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Zorluk: Çok zorRouting Concepts and Dynamic Routing Protocols

An enterprise edge router running single-area OSPF detects an unexpected physical link failure on a directly connected network segment. Order the following operational events chronologically from the moment the topology change is detected until traffic forwarding paths are updated across the area. What is the correct sequence of events?

  1. 1The local router generates an updated Type 1 Router LSA containing the modified link state and metric details.
  2. 2The local router floods a Link-State Update (LSU) packet containing the modified LSA out all remaining active OSPF-enabled interfaces.
  3. 3Receiving adjacent routers update their local Link-State Database (LSDB) and transmit a Link-State Acknowledgment (LSAck) packet.
  4. 4Each router in the area executes Dijkstra's Shortest Path First (SPF) algorithm against its updated LSDB to compute a new shortest-path tree.
  5. 5The router updates the IP Routing Information Base (RIB) with the newly calculated optimal next-hop routes.

Cevap

The correct sequence begins with the local router generating an updated Type 1 LSA, followed by flooding the LSU to adjacent peers. Next, neighbors update their LSDB and send an LSAck. Then, routers run Dijkstra's SPF algorithm on the updated LSDB, and finally, the IP Routing Information Base (RIB) is updated with the new best paths.
The precise operational sequence for OSPF convergence starts at the point of failure: the router generates a new Type 1 LSA reflecting the down interface. This LSA is flooded in an LSU packet to adjacent peers. Peers update their local LSDB and return an LSAck for reliable transport. Once the LSDB is synchronized, Dijkstra's SPF algorithm runs to construct the new shortest-path tree, and finally, optimal paths are installed into the Routing Information Base (RIB).

Adım Adım Çözüm

1
Identify the initial event immediately following a link failure.
The local router detects the failure and generates a fresh Type 1 Router LSA describing the link change.
An OSPF router must first construct the new LSA locally before it can inform neighboring routers about the topology change.
2
Determine how the topology change information is distributed across the area.
The router encapsulates the new LSA into a Link-State Update (LSU) packet and floods it to adjacent routers.
Link-State updates are the mechanisms OSPF uses to distribute LSAs to maintain area-wide LSDB synchronization.
3
Identify the action taken by receiving peer routers upon receiving the update.
Adjacent routers accept the LSU, update their local Link-State Database (LSDB), and reply with a Link-State Acknowledgment (LSAck).
OSPF requires explicit acknowledgment (LSAck) for reliable flooding and immediate database update.
4
Determine when path re-computation takes place.
Each router runs Dijkstra's Shortest Path First (SPF) algorithm using the newly synchronized LSDB.
SPF algorithm calculation depends on an updated LSDB; running it prematurely on stale data leads to incorrect routing trees.
5
Determine the final step in restoring valid packet forwarding.
The best routes derived from the SPF tree are committed to the IP Routing Information Base (RIB).
The routing table (RIB) can only be updated once SPF calculation completes and identifies the optimal next hops.

Anahtar Kavram

OSPF Convergence and LSA Propagation Sequence
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Routing Concepts and Dynamic Routing Protocols

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