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Nokia 4A0-113 OSPF study materials
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Nokia 4A0-113 OSPF practice questions

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Question 1OSPF Fundamentals and Protocol Operation

Which protocol number is carried in the IPv4 header of an OSPFv2 packet?

  1. 89 Correct answer.
  2. 88
  3. 112
  4. 103

Explanation

OSPFv2 for IPv4 is identified by IP protocol number 89, and routers use this value to hand received packets to the OSPF process. The other values belong to EIGRP (88), VRRP (112) and PIM (103), so they never identify OSPF traffic.

Question 2OSPF Fundamentals and Protocol Operation

Which transport mechanism does OSPFv2 use to deliver its messages to neighboring routers?

  1. TCP port 2604
  2. IP protocol 89 directly over IP Correct answer.
  3. TCP port 89
  4. UDP port 89

Explanation

OSPF runs directly over IP with protocol number 89 and has no TCP or UDP header, which is why it must implement its own reliability, acknowledgments and retransmissions. TCP port 2604 is used by Zebra/Quagga management, and no OSPF variant listens on a UDP port.

Question 3OSPF Fundamentals and Protocol Operation

Which OSPF packet type is used to confirm that a neighbor received a link-state update reliably?

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

Explanation

Link State Acknowledgment packets explicitly acknowledge received LSAs, providing the reliability that OSPF needs because it runs over unreliable IP. Hellos maintain adjacency, Database Description packets summarize the database, and Link State Request packets ask for specific LSAs.

Question 4OSPF Fundamentals and Protocol Operation

Which field of the OSPF hello packet must match on both routers for a neighbor relationship to form?

  1. Router priority
  2. Hello interval Correct answer.
  3. Router ID
  4. Dead interval

Explanation

Hello and dead intervals, area ID, authentication parameters and stub flags must match before two routers will accept each other's hellos. Router priority and router ID may differ, because priority only influences DR election and the router ID only identifies the router.

Question 5OSPF Fundamentals and Protocol Operation

What is the default hello interval on an OSPF broadcast network?

  1. 30 seconds
  2. 5 seconds
  3. 10 seconds Correct answer.
  4. 40 seconds

Explanation

On broadcast and point-to-point interfaces the default hello interval is 10 seconds and the dead interval is 40 seconds. The 30-second hello applies to NBMA and point-to-multipoint networks, where neighbors are not discovered automatically.

Question 6OSPF Fundamentals and Protocol Operation

Which statement describes the OSPF router ID selection when no router-id command is configured?

  1. It is the highest address on the interface that first receives a hello.
  2. It is the highest loopback address, or the highest active interface address if no loopback exists. Correct answer.
  3. It is the MAC address of the lowest-numbered interface.
  4. It is always the lowest IP address on any interface.

Explanation

Without an explicit router-id, OSPF prefers the highest IP address among loopback interfaces and falls back to the highest active physical interface address. The ID is fixed once the process selects it, so later interface changes do not alter it until OSPF is restarted or the ID is cleared.

Question 7OSPF Fundamentals and Protocol Operation

Which OSPF packet type carries the complete list of LSAs that a router wants its neighbor to send?

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

Explanation

After database exchange, a router compares the neighbor's Database Description summary with its own database and sends Link State Request packets listing the LSAs it is missing or has an older instance of. The neighbor answers with Link State Update packets containing those LSAs.

Question 8OSPF Fundamentals and Protocol Operation

Which two routers become adjacent on a broadcast multi-access segment when DR election is complete?

  1. Every router forms a full adjacency with every other router.
  2. DROthers form full adjacencies only among themselves.
  3. Only the DR and BDR form full adjacencies with all other routers. Correct answer.
  4. Only the DR forms full adjacencies; the BDR stays in 2-Way.

Explanation

On a broadcast segment all routers form full adjacencies with the DR and BDR, while DROthers remain in 2-Way state with each other. This reduces the number of adjacencies from n(n-1)/2 to roughly 2n and limits LSA flooding.

Question 9OSPF Fundamentals and Protocol Operation

A router interface is configured with ip ospf priority 0. What is the effect on a broadcast segment?

  1. The router can never become DR or BDR on that interface. Correct answer.
  2. The router is excluded from hello processing.
  3. The router becomes the DR immediately.
  4. The router becomes the BDR by default.

Explanation

Priority 0 removes the interface from DR and BDR election, although the router still forms full adjacencies with the DR and BDR. This is commonly used on routers that should never carry the segment's database synchronization role.

