Which of the following descriptions of the router's LSA processing principles are correct? (Multiple choice)

Question

Options

• AIf multiple instances of an LSA are received, the highest priority is to check whether the checksums are the same. If they are not the same, the larger one is preferred.
• BIf the received LSA is damaged, such as Checksumn error, the LSA will not be received.
• CIf an LSA already exists in the local LSDB of the router and an updated LSA is received at this time, the local LSDB will be updated and the updated LSA will be flooded out.
• DIf the router receives an LSA that is not available locally, it updates the LSDB and uses DD messages to flood the LSDB.

• A
  13% (4)
• B
  77% (23)
• C
  10% (3)

Explanation

Options B and D are correct because they accurately reflect how OSPF routers handle LSA integrity and new information: a router discards any LSA that fails checksum validation (B) rather than risk corrupting the LSDB with bad data, and when a router receives an LSA it has no local copy of, it installs it into the LSDB and propagates the updated database state to neighbors (D).

Option A is wrong because sequence number - not checksum - has the highest comparison priority when evaluating LSA instances. The router compares sequence numbers first (higher = more recent), uses checksum only as a secondary tiebreaker when sequence numbers are equal, and considers age last. Jumping to checksum comparison first violates this order.

Option C is wrong because it presents LSDB update and flooding as automatic and unconditional. In reality, when a router receives an LSA that already exists in its LSDB, it must first compare the two instances (sequence number → checksum → age) to determine which is newer. Only if the received copy is confirmed more recent does the router update the LSDB and flood it; if the received copy is older or identical, the router instead sends the local copy back or just acknowledges - no update, no flood.

Memory tip: Use "SCA" to remember LSA comparison order - Sequence number, Checksum, Age. And anchor option B with a simple rule: corrupted in = discarded, never installed.

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