JNCIS-ENT OSPF Practice Test 3 — 15 Questions

Question 1

Which command allows an administrator to view the OSPF link-state database (LSDB) summary on a Junos device?

Accepted Answer

show ospf database. The command 'show ospf database' displays the OSPF link-state database (LSDB) summary, including all LSAs (Router, Network, Summary, External) and their details. This is the correct command to view the LSDB on a Junos device.

Answer

show ospf neighbor

Answer

show route protocol ospf

Answer

show ospf interface

Question 2

While reviewing OSPF logs on a Junos device, you notice frequent adjacency flaps. You suspect a mismatch in the OSPF timer configurations. Which two timers must match identically on adjacent OSPF interfaces for an adjacency to form?

Accepted Answer

Hello interval and Dead interval. OSPF Hello and Dead intervals are critical parameters that must match identically on connected interfaces; otherwise, hello packets are rejected or dead timers expire prematurely.

Answer

Reference bandwidth and Cost metric

Answer

SPF delay timer and Hold timer

Answer

Retransmit interval and Poll interval

Question 3

An OSPF area contains routers running Junos OS where external routes must be imported into the OSPF domain from a BGP core. However, the area must not accept external AS-external LSAs (Type 5) from other areas, but must allow routers within this area to originate external routes themselves. Which area type fulfills this requirement?

Accepted Answer

Not-So-Stubby Area (NSSA). A Not-So-Stubby Area (NSSA) is designed to allow external routes to be imported into the OSPF domain from a non-OSPF protocol (like BGP) while preventing Type 5 AS-external LSAs from other areas. NSSAs use Type 7 LSAs for external routes originated within the area, which are translated to Type 5 at the NSSA ABR. This fulfills the requirement: the area does not accept Type 5 LSAs from other areas, but routers within can originate external routes.

Answer

Backbone Area (Area 0)

Answer

Totally Stubby Area

Answer

Standard Stub Area

Question 4

You are troubleshooting an OSPF adjacency issue between two Juniper EX Series switches. One switch logs an error stating 'OSPF packet received with mismatched area ID'. Where in the Junos CLI should you check and correct this parameter?

Accepted Answer

[edit protocols ospf area interface ]. The OSPF area ID is configured per interface under '[edit protocols ospf area interface ]'. If two routers have mismatched area IDs on the same link, the adjacency will not form and the 'mismatched area ID' error is logged. Correcting the area ID on the interface under the correct area hierarchy resolves the issue.

Answer

[edit system services ospf]

Answer

[edit protocols ospf area <area-id> stub]

Answer

[edit routing-options router-id <ip-address>]

Question 5

Which OSPF packet type is responsible for discovering neighbors and maintaining adjacencies on a Junos device?

Accepted Answer

Hello (Type 1). OSPF Hello packets (Type 1) are responsible for neighbor discovery and maintaining adjacencies. They are sent periodically on OSPF-enabled interfaces, and they carry parameters like router ID, area ID, hello/dead intervals, and neighbor lists. If Hellos stop arriving within the dead interval, the adjacency is torn down.

Answer

Link-State Update (Type 4)

Answer

Link-State Request (Type 3)

Answer

Database Description (Type 2)

Question 6

An administrator is configuring OSPF on a Junos device and notices that two routers on a broadcast segment are stuck in the 2-Way state. What is the most likely cause of this behavior on a broadcast network?

Accepted Answer

The routers are neither the DR nor the BDR on a multi-access network.. On a broadcast multi-access network, OSPF routers that are neither the DR nor the BDR form Full adjacencies only with the DR and BDR, and remain in 2-Way with each other. This is normal OSPF behavior designed to reduce LSA flooding, so two routers stuck in 2-Way are most likely both DROthers.

Answer

There is a mismatch in the OSPF hello interval configuration.

Answer

Authentication keys differ between the neighboring interfaces.

Answer

The area type is incorrectly configured as a totally stubby area.

Question 7

Which LSA type is originated by a Designated Router (DR) on a broadcast or NBMA network to describe all the routers attached to that multi-access segment?

Accepted Answer

Type 2 (Network LSA). The Type 2 Network LSA is originated by the Designated Router (DR) on a broadcast or NBMA multi-access segment and lists all routers that are fully adjacent to the DR on that segment, including the DR itself. It describes the multi-access network as a pseudo-node, allowing other routers to compute the topology without needing full-mesh adjacencies. The Network LSA's Link State ID is the DR's interface IP address on that segment.

Answer

Type 3 (Summary LSA)

Answer

Type 4 (ASBR Summary LSA)

Answer

Type 1 (Router LSA)

Question 8

What type of LSA is generated by an OSPF Area Border Router (ABR) to advertise networks from one area to another area?

Accepted Answer

Type 3 (Summary LSA). Type 3 Summary LSAs are generated by ABRs to advertise inter-area routes — networks from one area injected into another. The ABR originates a Type 3 LSA for each prefix it knows from a different area, allowing routers in the receiving area to reach those destinations without running SPF for the source area's topology.

