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Nokia NRS II Composite (OSPF) study materials
Nokia NRS II Composite (OSPF) study resources

Nokia NRS II Composite (OSPF) practice questions

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Question 1OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF packet type is used to discover neighbors on a multi-access segment and to elect the Designated Router and Backup Designated Router?

  1. Link State Request packet
  2. Database Description packet
  3. Hello packet Correct answer.
  4. Link State Update packet

Explanation

Hello packets are sent periodically on every OSPF-enabled interface. On broadcast and NBMA segments they carry the Router Priority, DR and BDR fields that drive the DR/BDR election, and they also perform neighbor discovery and keepalive functions.

Question 2OSPF Fundamentals, Packet Types and Neighbor State Machine

In the OSPF neighbor state machine, which state indicates that bidirectional communication has been confirmed because the router saw its own Router ID in a neighbor's Hello packet?

  1. Exchange
  2. 2-Way Correct answer.
  3. ExStart
  4. Init

Explanation

A router moves a neighbor to Init when it receives a Hello but does not yet see its own Router ID listed. Seeing its own Router ID in the neighbor's Hello proves two-way reachability and moves the neighbor to 2-Way.

Question 3OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF packet type carries the master/slave negotiation, the initial DD sequence number, and the MTU value used during adjacency formation?

  1. Database Description Correct answer.
  2. Link State Acknowledgment
  3. Link State Request
  4. Hello

Explanation

Database Description packets start with an empty, Init-bit-set DD that carries the proposed DD sequence number and the interface MTU. The router with the higher Router ID becomes master, and subsequent DD packets carry LSA headers.

Question 4OSPF Fundamentals, Packet Types and Neighbor State Machine

Two routers are connected and their Hello packets are exchanged, but they remain stuck in ExStart. Which condition most directly explains this behavior?

  1. The area IDs configured on the two interfaces differ
  2. The network type on both sides is point-to-point
  3. The dead interval on one side is shorter than the hello interval on the other
  4. An MTU mismatch exists between the two interfaces Correct answer.

Explanation

During ExStart the routers exchange DD packets that include the interface MTU. If the MTU values differ, the DD exchange fails and the adjacency stalls in ExStart; Cisco and Nokia devices typically log an MTU mismatch message.

Question 5OSPF Fundamentals, Packet Types and Neighbor State Machine

On a broadcast multi-access segment, which routers form a Full adjacency with the Designated Router?

  1. Only routers configured as point-to-point neighbors
  2. Only the Backup Designated Router
  3. Only routers whose priority is higher than the DR's priority
  4. All other routers on the segment, including the BDR Correct answer.

Explanation

On a broadcast segment, every OSPF router forms a Full adjacency with the DR and the BDR. Non-DR and non-BDR routers stay in 2-Way with each other, which reduces the number of adjacencies and LSA flooding.

Question 6OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF packet is sent as a unicast response to a Link State Request and contains the complete LSAs the neighbor asked for?

  1. Hello
  2. Link State Acknowledgment
  3. Database Description
  4. Link State Update Correct answer.

Explanation

A Link State Request identifies LSAs by type, Link State ID and advertising router. The neighbor answers with a Link State Update carrying those full LSAs, which the requester then acknowledges.

Question 7OSPF Fundamentals, Packet Types and Neighbor State Machine

A router interface is configured with a hello interval of 10 seconds and a dead interval of 40 seconds. What happens when it receives a Hello from a neighbor advertising a hello interval of 30 seconds and a dead interval of 120 seconds?

  1. The Hello is rejected because the hello and dead intervals do not match Correct answer.
  2. The Hello is accepted, but the dead interval is negotiated down to 40 seconds
  3. The Hello is accepted only if the neighbor's Router ID is higher
  4. The Hello is accepted and the adjacency proceeds normally

Explanation

For a Hello to be accepted, the hello interval, dead interval, area ID, authentication and stub area flag must match on both sides. A mismatch in the hello or dead interval causes the Hello to be discarded, so no adjacency forms.

Question 8OSPF Fundamentals, Packet Types and Neighbor State Machine

Which field in the OSPF Hello packet is used to verify that both routers belong to the same area?

