicast address 224.0.0.5
  • Allow routers to discover each other and determine whether to form an adjacency
• DBD Packets:
  • Send a summary of LSAs in the LSDB
  • Spare routers from sending and receiving the full LSDB
  • Routers can request specific LSAs they need
• Link State Request Packets:
  • Used to request specific LSAs from another router
• LSU Packets:
  • Send the requested LSAs to the requesting router
• LS ACK Packets:
  • Confirm receipt of LSU packets

OSPF Tables

• OSPF uses three tables to operate: Neighbor Table, Topology Table (Link State Database or LSDB), and Routing Table
• The Neighbor Table is maintained by every OSPF router, listing all directly connected OSPF routers and their adjacency states
• The Topology Table (LSDB) stores the entire network topology, with each router having an identical LSDB for a shared network view
• The Routing Table calculates best routes from the Topology Table and makes them available for routing decisions

OSPF Packets

• 5 packet types are used in OSPF conversations: Hello, Database Descriptor (DBD), Link State Request, Link State Update (LSU), and Link State Acknowledgement (LS ACK) packets
• Hello Packets are periodically sent to the 224.0.0.5 multicast address to discover routers and form adjacencies
• Database Descriptor (DBD) Packets summarize LSAs in the LSDB, allowing routers to request specific LSAs and avoiding full LSDB transfers
• Link State Request Packets are used to request specific LSAs from other routers
• Link State Update (LSU) Packets send requested LSAs to the requesting router
• Link State Acknowledgement (LS ACK) Packets confirm receipt of LSU packets