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Q: What is the OSPF (Open Shortest Path First) protocol and how does it work?

A: OSPF is a routing protocol that uses the Shortest Path First (SPF) algorithm to calculate the best path for data packets within a single Autonomous System (AS).

How OSPF works:

  1. Neighbour Discovery: OSPF routers establish neighbour relationships by exchanging Hello messages. Once a neighbour adjacency is formed, they begin sharing routing information.
  2. Link State Database Synchronisation: Each OSPF router stores its link-state information in a local Link State Database (LSDB). When two routers become neighbours, they exchange their LSDB information to synchronise their view of the network.
  3. SPF Calculation: With a complete LSDB, each router runs the SPF algorithm to build a Shortest Path Tree (SPT), which is then stored in the routing table. This tree represents the optimal path from the router to all other destinations.
  4. Route Updates: Routers periodically send update messages to notify neighbours of any changes in their link states. Upon receiving an update, routers update their LSDB and recalculate the SPT.
  5. Data Forwarding: When a data packet arrives, the router consults its SPT to determine the next-hop router and forwards the packet accordingly.