Question 10OSPF Fundamentals and Protocol Operation

Two routers on a broadcast segment are tied with the same DR priority. Which value breaks the tie?

  1. The lower router ID
  2. The higher router ID Correct answer.
  3. The lower MAC address
  4. The higher interface IP address

Explanation

When priorities are equal, the router with the highest router ID wins the DR election and the second highest becomes BDR. Router ID, not interface address, is the tiebreaker, and the election is not preemptive once the DR is established.

Question 11OSPF Fundamentals and Protocol Operation

Which sequence correctly describes the states a router passes through when forming an adjacency with a neighbor?

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

Explanation

The neighbor state machine moves from Down through Init and 2-Way, then negotiates master/slave in ExStart, exchanges database summaries in Exchange, requests missing LSAs in Loading, and reaches Full when databases are synchronized. Any deviation from this order indicates a problem such as an MTU mismatch or an LSA that never arrives.

Question 12OSPF Fundamentals and Protocol Operation

Two routers remain stuck in ExStart state. Which condition most likely explains this?

  1. A mismatch in the MTU of the two interfaces Correct answer.
  2. Different OSPF areas on the two interfaces
  3. A mismatch in the dead interval
  4. Duplicate router IDs on the segment

Explanation

ExStart is where the master/slave relationship is negotiated using Database Description packets, and an MTU mismatch causes the larger DBD packets to be dropped, leaving neighbors stuck there. Timer or area mismatches prevent the adjacency much earlier, so the routers would never reach ExStart.

Question 13OSPF Fundamentals and Protocol Operation

What does the OSPF neighbor state 2-Way indicate on a broadcast network?

  1. The routers have exchanged full databases and are adjacent.
  2. The router has received a DBD packet and is negotiating master/slave.
  3. The router is waiting for a hello from the neighbor.
  4. Bidirectional communication is confirmed, but the routers will not exchange LSAs unless one is DR or BDR. Correct answer.

Explanation

2-Way means each router has seen its own router ID in the neighbor's hello, proving two-way communication. On a broadcast segment DROthers intentionally remain in 2-Way with each other, so this state is normal rather than a fault.

Question 14OSPF Fundamentals and Protocol Operation

Which LSA type describes a router's own interfaces and is flooded within a single area only?

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

Explanation

Every router originates a Type 1 Router LSA listing its links, costs and neighbor relationships, and it is flooded only inside its area. Type 2 is generated by the DR, Type 3 by ABRs, and Type 5 by ASBRs to describe external destinations.

Question 15OSPF Fundamentals and Protocol Operation

Which router generates a Type 2 Network LSA?

  1. The DR of the segment Correct answer.
  2. Every router on the multi-access segment
  3. The ABR closest to the segment
  4. The ASBR that redistributes the segment

Explanation

The DR of a broadcast or NBMA segment originates the Type 2 Network LSA, which lists all routers attached to that segment including the DR itself. This represents the multi-access network as a pseudo-node in the topology so that the SPF tree stays accurate.

Question 16OSPF Fundamentals and Protocol Operation

What is the purpose of the OSPF cost metric?

  1. It counts the number of routers a packet must cross.
  2. It is derived from interface bandwidth and is accumulated along the path. Correct answer.
  3. It reflects the administrative distance of the route.
  4. It measures the round-trip delay to the destination.

Explanation

The default OSPF cost of an interface is the reference bandwidth divided by the interface bandwidth, and the cost of a route is the sum of outgoing interface costs along the path. Hop count, delay and administrative distance are not part of OSPF path selection.

Question 17OSPF Fundamentals and Protocol Operation

The reference bandwidth is 100 Mbps and an interface runs at 1 Gbps. What is the default OSPF cost of that interface?

  1. 1 Correct answer.
  2. 1000
  3. 100
  4. 10

Explanation

Cost equals reference bandwidth divided by interface bandwidth, so 100 Mbps divided by 1000 Mbps gives 0.1, which is rounded up to the minimum value of 1. Raising the reference bandwidth is required to differentiate links faster than the reference.

Question 18OSPF Fundamentals and Protocol Operation

Which condition causes two OSPF routers to fail to become neighbors because of an area mismatch?

  1. The interfaces are assigned to different OSPF areas. Correct answer.
  2. The interfaces have different router priorities.
  3. The interfaces have different IP subnet masks.
  4. The interfaces use different OSPF cost values.

Explanation

The area ID is carried in the hello and must match, because routers in different areas do not share a link-state database and must communicate through an ABR. Priority, cost and mask differences do not by themselves prevent hello processing.

