What is the difference between basic IS-IS and OSPF packet types?

Question

Options

• AIS-IS and OSPF use area packets, but only IS-IS uses sequence number packets.
• BIS-IS and OSPF use link-state update packets, but only OSPF uses link-state ACK packets.
• CIS-IS and OSPF use Hello packets, but only OSPF uses DBD packets.
• DIS-IS and OSPF use link-state update packets, but only IS-IS uses DBD packets.

• B
  3% (1)
• C
  94% (33)
• D
  3% (1)

Explanation

C is correct because both IS-IS and OSPF use Hello packets to discover and maintain neighbor relationships - this is a shared fundamental mechanism. However, OSPF uses Database Description (DBD) packets to summarize its LSDB during adjacency formation, while IS-IS has no DBD equivalent; instead, IS-IS uses CSNP (Complete Sequence Number PDU) and PSNP (Partial Sequence Number PDU) to synchronize databases.

Why the distractors fail:

• A is wrong because "area packets" is not a real packet type in either protocol; while IS-IS does use sequence number PDUs (CSNP/PSNP), OSPF does not use them at all - not the other way around.
• B is misleading: OSPF does use dedicated LSAck packets and IS-IS does not, but it also implies both share the same update mechanism label, which obscures the deeper structural difference - making C the more precise and exam-relevant answer.
• D reverses the truth - it's OSPF, not IS-IS, that uses DBD packets.

Memory tip: "OSPF Does Big Descriptions" - the DBD packet belongs exclusively to OSPF. IS-IS skips that step and relies on sequence number PDUs (CSNP/PSNP) to sync its link-state database.

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