  1. Router Priority
  2. Area ID Correct answer.
  3. Options field
  4. Authentication Type

Explanation

The Area ID field in the Hello must match on both sides for the Hello to be accepted. Routers in different areas can be physically connected but will not form an adjacency.

Question 9OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF neighbor state is reached when the master/slave relationship has been negotiated and the routers are exchanging Database Description packets containing LSA headers?

  1. Full
  2. Loading
  3. ExStart
  4. Exchange Correct answer.

Explanation

In ExStart the routers negotiate the master/slave relationship and DD sequence number. Once that is complete they enter Exchange, where they send DD packets containing LSA headers describing their databases.

Question 10OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF packet type is used to flood a newly originated Router LSA to all neighbors on an interface?

  1. Link State Request
  2. Database Description
  3. Link State Update Correct answer.
  4. Hello

Explanation

Link State Update packets carry one or more complete LSAs and are used to flood new or changed LSAs. On multi-access segments, the update is multicast to AllSPFRouters and to AllDRouters so the DR and BDR receive it.

Question 11OSPF Fundamentals, Packet Types and Neighbor State Machine

A router is configured as an OSPF stub router on a multi-access segment. Which statement describes the DR election behavior for that interface?

  1. The router can become DR if all other routers have priority 0
  2. The router becomes DR automatically because stub routers have highest priority
  3. The router becomes DR only if it has the highest Router ID
  4. The router never becomes DR or BDR regardless of other priorities Correct answer.

Explanation

A router with OSPF priority 0 is not eligible to become DR or BDR. A stub router is configured with priority 0 so it does not attract transit traffic, and it will not be elected even if it has the highest Router ID.

Question 12OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF packet type is used to acknowledge receipt of LSAs and to make flooding more reliable?

  1. Hello
  2. Link State Acknowledgment Correct answer.
  3. Database Description
  4. Link State Update

Explanation

Link State Acknowledgment packets list the LSA headers being acknowledged, confirming that a Link State Update was received. They make the flooding process reliable, since unacknowledged LSAs are retransmitted.

Question 13OSPF Fundamentals, Packet Types and Neighbor State Machine

Two routers on a broadcast segment both have OSPF priority 1. Router A has Router ID 10.0.0.1 and Router B has Router ID 10.0.0.2. Which router becomes the Designated Router?

  1. The router that sends the first Hello packet
  2. Neither; a new election is triggered until priorities differ
  3. Router A, because it has the lower Router ID
  4. Router B, because it has the higher Router ID Correct answer.

Explanation

When priorities are equal, the router with the highest Router ID wins the DR election. Since both have priority 1, Router B with Router ID 10.0.0.2 becomes DR and Router A becomes BDR.

Question 14OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF neighbor state is reached when a router has sent Link State Requests for missing LSAs and is waiting for the corresponding Link State Updates?

  1. Exchange
  2. Loading Correct answer.
  3. 2-Way
  4. Full

Explanation

After the Exchange phase, a router compares the received database summary with its own database. If LSAs are missing it sends Link State Requests and enters Loading until the requested LSAs arrive.

Question 15OSPF Fundamentals, Packet Types and Neighbor State Machine

Which two multicast addresses are used by OSPFv2 to send Hello packets on a broadcast segment?

  1. 224.0.0.5 and 224.0.0.6 Correct answer.
  2. 239.0.0.5 and 239.0.0.6
  3. 224.0.0.1 and 224.0.0.2
  4. 224.0.0.9 and 224.0.0.10

Explanation

OSPFv2 uses 224.0.0.5 (AllSPFRouters) for Hellos and general flooding, and 224.0.0.6 (AllDRouters) for updates sent to the DR and BDR. These addresses are link-local in scope and are not forwarded by routers.

Question 16OSPF Fundamentals, Packet Types and Neighbor State Machine

A router's interface is configured as OSPF passive. What is the effect on that interface?

  1. Hellos are sent at a reduced interval to save bandwidth
  2. The interface is removed from the OSPF process entirely and not advertised
  3. Hellos are sent but the interface never becomes DR
  4. Hellos are suppressed, so no neighbor adjacency forms, but the connected network is still advertised Correct answer.

