Question

Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

A network engineer is observing the convergence process of an OSPF network after a core router interface experiences a sudden link loss. Arrange the events of link-state convergence into the correct chronological order from first to last.

  1. 1The local router detects a physical link failure on an active interface.
  2. 2The router generates an updated Link-State Advertisement (LSA) and floods it to adjacent neighbors.
  3. 3Neighboring routers receive the LSA and update their local Link-State Database (LSDB).
  4. 4Routers execute Dijkstra's Shortest Path First (SPF) algorithm using the revised LSDB.
  5. 5Routers update their Routing Information Base (RIB) with the recalculated next-hop paths.

Answer

The correct chronological sequence of OSPF convergence events is: 1) Physical link failure detection, 2) LSA generation and flooding, 3) Neighbor LSDB update, 4) Dijkstra's SPF algorithm recalculation, and 5) RIB/Routing table path updates.
The convergence process in link-state protocols follows a strict operational order: local failure detection occurs first, followed by LSA creation and flooding to communicate the failure. Next, receiving routers update their local LSDB map, execute Dijkstra's SPF algorithm on the updated database to determine new optimal paths, and finally update the Routing Information Base (RIB) with the newly computed routes.

Step-by-Step Solution

1
Identify the initial trigger event for link-state convergence.
The local router detects a link failure on an active interface.
Network convergence cannot start until a hardware or protocol-level failure is detected by the router.
2
Determine how topology changes are communicated across the OSPF area.
The router constructs an updated LSA and floods it to adjacent neighbors.
Link-state dynamic routing protocols rely on flooding LSAs so every router in an area has accurate topology information.
3
Trace the database update process on receiving routers.
Adjacent routers update their local Link-State Database (LSDB).
Routers must synchronize their database entries before calculating new paths.
4
Identify the path calculation algorithm executed after database synchronization.
Routers execute Dijkstra's Shortest Path First (SPF) algorithm.
Link-state protocols require running the SPF algorithm against the updated LSDB map to find the lowest-cost paths.
5
Finalize path installation into the routing table.
The new best paths are written into the Routing Information Base (RIB).
Packet forwarding decisions rely on the RIB, which is updated only after SPF computation completes.

Key Concept

OSPF Link-State Convergence Sequence and SPF Calculation
Estimated Time:1m 30s
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