Question 19OSPF Fundamentals and Protocol Operation

A router with a single interface in area 0 and another interface in area 1 is best described as which role?

  1. Area Border Router Correct answer.
  2. Backbone router
  3. Internal router
  4. Autonomous System Boundary Router

Explanation

A router with interfaces in more than one area is an ABR, and it maintains a separate link-state database for each area while summarizing prefixes between them. An ASBR redistributes external routes, and an internal router has all interfaces in one area.

Question 20OSPF Fundamentals and Protocol Operation

Which LSA type is originated by an ASBR to advertise routes redistributed from outside the OSPF domain?

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

Explanation

Type 5 External LSAs carry redistributed external prefixes and are flooded throughout the OSPF domain, with the forwarding address and external metric in the body. Type 4 merely tells other areas where the ASBR is, and Type 7 is confined to NSSA areas.

Question 21OSPF Fundamentals and Protocol Operation

How does OSPF prevent routing loops between areas?

  1. Area IDs are included in every data packet and checked at each hop.
  2. Every router runs a full SPF calculation on the entire domain database.
  3. All inter-area traffic must pass through the backbone area 0, and Type 3 LSAs are only generated by ABRs. Correct answer.
  4. External routes are always preferred over inter-area routes.

Explanation

OSPF enforces a two-level hierarchy in which ABRs inject summarized Type 3 LSAs and all inter-area paths transit area 0, so distance-vector-like loop prevention applies between areas. Within an area, the synchronized link-state database and SPF guarantee loop-free paths.

Question 22OSPF Fundamentals and Protocol Operation

Which condition must be satisfied before two routers on a point-to-point link can become fully adjacent?

  1. Both must have identical OSPF cost values on the link.
  2. Both must be configured as ASBRs.
  3. Both must have the same router priority value.
  4. Both must agree on the network type and be in the same area with matching hello parameters. Correct answer.

Explanation

Adjacency requires matching area ID, hello and dead intervals, authentication and stub flags, plus compatible network types so that both sides expect the same adjacency behavior. Priority, ASBR status and interface cost do not gate adjacency formation.

Question 23OSPF Fundamentals and Protocol Operation

A network administrator observes that the DR on a segment has not changed even though a newly added router has a higher priority. What explains this?

  1. OSPF DR election is preemptive and should have changed immediately.
  2. The DR election is not preemptive; the new router only takes over after the current DR fails. Correct answer.
  3. The new router must have priority 0.
  4. The new router is in a different area.

Explanation

Once elected, the DR and BDR keep their roles until they fail or the OSPF process is restarted, so a later router with higher priority does not displace them. This non-preemptive behavior avoids constant re-election and repeated LSA flooding on stable segments.

Question 24OSPF Fundamentals and Protocol Operation

What information does a Type 3 Summary LSA carry when an ABR floods it into another area?

  1. A destination prefix and the cost to reach it from the ABR Correct answer.
  2. The MAC address of the destination host
  3. The full Router LSA of every router in the source area
  4. The complete link-state database of area 0

Explanation

A Type 3 LSA advertises a network prefix together with the cost from the advertising ABR, hiding the source area's internal topology from the receiving area. Routers in the receiving area therefore compute paths to the ABR rather than to the original router.

Question 25OSPF Fundamentals and Protocol Operation

Which OSPF feature ensures that a router does not accept routing information from a neighbor until bidirectional communication is verified?

  1. The hello packet's neighbor list, which must contain the receiving router's router ID Correct answer.
  2. The authentication trailer in every hello
  3. The DBD sequence number exchange
  4. The LSA checksum verification

Explanation

A router moves from Init to 2-Way only after it sees its own router ID listed in a neighbor's hello, which proves the neighbor can hear it as well. This two-way check prevents one-way links from producing unusable adjacencies.

Question 26OSPF Fundamentals and Protocol Operation

Which field in the OSPF Hello packet must match between two routers before they can form a neighbor relationship on a common link?

  1. The link-state sequence number and the checksum
  2. The router priority and the router ID
  3. The Hello interval and the router dead interval Correct answer.
  4. The interface MTU and the maximum age

Explanation

OSPF Hello packets carry the Hello interval and the Router Dead interval, and both values must be identical on the two interfaces for the routers to accept each other as neighbors. Mismatched timers cause the neighbor to be rejected or to time out repeatedly. The other listed fields are either not carried in the Hello or not used as a matching condition.