Answer

Type 2 (Network LSA)

Answer

Type 1 (Router LSA)

Answer

Type 5 (AS-external LSA)

Question 9

You are configuring OSPF route summarization on an ABR in a Junos environment. You want to summarize routes from Area 1 into Area 0. Which Junos configuration hierarchy is used to define this summary?

Accepted Answer

set protocols ospf area 0.0.0.1 range 192.168.0.0/16. To summarize routes from Area 1 into Area 0 on an ABR in Junos, you configure the `range` statement under the OSPF area that contains the routes to be summarized. The correct hierarchy is `set protocols ospf area 0.0.0.1 range 192.168.0.0/16`, where 0.0.0.1 is Area 1. This instructs the ABR to advertise the summary route into other areas.

Answer

set routing-options static route 192.168.0.0/16 discard

Answer

set protocols ospf summary-prefix 192.168.0.0/16

Answer

set protocols ospf export summarize-policy

Question 10

When calculating OSPF path costs in a network containing high-speed links (such as 10Gbps and 40Gbps interfaces), what default reference bandwidth does Junos use, and what configuration adjustment is typically required to differentiate costs accurately?

Accepted Answer

100 Mbps default; reference bandwidth must be increased to accurately cost high-speed links.. Junos OSPF uses a default reference bandwidth of 100 Mbps, so any interface at 100 Mbps or faster gets a cost of 1, making 1G, 10G, and 40G links indistinguishable. To differentiate high-speed links, the reference bandwidth must be increased (e.g., to 10G or higher) using the reference-bandwidth statement under protocols ospf.

Answer

100 Gbps default; reference bandwidth must be decreased for legacy interfaces.

Answer

10 Gbps default; no adjustment is needed for modern networks.

Answer

1 Gbps default; interfaces above 1 Gbps are automatically assigned fractional costs.

Question 11

You are troubleshooting an OSPF network where an ASBR is injecting external routes. Routers in other areas need to reach this ASBR. Which LSA type provides the reachability information to the ASBR itself?

Accepted Answer

Type 4 (ASBR Summary LSA). Type 4 (ASBR Summary LSA) is generated by the ABR to advertise the reachability of an ASBR to routers in other areas. Since Type 5 LSAs are flooded throughout the OSPF domain but do not contain the ASBR's router ID reachability information, routers in non-backbone areas need a Type 4 LSA to know how to reach the ASBR that originated the Type 5 LSAs.

Answer

Type 2 (Network LSA)

Answer

Type 7 (NSSA External LSA)

Answer

Type 5 (AS-external LSA)

Question 12

Which OSPF packet type is used to acknowledge the receipt of Link-State Updates (LSUs) to ensure reliable flooding?

Accepted Answer

Link-State Acknowledgment (Type 5). OSPF uses Link-State Acknowledgment (Type 5) packets to explicitly acknowledge the receipt of Link-State Update (LSU) packets, ensuring reliable flooding of LSAs. When a router receives an LSU, it sends an LSAck to confirm receipt; if not acknowledged, the LSU is retransmitted. This mechanism guarantees that all routers in an area receive LSAs reliably.

Answer

Database Description (Type 2)

Answer

Link-State Request (Type 3)

Answer

Hello (Type 1)

Question 13

What is the OSPF router ID format used by Junos OS when initializing the OSPF routing process if no explicit router ID is configured?

Accepted Answer

A 32-bit IPv4 address format (dotted quad). OSPF router IDs are formatted as a 32-bit dotted-quad number, identical in structure to an IPv4 address.

Answer

A 48-bit MAC address

Answer

An integer value between 1 and 65535

Answer

A 128-bit IPv6 address

Question 14

In an OSPF NSSA configuration on a Junos device, which router is specifically responsible for translating Type 7 LSAs into Type 5 LSAs when propagating routes out of the NSSA into the backbone?

Accepted Answer

The NSSA ABR with the highest router ID. In an OSPF NSSA, Type 7 LSAs are generated by the ASBR for external routes. The NSSA ABR is responsible for translating Type 7 LSAs into Type 5 LSAs when propagating them into the backbone area. If multiple ABRs exist, the one with the highest router ID is elected as the translator to avoid duplicate translation.

Answer

The OSPF Designated Router (DR) on the NSSA segment

Answer

The ASBR that originated the external route

Answer

Any internal NSSA router running Junos OS

Question 15

You are configuring authentication for OSPF on a Junos router using MD5. Where must the MD5 key ID and password be defined in the configuration hierarchy?

Accepted Answer

[edit protocols ospf area interface authentication md5]. On Junos, OSPF MD5 authentication is configured at the interface level under the OSPF area hierarchy. The correct path is [edit protocols ospf area interface authentication md5], where you specify the key ID and password. This binds the authentication to the specific OSPF interface, which is required because OSPF authentication is per-interface, not global.

Answer

[edit protocols bgp group <group> authentication-key]

Answer

[edit routing-options autonomous-system]

Answer

[edit system authentication-order]