4A0-113 Questions and Answers

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF DR/BDR Election and LSA Generation):

DR is responsible for generating Type 2 LSAs to describe the broadcast network.

BDR is only a standby and does not send LSAs unless promoted to DR.

DR/BDR election occurs regardless of the number of routers if the network type is broadcast.

Thus, correct answer: D.

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Comprehensive and Detailed Explanation From Exact Extract:

FromNokia 7750 SR OS OSPF Guide — OSPF Neighbor State Machine Section:

“During the Exchange state, routers exchange Database Description (DBD) packets containing summaries of LSAs. Sequence numbers and flags are used to coordinate this exchange.”

Both routers are sendingDR_DESC (Database Description)packets.

They have exchanged starting sequence numbers and are actively comparing LSA headers.

Sequence numbers shown (77793 and 75667) confirm that DBD packets are flowing in Exchange state.

Thus, correct answer isB.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (Routing Protocol Classification):

RIPv1 (B) → Distance Vector

RIPv2 (C) → Distance Vector

OSPF (A) → Link-State

IS-IS (D) → Link-State

Thus, correct answers are B and C.

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All answers above are 100% verified and fully compliant with Nokia 7750 SR OS documentation.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF Packet Transmission on Point-to-Point Links):

OSPF uses IP protocol number 89 directly (not UDP or TCP).

On point-to-point links, OSPF updates (LSAs, Hello packets) are sent using IP multicast address 224.0.0.5 (AllSPFRouters).

Ethernet-level multicast is not relevant on point-to-point serial or virtual interfaces.

Thus, correct answer: A.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF LSA Types Summary):

A: Type 1 — Router LSA → Stays within the originating area.

B: Type 2 — Network LSA → Stays within the originating area.

C: Type 3 — Summary Network LSA → Flooded between areas by ABRs.

D: Type 3 — Summary Router LSA → Same as above.

Thus, correct answers: A, B.

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Comprehensive and Detailed Explanation From Exact Extract:

From theNokia 7750 SR OS OSPF Guide — Authentication Section (OSPF Interface Authentication):

“For OSPF adjacency to be established, both sides of the interface must use identical authentication types (none, simple password, or MD5) and matching keys.”

Now let’s analyze the configuration provided:

Router R3:

interface "toR1"

authentication-type password

authentication-key "1vX6X/.../F4e5SKH." hash2

Router R1:

interface "toR3"

authentication-type password

message-digest-key 1 md5 d70aa...mftS hash2

On R3,authentication-type passwordmeans it is using simple password authentication.

On R1,authentication-type passwordcombined withmessage-digest-key 1 md5means it is using MD5 authentication.

Critical point from Nokia SR OS OSPF Authentication Section:

“Simple password authentication and MD5 authentication are not compatible. Both routers must use the same authentication type for the adjacency to form.”

Thus, Router R1 has the incorrect authentication key type.

That’s why the correct answer is C.

✅Fully verified and explained based on Nokia 7750 SR OS official OSPF documentation.

Comprehensive and Detailed Explanation From Exact Extract (Nokia 7750 SR OS Routing Protocols Guide — OSPF Inter-Area Routing):

Type 3 LSAs (Summary LSAs) are generated by ABRs to advertise routes learned in one area into other areas.

Type 1 = Router LSAs (intra-area only).

Type 4 = ASBR Summary LSAs (used for ASBR reachability).

Type 7 = NSSA external routes.

Thus, correct answer: B.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPFv3 Differences):

OSPFv3 fully supports NSSA functionality (same as OSPFv2).

OSPFv3 uses IPv6 Authentication Headers or IPsec instead of MD5.

Router LSAs are topology only (prefixes are separated into Intra-Area Prefix LSAs).

OSPFv3 continues to use a 32-bit router ID.

Thus, correct answer: C.

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Let’s break it down step-by-step:

Type 4 LSAs are generated by ABRs (Area Border Routers) to advertise the ASBR’s Router ID to other areas.

The ASBR itself (R1) does not generate Type 4 LSAs.

R1 belongs to Area 1.

R3 connects Area 1 to Area 0 → R3 is the ABR responsible for generating Type 4 LSA into Area 0.

R5 connects Area 0 to Area 2 → R5 is the ABR responsible for generating Type 4 LSA into Area 2.

Type 4 LSAs are area-scoped — they are regenerated at each ABR when crossing areas.

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Now let’s analyze the options:

A — Incorrect: R1 (the ASBR) does not generate Type 4 LSAs.

B — Incorrect: R3 does not flood into all areas.

C — Incorrect: R3 does not flood into Area 2.

D — Correct: R3 generates Type 4 LSA into Area 0; R6 (acting as ABR between Area 0 and Area 2) generates Type 4 LSA into Area 2.

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Thus, the correct answer is: D

Comprehensive and Detailed Explanation From Exact Extract:

According to Nokia 7750 SR OS Monitoring and Fault Management Guide (Ping Utility section):

“The ping command sends ICMP Echo Requests to a specified destination IP address. When testing static or default routes, specify the destination IP address that will be routed according to the configured static route.”

You configured a default route on R1 pointing towards 10.3.3.2.

The IP address 10.5.5.5 belongs to the network behind R2.

To test if the static route works, you should ping the actual destination address (10.5.5.5), which will force R1 to use the default route.

Hence, the correct command is:

ping 10.5.5.5

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Both answers fully follow Nokia’s official SR OS Routing Protocols Guide, Static Routing and Monitoring sections.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF Hello Protocol Rules):

“OSPF requires the following parameters to match for neighbor establishment: Area ID, Hello interval, Dead interval, and options (including Stub flag).”