Question 27OSPF Fundamentals and Protocol Operation

An OSPF router receives an LSA whose sequence number equals the one already stored in its link-state database. What does the router do with the newly received copy?

  1. It discards the received LSA and does not flood it further Correct answer.
  2. It deletes the existing LSA and requests a fresh copy
  3. It floods the LSA back out of every interface immediately
  4. It increments the sequence number and re-originates the LSA

Explanation

When a received LSA has the same sequence number, checksum, and age as the instance already in the database, it is considered a duplicate and is silently dropped without being flooded. Flooding or re-origination would only be triggered if the received instance were newer. This duplicate-detection rule keeps the flooding process from looping.

Question 28OSPF Fundamentals and Protocol Operation

A router has two equal-cost OSPF routes to the same destination, one learned through an intra-area path and one through an inter-area Type 3 LSA. How does OSPF prefer the paths?

  1. It installs neither route until the costs are made unequal
  2. It installs only the intra-area route, because intra-area paths are preferred over inter-area paths Correct answer.
  3. It installs only the inter-area route, because summary LSAs are more specific
  4. It installs both paths and load-balances traffic across them

Explanation

OSPF ranks path types in a fixed preference order: intra-area, then inter-area, then external Type 1, then external Type 2. When an intra-area and an inter-area path to the same prefix have equal cost, the intra-area path wins and the inter-area path is not installed. Equal-cost load balancing happens only among paths of the same type.

Question 29OSPF Fundamentals and Protocol Operation

Which condition explains why an OSPF router keeps receiving LSAs with an unexpectedly high sequence number from a neighbor?

  1. The neighbor's interface has a lower OSPF cost than expected
  2. The neighbor's link-state database was restored from an old backup Correct answer.
  3. The neighbor uses a different Hello interval on the link
  4. The neighbor has a different reference bandwidth configured

Explanation

Sequence numbers for an LSA are tracked by the originating router, so a restored or stale database can make a router re-originate an LSA with a sequence number far ahead of what the network currently uses. Neighbors then accept the higher sequence number as newer. Reference bandwidth, Hello interval, and interface cost do not affect LSA sequence numbering.

Question 30OSPF Fundamentals and Protocol Operation

An ABR has an active Type 3 Summary LSA for a prefix in area 1, and a new more-specific intra-area route to the same prefix appears in area 0. Which statement describes the ABR's behavior?

  1. It converts the Type 3 LSA into a Type 5 AS-external LSA
  2. It withdraws the prefix from area 0 until area 1 reconverges
  3. It keeps the Type 3 LSA and prefers the area 1 summary path for all traffic
  4. It suppresses the Type 3 LSA for area 1 and advertises the area 0 prefix as an intra-area route Correct answer.

Explanation

An ABR does not advertise a summary LSA into an area for a prefix that it already knows as an intra-area route in that area's own topology, and it stops advertising a summary when the prefix becomes reachable intra-area. The intra-area route is preferred, so the summary is suppressed rather than kept. Type 5 LSAs are generated only by an ASBR for redistributed external routes.

Question 31OSPF Neighbor Adjacencies and Network Types

Which OSPF packet type is used to discover neighbors and confirm bidirectional reachability before any database exchange begins?

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

Explanation

Hello packets are sent periodically on every OSPF-enabled interface; they carry the router ID, area ID, hello/dead intervals, and a list of seen neighbors. Two routers only progress beyond Init once each sees its own router ID in the other's Hello, proving bidirectional communication.

Question 32OSPF Neighbor Adjacencies and Network Types

Two routers are cabled directly together on an Ethernet segment but remain stuck in the Init state. What is the most likely cause?

  1. The authentication keys match on both sides
  2. The MTU values differ between the interfaces
  3. Hello packets are being received in one direction only Correct answer.
  4. The OSPF network type is set to point-to-point

Explanation

The Init state means a router has received a Hello from the neighbor but has not yet seen its own router ID listed in that neighbor's Hello. That happens when Hellos arrive in only one direction, for example due to a one-way filter or a broken transmit path, so the two-way handshake never completes.

Question 33OSPF Neighbor Adjacencies and Network Types

Which fields must match between two Hello packets for an adjacency to form on a broadcast segment?

  1. Link state database sequence number
  2. Hello interval, dead interval, area ID, and authentication Correct answer.
  3. Router priority and router ID
  4. Interface IP address and subnet mask only

Explanation

For Hello packets to be accepted and an adjacency to progress, the area ID, hello interval, dead interval, authentication type and credentials, and stub area flag must all agree. Router priority and router ID may differ because they only influence DR/BDR election and identity.