Explanation

A passive interface does not send or process Hello packets, so no adjacency can form on it. However, the connected subnet is still included in the router's Router LSA and advertised into OSPF.

Question 17OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF neighbor state indicates that the router has received a Hello from a neighbor but has not yet seen its own Router ID in that neighbor's Hello?

  1. 2-Way
  2. Down
  3. Init Correct answer.
  4. ExStart

Explanation

Init means a Hello was received from the neighbor, but the local Router ID was not listed in the neighbor's Hello. This indicates one-way communication; when the local Router ID appears, the state moves to 2-Way.

Question 18OSPF Fundamentals, Packet Types and Neighbor State Machine

On a point-to-point OSPF link, which statement correctly describes DR and BDR behavior?

  1. A DR is elected but no BDR is elected
  2. DR election occurs only if the link is configured as point-to-multipoint
  3. A DR and BDR are elected, but only the DR forms a Full adjacency
  4. No DR or BDR election occurs, and the two routers form a Full adjacency directly Correct answer.

Explanation

On point-to-point and point-to-multipoint network types, the DR/BDR concept does not apply. The two routers form a direct Full adjacency, and the DR and BDR fields in the Hello are set to 0.0.0.0.

Question 19OSPF Fundamentals, Packet Types and Neighbor State Machine

Which field in the OSPF Hello packet prevents a router from accepting a Hello from a neighbor that is configured with a different authentication password?

  1. Checksum field
  2. Authentication field Correct answer.
  3. Router Priority field
  4. Options field

Explanation

The Authentication field in the Hello carries the authentication type and data, such as a cleartext password or a cryptographic hash. If the authentication does not match, the Hello is discarded and no adjacency forms.

Question 20OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF neighbor state is reached when the databases of two routers are fully synchronized and the adjacency is complete?

  1. Exchange
  2. Loading
  3. Full Correct answer.
  4. 2-Way

Explanation

Full is the final neighbor state and means the two routers have synchronized link state databases and can use each other for SPF computation. On broadcast segments, non-DR/non-BDR routers remain in 2-Way with each other.

Question 21OSPF Fundamentals, Packet Types and Neighbor State Machine

A router is configured with an OSPF priority of 0 on a broadcast interface. Which statement correctly describes the outcome of the DR election on that segment?

  1. The router can become BDR if no other routers are eligible
  2. The router cannot become DR or BDR, but it still forms Full adjacencies with the DR and BDR Correct answer.
  3. The router is excluded from the OSPF process on that interface
  4. The router becomes DR if it has the highest Router ID

Explanation

Priority 0 makes a router ineligible to become DR or BDR. It still participates in OSPF on that segment and forms Full adjacencies with the elected DR and BDR, but it never holds either role.

Question 22OSPF Fundamentals, Packet Types and Neighbor State Machine

Which sequence correctly orders the OSPF neighbor states from initial discovery to full adjacency?

  1. Down, 2-Way, Init, ExStart, Exchange, Loading, Full
  2. Down, Init, 2-Way, ExStart, Exchange, Loading, Full Correct answer.
  3. Init, Down, 2-Way, Exchange, ExStart, Loading, Full
  4. Down, Init, ExStart, 2-Way, Exchange, Loading, Full

Explanation

The standard progression is Down, Init (Hello received), 2-Way (own Router ID seen), ExStart (master/slave negotiation), Exchange (DD packets with LSA headers), Loading (LSRs and LSUs), and Full (databases synchronized).

Question 23OSPF Fundamentals, Packet Types and Neighbor State Machine

A router receives a Hello packet from a neighbor whose Router ID is already listed in the neighbor's own Hello. What does this indicate about the neighbor's view of the adjacency?

  1. The neighbor considers the link to be in 2-Way state with the local router Correct answer.
  2. The neighbor considers the link to be in Init state
  3. The neighbor considers the link to be in Down state
  4. The neighbor considers the link to be in ExStart state

Explanation

When a Hello lists the local router's Router ID in the neighbor list, the sender has received a Hello from the local router and considers the adjacency to be at least 2-Way. Bidirectional communication is thereby confirmed on both sides.