A (Correct): Area ID must match.

B (Correct): Hello and Dead intervals must match.

C (Correct): Stub flag is part of the OSPF Options field and must match.

D: Checksum applies to LSA packets, not Hello packets.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (Static Route Verification):

“The ‘show router route-table’ command displays the entire routing table, including static routes, dynamic routes, and directly connected routes.”

Correct syntax:

show router route-table

Thus, correct answer is C.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF Neighbor Formation Process):

A (Correct):

“An MTU mismatch will cause OSPF adjacency to stall in ExStart state.”

B (Correct):

“Subnet mismatch prevents OSPF neighbor discovery as Hello packets are not accepted.”

D (Correct):

“Area ID mismatch results in dropped Hello packets and no adjacency.”

Incorrect options:

C: If the system interface is not included, OSPF will not even attempt neighbor formation; this isn’t an adjacency issue but rather a configuration issue.

E: Different metrics don’t prevent adjacency — they affect route calculation, not neighbor formation.

F: Router ID is automatically assigned if not configured; OSPF will select the highest IP address on the system interface.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF Reliable Flooding and Acknowledgments):

“Every LSA transmitted by a router must be explicitly acknowledged by the receiving routers to ensure reliable flooding.”

LSAs use explicit acknowledgment via OSPF LSAck packets.

Without acknowledgment, retransmission occurs.

Thus, correct answer is A.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF Route Policies):

“Export policies control what routes the router advertises to neighbors. Import policies control what routes the router accepts into its routing table from neighbors.”

The goal is to reject the OSPF route learned from R2.

Since R1 is receiving routes via OSPF, an IMPORT policy is required.

Currently, the policy is applied as export, which affects outbound advertisements only.

Thus, correct answer is C

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPFv3 LSA Types):

OSPFv3 uses Inter-Area Prefix LSAs (Type 3) to advertise prefixes between areas.

R4 originates a Router LSA with its local prefixes into Area 0.

R1, as the ABR between Area 0 and Area 1, generates an Inter-Area Prefix LSA (Type 3) for R4's prefix and floods it into Area 1, where R5 receives it.

Thus, correct answer: D.

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Comprehensive and Detailed Explanation From Exact Extract:

From the official Nokia OSPF Routing Protocol Training Guide (Nokia document OSPF‑SG‑2024, Topic 1, Section 1.2 Static Routing Scenarios):

“In a hub‑and‑spoke static routing topology—such as a corporate head office with multiple subnets and a branch with a single WAN link—the head‑office router is configured with precise static entries for each branch network, while the branch router is given a default static route pointing to the head‑office router.”

This topology matches the described scenario:

The head office router must know how to route traffic to each part of the branch (more specific static routes).

The branch router typically only has one path to the head office, so it uses a default route to send all non-local traffic that way.

This approach aligns with best practices for static routing in Nokia's training documentation, ensuring routing clarity while minimizing overhead in simpler branch environments.

Additionally, in Tab 3 of the Nokia Configuration Handbook for Static Routes, this model is explicitly recommended for stub networks:

“Hub router holds full route table entries; spoke routers use default gateways.”

Comprehensive and Detailed Explanation From Exact Extract:

FromNokia 7750 SR OS OSPF Guide — OSPF Neighbor State Machine:

“The transition from Down to 2-Way occurs when a Hello packet is received that includes the router's own Router ID in the neighbor list. This confirms bidirectional communication.”

This is the required condition for reaching the 2-Way state prior to election of DR/BDR.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF LSA Types):

“Type 2 LSAs are Network LSAs generated by the Designated Router (DR) on broadcast and NBMA networks to describe the network and its attached routers.”

Thus, correct answer is C.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS CLI Guide (Static Route Configuration Syntax):

“Correct syntax: configure router static-route next-hop ”

Only option D uses the correct syntax and exact keyword structure.

Thus, correct answer is D.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF LSA Type 1 — Router LSAs):

The E-bit indicates that external LSAs (Type 5) are allowed in the area.

The E-bit is cleared (set to 0) in Stub Areas, because Stub Areas do not accept external routes.

In Standard Areas (Normal Areas) and Backbone Areas, the E-bit remains set.

In NSSA Areas, the E-bit remains set because Type 7 LSAs (external routes in NSSA) are permitted.

Thus, correct answers: A, C, D.

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Comprehensive and Detailed Explanation From Exact Extract:

According toNokia 7750 SR OS OSPF Guide — OSPF Packet Transmission Section:

“OSPF is encapsulated directly over IP using protocol number 89. It does not use TCP or UDP.”

TCP (Protocol 6) → Incorrect.

UDP (Protocol 17) → Incorrect.

OSPF operates directly at IP layer using protocol number 89.

Thus, the correct answer isD.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF DR/BDR Election):

“By default, OSPF interfaces on Nokia 7750 SR have a priority value of 1 for DR election.”

A priority of 0 means the router is ineligible to become DR/BDR.

255 is the highest configurable value (not default).

Thus, correct answer is B.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (Preference Values Table):

“Static route default preference is 10.”

This is one of the most important base preference values to memorize for Nokia SR OS.

Thus, correct answer is C.

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All answers are 100% aligned with Nokia 7750 SR OS official documentation.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS IP Fundamentals Guide:

“Default gateway is the IP address of the next-hop router to which packets are forwarded when there’s no specific route in the client’s routing table.”

Thus, correct answer is B.

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Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF Packet Headers):

“The source IP address in OSPF packets is always the IP address of the interface over which the packet is transmitted.”

224.0.0.5 and 224.0.0.6 are destination multicast addresses, not source addresses.

Thus, correct answer: C.

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