Question 34OSPF Neighbor Adjacencies and Network Types

On a broadcast multi-access network, which routers form a full adjacency with the DR and BDR?

  1. Only routers configured with the highest priorities
  2. Only the DR and BDR with each other
  3. All routers on the segment
  4. All non-DR/BDR routers with the DR and BDR, but not with each other Correct answer.

Explanation

On broadcast and NBMA networks, OSPF builds full adjacencies between each DROther and both the DR and the BDR. DROther routers remain in the 2-Way state with one another, which reduces the number of adjacencies and LSA flooding on the segment.

Question 35OSPF Neighbor Adjacencies and Network Types

A router interface is configured with ip ospf network point-to-multipoint. How does OSPF treat neighbor adjacencies on that interface?

  1. Neighbors remain in 2-Way until manually cleared
  2. Only one neighbor can be adjacent at a time
  3. Each neighbor forms a point-to-point adjacency and no DR/BDR is elected Correct answer.
  4. A DR and BDR are elected as on a broadcast segment

Explanation

The point-to-multipoint network type models the interface as a collection of point-to-point links, so a full adjacency is formed with every reachable neighbor and no DR/BDR election occurs. It also treats the interface as a single IP subnet with host routes advertised for each neighbor.

Question 36OSPF Neighbor Adjacencies and Network Types

Which condition causes two OSPF routers on a point-to-point link to fail during the Exchange state?

  1. Different router IDs
  2. Different interface descriptions
  3. Mismatched router priorities
  4. Mismatched MTU values on the interfaces Correct answer.

Explanation

During the Exchange state, each router includes its interface MTU in Database Description packets. If the MTU values differ, the receiving router rejects the DD packet and the adjacency stalls in ExStart or Exchange; this is why MTU mismatches are a classic cause of stuck adjacencies.

Question 37OSPF Neighbor Adjacencies and Network Types

What is the purpose of the Master/Slave negotiation during the ExStart state?

  1. To determine which router becomes the BDR
  2. To select the router that will originate the network LSA
  3. To decide which router sends Database Description packets first and controls the sequence number Correct answer.
  4. To elect the DR for the segment

Explanation

In ExStart the two neighbors compare router IDs and the higher ID becomes Master, which initiates DD exchange and controls the DD sequence number. This ensures an orderly, collision-free database summary exchange between the pair.

Question 38OSPF Neighbor Adjacencies and Network Types

Two routers on a broadcast LAN show neighbor state 2-Way with each other. Which explanation fits?

  1. They are both DROther routers on the segment Correct answer.
  2. Their authentication keys are mismatched
  3. Their hello intervals are mismatched
  4. Their area IDs are mismatched

Explanation

On a broadcast segment, DROther routers deliberately stop at 2-Way with each other because they only need full adjacencies with the DR and BDR. This is expected and stable behavior, not a fault, as long as both are fully adjacent with the DR and BDR.

Question 39OSPF Neighbor Adjacencies and Network Types

Which OSPF network type does not elect a DR or BDR and forms adjacencies with all neighbors on the segment?

  1. Point-to-multipoint non-broadcast
  2. Non-broadcast multi-access
  3. Broadcast
  4. Point-to-point Correct answer.

Explanation

Point-to-point interfaces have exactly one possible neighbor and therefore never elect a DR or BDR. The point-to-multipoint types also skip election, but among the choices given only point-to-point is the classic single-neighbor case; the question asks for the type that forms an adjacency with its one peer directly.

Question 40OSPF Neighbor Adjacencies and Network Types

A router is configured with ip ospf network non-broadcast on an Ethernet interface. What must be configured for neighbors to become adjacent?

  1. Nothing extra; multicast Hellos are still sent
  2. The neighbor statements identifying each peer address Correct answer.
  3. A higher router priority than all peers
  4. The interface must be shut down and restarted

Explanation

The non-broadcast network type disables multicast Hello transmission and requires unicast Hellos, so each neighbor must be statically identified with a neighbor statement. This is common on NBMA media such as Frame Relay or when multicast is filtered on Ethernet.

Question 41OSPF Neighbor Adjacencies and Network Types

Which router becomes the DR when all routers on a broadcast segment have the default priority and the election has just started?