Question 24OSPF Fundamentals, Packet Types and Neighbor State Machine

On an OSPF broadcast segment, the DR fails. Which router takes over as DR, and what happens to the previous BDR?

  1. No DR is elected until the original DR returns
  2. The BDR becomes DR, and a new BDR is elected among the remaining eligible routers Correct answer.
  3. A new DR is elected among all routers, and the BDR remains BDR
  4. The router with the highest Router ID becomes DR, and the BDR is unchanged

Explanation

When the DR fails, the BDR is promoted to DR without a full election. A new BDR is then elected among the remaining routers with non-zero priority, using priority and Router ID as tiebreakers.

Question 25OSPF Fundamentals, Packet Types and Neighbor State Machine

A router's interface is configured with an OSPF dead interval of 40 seconds. How long after the last Hello from a neighbor does the router declare the neighbor down?

  1. After 10 seconds without receiving a Hello from that neighbor
  2. Immediately upon the next missed Hello
  3. After 120 seconds without receiving a Hello from that neighbor
  4. After 40 seconds without receiving a Hello from that neighbor Correct answer.

Explanation

The dead interval is the period during which no Hello is received before the neighbor is declared down. With a dead interval of 40 seconds, the neighbor is declared down after 40 seconds of silence.

Question 26OSPF Fundamentals, Packet Types and Neighbor State Machine

Which OSPF packet type is used to carry individual Link State Advertisements between routers during database synchronization?

  1. Link State Update (Type 4) Correct answer.
  2. Database Description (Type 2)
  3. Hello (Type 1)
  4. Link State Request (Type 3)

Explanation

The Link State Update packet (Type 4) carries one or more complete LSAs and is used both for initial database exchange and for flooding new or changed LSAs. Database Description packets only summarize LSA headers, Link State Request packets ask for specific LSAs, and Hello packets discover and maintain neighbors.

Question 27OSPF Fundamentals, Packet Types and Neighbor State Machine

A router is stuck in the ExStart state with a neighbor on a point-to-point link. Which condition most directly prevents progression to the Exchange state?

  1. The two routers have different MTU values on the interface, causing the Database Description packet exchange to fail. Correct answer.
  2. The neighbor's Hello packets are being received but the Router ID in the neighbor field does not match.
  3. The interface is configured as passive, so no Hellos are sent.
  4. The area IDs on the two interfaces do not match.

Explanation

In ExStart the routers negotiate master/slave roles using Database Description packets with the Init bit set. If the MTU values on the two interfaces differ, the routers cannot exchange these packets successfully and the adjacency remains in ExStart. Passive interfaces and area mismatches prevent the adjacency from forming at all, so the router would not reach ExStart.

Question 28OSPF Fundamentals, Packet Types and Neighbor State Machine

Two routers are connected over a broadcast segment and both are DR-eligible. After the DR and BDR are elected, a third router with a higher OSPF priority joins the segment. What is the effect on the existing DR and BDR?

  1. The existing DR and BDR remain unchanged until the DR fails or the interface is reset. Correct answer.
  2. The new router immediately becomes DR and the former DR becomes BDR.
  3. The new router becomes DR and the former BDR becomes BDR.
  4. The new router becomes BDR and the former BDR becomes DR.

Explanation

OSPF DR election is not preemptive: once a DR and BDR are elected, a newly arriving router with a higher priority does not displace them. The new router becomes a DROther until the DR fails or the OSPF process on the DR is restarted. This behavior prevents unnecessary LSA flooding caused by frequent DR changes.

Question 29Link-State Database, LSAs and Area Types

In an OSPFv2 router's link-state database, which LSA type is originated by every router to describe its own interfaces, neighbors and link costs within a single area?

  1. Type 3 Summary LSA
  2. Type 1 Router LSA Correct answer.
  3. Type 2 Network LSA
  4. Type 5 AS External LSA

Explanation

The Type 1 Router LSA is generated by each router and flooded only inside its area. It lists the router's links with cost, type and the neighbor router IDs reachable on each link, forming the per-area topology graph used by SPF.