  1. The router with the lowest router ID
  2. The router with the highest router ID Correct answer.
  3. The router with the highest interface IP address
  4. The router that has been up the longest regardless of ID

Explanation

When priorities are equal, the DR election is broken by the highest router ID, which is the highest loopback or interface IP used as the OSPF identifier. Uptime does not matter in OSPF DR election, unlike some other protocols.

Question 42OSPF Neighbor Adjacencies and Network Types

A new router with priority 200 joins a broadcast segment where a DR and BDR are already established. What happens immediately?

  1. It remains a DROther until the existing DR fails or is cleared Correct answer.
  2. It forces a full re-election and all adjacencies reset
  3. It becomes the DR and the old DR becomes BDR
  4. It becomes the BDR and the old BDR becomes DROther

Explanation

OSPF DR election is not preemptive: a newly arriving router with a higher priority does not displace an existing DR. The new router stays a DROther and only takes over if the DR fails or the OSPF process is cleared.

Question 43OSPF Neighbor Adjacencies and Network Types

What is the default OSPF hello interval on a broadcast Ethernet interface?

  1. 1 second
  2. 10 seconds Correct answer.
  3. 30 seconds
  4. 5 seconds

Explanation

On broadcast and point-to-point interfaces the default hello interval is 10 seconds and the dead interval is 40 seconds. These defaults are exchanged in Hello packets and must match between neighbors.

Question 44OSPF Neighbor Adjacencies and Network Types

Two neighbors show mismatched dead intervals. What is the direct result?

  1. The routers fall back to point-to-point mode
  2. The adjacency forms but floods slowly
  3. The DR election is skipped
  4. The Hello packets are rejected and no adjacency forms Correct answer.

Explanation

The dead interval is one of the Hello parameters that must match exactly. If it differs, the receiving router discards the Hello, so the neighbor is never discovered and no adjacency can form.

Question 45OSPF Neighbor Adjacencies and Network Types

Which state indicates that a router has received a Hello from a neighbor and has seen its own router ID in that neighbor's Hello?

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

Explanation

2-Way means bidirectional communication is confirmed because each router sees its own router ID listed in the other's Hello. Init means a Hello was received but the local router ID was not present, so two-way communication is not yet proven.

Question 46OSPF Neighbor Adjacencies and Network Types

In which neighbor state has the link state database synchronization fully completed?

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

Explanation

Full means the routers have completed database exchange and their link state databases are synchronized. Loading is the stage where Link State Requests are still outstanding, and Exchange is where DD packets are being swapped.

Question 47OSPF Neighbor Adjacencies and Network Types

A point-to-point link between two routers shows neighbor state ExStart and never progresses. Which cause is consistent with this symptom?

  1. A mismatched authentication key
  2. Duplicate router IDs on the two routers Correct answer.
  3. A hello interval mismatch
  4. A mismatched area ID

Explanation

Duplicate router IDs cause the Master/Slave negotiation in ExStart to fail because each router sees the same ID and neither can reliably win the election. Area, authentication, and timer mismatches prevent Hellos from being accepted, so the neighbor would never reach ExStart.

Question 48OSPF Neighbor Adjacencies and Network Types

Which two routers on a broadcast segment will remain in 2-Way with each other under normal operation?

  1. The DR and BDR
  2. The BDR and a DROther
  3. The DR and a DROther
  4. Two DROther routers Correct answer.

Explanation

DROther routers only maintain full adjacencies with the DR and BDR; with each other they stay in 2-Way. This design reduces the number of full adjacencies on a multi-access segment from n(n-1)/2 to roughly 2n.

Question 49OSPF Neighbor Adjacencies and Network Types

Which OSPF network type requires manual neighbor configuration and elects a DR and BDR?

  1. Point-to-multipoint
  2. Broadcast
  3. Non-broadcast multi-access Correct answer.
  4. Point-to-point

Explanation

NBMA networks such as Frame Relay do not support multicast, so neighbors must be configured manually with neighbor statements, and a DR/BDR is still elected. Broadcast uses multicast Hellos, while point-to-point and point-to-multipoint skip DR election.

Question 50OSPF Neighbor Adjacencies and Network Types

A router on an NBMA hub-and-spoke network is configured as point-to-multipoint. How are Hellos sent to the spokes?

  1. As anycast to the DR address
  2. As multicast to 224.0.0.5
  3. As unicast to each configured or discovered neighbor Correct answer.
  4. As broadcast to 255.255.255.255

Explanation

Point-to-multipoint interfaces send Hellos as unicast to each neighbor because the underlying medium may not support multicast. This allows spokes that cannot reach each other directly to still form adjacencies with the hub.