Question 30Link-State Database, LSAs and Area Types

Which router generates a Type 2 Network LSA on a multi-access OSPF segment?

  1. The Area Border Router with the highest router ID
  2. The Autonomous System Boundary Router
  3. The Designated Router of that segment Correct answer.
  4. Every router on the segment

Explanation

On broadcast and NBMA networks the elected Designated Router originates the Type 2 Network LSA. It advertises the subnet mask and the list of routers currently attached to the segment, allowing SPF to model the shared link as a pseudo-node.

Question 31Link-State Database, LSAs and Area Types

Which LSA type carries inter-area prefix information between areas in an OSPFv2 domain?

  1. Type 7
  2. Type 4
  3. Type 9
  4. Type 3 Correct answer.

Explanation

Type 3 Summary LSAs are originated by Area Border Routers to advertise prefixes from one area into another. They describe reachability and cost rather than full topology, which is why inter-area routes are computed from the ABR rather than by end-to-end SPF.

Question 32Link-State Database, LSAs and Area Types

Which LSA type is flooded only within a not-so-stubby area to carry external routing information that the ABR later translates?

  1. Type 7 Correct answer.
  2. Type 3
  3. Type 5
  4. Type 2

Explanation

NSSAs allow external information to be carried inside the area using Type 7 NSSA External LSAs, which stay within the NSSA. The ABR translates selected Type 7 LSAs into Type 5 LSAs for the rest of the domain.

Question 33Link-State Database, LSAs and Area Types

What is the defining characteristic of a totally stubby area in OSPF?

  1. It allows Type 7 LSAs and performs translation at the ABR
  2. It blocks only Type 5 LSAs while still accepting Type 3 and Type 4 LSAs
  3. It permits external LSAs but suppresses summary LSAs
  4. It blocks Type 3, Type 4 and Type 5 LSAs, relying on a default route injected by the ABR Correct answer.

Explanation

A totally stubby area additionally suppresses Type 3 and Type 4 summary LSAs beyond the normal stub behavior, so internal routers know only intra-area routes plus a default route from the ABR. This shrinks the LSDB and SPF workload on access routers.

Question 34Link-State Database, LSAs and Area Types

Which two fields together uniquely identify an LSA within an OSPF link-state database?

  1. Options field and Link State Type
  2. Advertising Router and Checksum
  3. Link State ID and Advertising Router Correct answer.
  4. Sequence Number and Age

Explanation

An LSA instance is keyed by the combination of Link State Type, Link State ID and Advertising Router. Among the offered pairs, Link State ID plus Advertising Router is the identifying combination used when comparing and storing LSAs.

Question 35Link-State Database, LSAs and Area Types

What does the Link State Age field in an OSPF LSA header represent?

  1. The interval between consecutive SPF runs
  2. The time since the LSA was originated or last refreshed Correct answer.
  3. The time since the LSA was last flooded
  4. The remaining time before the LSA is purged from the domain

Explanation

The age counts seconds since the LSA was originated or last re-originated, and it is incremented as the LSA sits in the database and while flooded. When it reaches MaxAge the LSA is flushed, and the originator refreshes it well before that point.

Question 36Link-State Database, LSAs and Area Types

Which area type is the only one in which Type 5 AS External LSAs may be flooded?

  1. Not-so-stubby area
  2. Normal (non-stub) area Correct answer.
  3. Stub area
  4. Totally stubby area

Explanation

Type 5 LSAs are flooded throughout the domain but are blocked at stub, totally stubby and NSSA boundaries. Therefore only normal, non-stub areas carry Type 5 LSAs in their databases.

Question 37Link-State Database, LSAs and Area Types

A router's Type 1 LSA shows two links to the same transit network, one as a transit link and one as a stub link. What does the stub link most likely represent?

  1. The virtual link to the backbone
  2. The subnet address of the transit segment itself Correct answer.
  3. The loopback address of the DR
  4. A redistributed external prefix

Explanation

When a router is attached to a transit network it advertises a transit link pointing at the DR's Network LSA plus a stub link for the segment's own IP subnet. The stub link makes the directly connected prefix reachable even though the topology is represented by the pseudo-node.

Question 38Link-State Database, LSAs and Area Types

An ABR receives a Type 3 Summary LSA for a prefix that also exists as an intra-area route in the receiving area. How does OSPF route selection resolve this?

  1. The inter-area route is preferred because it is more specific
  2. The intra-area route is preferred over the inter-area route Correct answer.
  3. The two routes are load-balanced across the ABR
  4. The route with the lower forwarding address is selected

Explanation

OSPF applies a strict preference order of intra-area, then inter-area, then external routes, regardless of metric comparison across those categories. A prefix known intra-area therefore wins over the same prefix learned through a Type 3 LSA.

Question 39Link-State Database, LSAs and Area Types

Which LSA does an ABR generate so that routers inside a stub area can reach an ASBR located in another area?

  1. Type 7 NSSA External LSA
  2. Type 4 Summary ASBR LSA Correct answer.
  3. Type 3 Summary LSA
  4. Type 5 AS External LSA

Explanation

The Type 4 Summary ASBR LSA tells routers in other areas how to reach the ASBR itself, which is needed before any Type 5 external route can be resolved. Stub areas block Type 5 but still rely on Type 4 information when external reachability is summarized by a default.

Question 40Link-State Database, LSAs and Area Types

A network engineer configures an area as NSSA but the ABR is not translating Type 7 LSAs into Type 5. Which condition should be checked first?

  1. The P-bit in the Type 7 LSA Options field and the translator role election Correct answer.
  2. The hello interval mismatch between the ABR and internal routers
  3. The MaxAge of the Type 3 LSAs in the backbone
  4. The DR priority on the NSSA transit segment

Explanation

Translation occurs only for Type 7 LSAs whose P-bit is set and only on the elected translator, normally the ABR with the highest router ID. If the P-bit is clear or translator election has not converged, no Type 5 LSA is produced.

Question 41Link-State Database, LSAs and Area Types

What is the effect of the no-summary keyword when configuring a stub area on an ABR?

  1. It suppresses the default route normally sent into the stub
  2. It allows Type 7 LSAs to be injected into the area
  3. It blocks Type 5 LSAs only, leaving summaries intact
  4. It blocks Type 3 and Type 4 LSAs in addition to Type 5, making the area totally stubby Correct answer.

Explanation

The no-summary option turns a stub area into a totally stubby area by filtering Type 3 and Type 4 LSAs at the ABR. Internal routers then hold only intra-area routes plus the default route advertised by the ABR.

Question 42Link-State Database, LSAs and Area Types

Two routers exchange database description packets and one sends a link-state request for an LSA. Which field in the LSA header does the requesting router use to specify the instance it wants?

  1. The Link State ID, Advertising Router and the sequence number seen in the summary Correct answer.
  2. The options field
  3. The age field only
  4. The checksum alone

Explanation

A link-state request identifies an LSA by type, Link State ID and Advertising Router, and the requester expects the current instance. The database description summary supplies those identifiers so the neighbor can send the matching full LSA.

Question 43Link-State Database, LSAs and Area Types

A router receives two instances of the same LSA, one with sequence number 0x80000005 and one with 0x80000006. Which instance is considered more recent?

  1. 0x80000006, because a higher sequence number is more recent Correct answer.
  2. 0x80000005, because a lower sequence is newer
  3. The one with the larger age is more recent
  4. They are equal and the checksum decides

Explanation

LSA sequence numbers are signed 32-bit values starting at 0x80000001, and a numerically higher sequence indicates a newer instance. Checksum and age are consulted only when sequence numbers are identical.

Question 44Link-State Database, LSAs and Area Types

Which statement describes how Type 3 LSAs affect the SPF computation inside a receiving area?

  1. They are converted into Type 1 LSAs by the receiving ABR
  2. They are discarded because SPF only uses Type 1 and Type 2 LSAs
  3. They are inserted as full router nodes and expanded by SPF
  4. They are treated as leaf prefixes attached to the advertising ABR, without expanding remote topology Correct answer.

Explanation

Summary LSAs do not describe remote topology, so SPF treats them as stub networks attached to the ABR that advertised them. The cost to the prefix is the cost to reach the ABR plus the metric carried in the LSA.

Question 45Link-State Database, LSAs and Area Types

Why must all routers in an area agree on the stub flag in the E-bit of the Options field?

  1. Because the E-bit controls the hello interval used on the segment
  2. Because the E-bit selects the DR on broadcast segments
  3. Because the E-bit determines the LSA refresh interval
  4. Because a mismatch prevents adjacency formation and keeps the LSDB consistent across the area Correct answer.

Explanation

The E-bit indicates whether the area can carry external LSAs, and routers that disagree will not form a full adjacency. This enforcement keeps every router's view of the area's LSA filtering consistent.

Question 46Link-State Database, LSAs and Area Types

A totally stubby area router needs to reach a destination outside the OSPF domain. What does its routing table contain for that destination?

  1. No route, because external reachability is impossible from a totally stubby area
  2. A default inter-area route learned from the ABR via a Type 3 LSA Correct answer.
  3. A Type 5 external route learned directly from the ASBR
  4. A Type 7 route translated locally

Explanation

The ABR injects a default route into the totally stubby area using a Type 3 Summary LSA with Link State ID 0.0.0.0. Internal routers forward unknown destinations to the ABR, which then applies its own more detailed routing.

Question 47Link-State Database, LSAs and Area Types

Which condition causes a router to flush an LSA by re-originating it with age set to MaxAge?

  1. The neighbor state has moved from Full to Two-Way
  2. The LSA has reached half of its refresh interval
  3. The interface or prefix described by the LSA has been removed from the router Correct answer.
  4. The LSA has been replaced by a newer instance from the same originator

Explanation

When the topology described by an LSA disappears, the originator prematurely ages the LSA to MaxAge so that other routers remove it promptly. The LSA is then flooded with MaxAge and eventually purged.

Question 48Link-State Database, LSAs and Area Types

An engineer observes that a summary route is present in the LSDB of the backbone but absent in a normal non-backbone area. Which explanation is most consistent with OSPF area rules?

  1. The prefix is carried in a Type 2 LSA that cannot cross an ABR
  2. The prefix originated in a stub area and cannot be summarized into other areas
  3. Type 3 LSAs are never flooded from the backbone into non-backbone areas
  4. The ABR did not generate a Type 3 LSA into that area, possibly due to filtering or a missing area range Correct answer.

Explanation

Inter-area prefixes reach a non-backbone area only when an attached ABR originates a Type 3 LSA for them. Absence usually points to summary-address filtering, a missing area range, or the ABR not having the prefix in its own table.

Question 49Link-State Database, LSAs and Area Types

A router in area 1 has two equal-cost paths to an ASBR, one intra-area and one inter-area. It also has two Type 5 LSAs for the same external prefix, one with metric type 1 and one with metric type 2. Which route does OSPF install?

  1. The E2 route via the intra-area path
  2. The E2 route via the inter-area path
  3. The E1 route via the intra-area path Correct answer.
  4. The E1 route via the inter-area path

Explanation

External type 1 is preferred over external type 2 when both describe the same prefix, and the E1 metric includes the internal cost to the ASBR. Among paths to the ASBR, the intra-area path is preferred, so the E1 route via the intra-area path is installed.

Question 50Link-State Database, LSAs and Area Types

An ABR has a Type 3 LSA for 10.0.0.0/8 with metric 20 and a Type 3 LSA for 10.1.0.0/16 with metric 50, both from different source areas. Which prefix does OSPF use for a packet destined to 10.1.5.5?

  1. 10.1.0.0/16, because the longest match is preferred before metric comparison Correct answer.
  2. Both, because they are equal-cost
  3. Neither, because Type 3 LSAs cannot overlap
  4. 10.0.0.0/8, because its metric is lower

Explanation

OSPF first selects the longest matching prefix among candidate routes, and only then compares metrics between routes for the same prefix. The /16 is therefore chosen for 10.1.5.5 even though its advertised metric is higher.