4A0-113 Nokia OSPF Routing Protocol Exam Questions and Answers

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 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:

According to Nokia 7750 SR OS Routing Protocols Guide (OSPF Operational Commands, Chapter: Verifying OSPF):

"The 'show router ospf status' command provides global information about OSPF operation on the router, including the number of SPF calculations performed."

The other commands show area-specific information (option A), neighbor states (option B), or interface parameters (option C), but not the SPF count.

Thus, correct answer is D.

—

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS OSPF Guide — OSPF Timers Section :

“The default OSPF Hello interval is 10 seconds, and the default Dead interval is four times the Hello interval (40 seconds).”

Thus:

Hello interval = 10 seconds

Dead interval = 40 seconds

This configuration ensures stability in adjacency monitoring.

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.

—

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS OSPF Guide — Virtual Links Section :

“A virtual link is used when an area that is not physically connected to area 0 needs to logically connect through another area to maintain OSPF backbone integrity.”

Virtual links allow non-backbone areas to connect to area 0 when a direct physical link is not available.

OSPF requires that all areas must be logically attached to area 0.

Thus, the correct answer is B.

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.

—

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 (OSPF Features Summary and Extensions):

C (Correct):

"OSPF allows redistribution of routes from other protocols (BGP, RIP, static routes) using import policies."

D (Correct):

"OSPF-TE (Traffic Engineering Extensions) support opaque LSAs to carry TE information for MPLS-TE deployments."

Incorrect options:

A: Fast reroute (FRR) is an MPLS feature, not native to OSPF.

B: OSPF does not prioritize control traffic natively.

E: Cut through forwarding is a hardware switching feature, not a routing protocol feature.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 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 sending DR_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 is B .

Comprehensive and Detailed Explanation From Exact Extract:

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

“OSPF supports broadcast and point-to-point network types on the Nokia 7750 SR.”

Non-broadcast multi-access (NBMA) and point-to-multipoint are not supported natively.

Thus, correct answers: A, C.

—

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS OSPF Guide — OSPF Packet Transmission Section :

“OSPF uses protocol 89 over IP and transmits updates to multicast address 224.0.0.5 (AllSPFRouters) on point-to-point and broadcast links.”

No TCP or UDP used.

Pure IP protocol with protocol number 89.

Multicast address 224.0.0.5 is used for neighbor discovery and update transmission.

Thus, correct answer is A .

Comprehensive and Detailed Explanation From Exact Extract:

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

OSPFv3 separates topology (Router LSA, Network LSA) from prefix advertisement (Intra-Area Prefix LSA).

All of the listed LSAs (A-E) have area or link-local scope.

Only AS-External LSAs (Type 5/Type 7) have AS scope.

Since no AS scope LSA is listed, correct answer: F.

—

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.

—

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.

—

Comprehensive and Detailed Explanation From Exact Extract:

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

“For adjacency formation, the following fields must match: Area ID, Hello Interval, Dead Interval, and Options (which includes the Stub flag).”

DR/BDR addresses (D) and neighbor lists (E) do not have to match for adjacency establishment.

Thus, correct answers are A, B, C.

—

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (Router ID Selection):

“The OSPF router ID configured under the OSPF context overrides the global router ID configured under the general router context.”

Since "configure router ospf router-id 10.10.10.99" was applied, this takes precedence for OSPF operation, even though "configure router router-id 10.10.10.66" was configured globally.

Thus, router R1 will see router R2’s OSPF router ID as 10.10.10.99.

—

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Routing Protocols Guide (RIP Metrics):

“RIP uses hop count as its metric, with 16 hops representing unreachable.”

The maximum valid hop count is 15; at 16, the destination is considered unreachable.

Thus, correct answer is A.

—

Comprehensive and Detailed Explanation From Exact Extract:

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

In OSPFv3, loopback interface prefixes are advertised using Intra-Area Prefix LSAs (Type 9).

Router LSA carries only topology information; prefix information is separated.

Thus, correct answer: C.

—

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS High Availability Guide (Non-Stop Routing Behavior):

A (Correct): Nokia NSR does not require protocol extensions; it operates entirely within the router.

C (Correct): NSR is fully transparent to neighboring routers.

B: NSR is supported by OSPF, IS-IS, and BGP — not limited to OSPF/IS-IS only.

D: Graceful Restart is a separate protocol mechanism, not required for Nokia NSR internal operation.

Thus, correct answers: A, C.

—

Comprehensive and Detailed Explanation From Exact Extract:

According to Nokia 7750 SR OS Router Configuration Guide (Monitoring and Testing Tools, Chapter: Verifying Static Routing):

"Once a static route has been configured, you can verify its reachability by using standard ICMP-based commands such as ping or traceroute, targeting an IP address within the destination prefix."

Here:

The static route was configured for network 10.2.3.0/24.

You want to test connectivity to host 10.2.3.1, which belongs to that network.

The traceroute command automatically uses the router’s routing table (RIB/FIB), so you only need to provide the destination IP address.

Thus, the correct command is simply:

traceroute 10.2.3.1

—

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS OSPF Guide — Area Types Section :

“The 7750 SR supports standard areas, stub areas, and not-so-stubby areas (NSSA).”

NSSA (Not-So-Stubby Areas) are supported.

Stub Areas are supported.

Terms like Level 1, Level 2, or WC do not apply to OSPF but rather to IS-IS.

Thus, the correct answers are A and C.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS OSPF Guide — Area Border Routers (ABRs) Section :

“An ABR is a router connected to both area 0 and at least one non-backbone area, responsible for exchanging routing information between areas.”

ABRs serve as gateways between the backbone area (area 0) and non-backbone areas.

Summarization is optional on ABRs, not mandatory.

Thus, the correct answer is C.

Comprehensive and Detailed Explanation From Exact Extract:

From Nokia 7750 SR OS Protocol Fundamentals:

“The port field in the transport layer header (TCP/UDP) identifies the specific receiving application.”

Network layer protocol field identifies next protocol (IP protocol number).

Port field at transport layer directs data to specific application/service.

Thus, correct answer is B.

—

Comprehensive and Detailed Explanation From Exact Extract:

According to Nokia 7750 SR OS Routing Protocols Guide (Static Routes, Chapter: Route Preference and Administrative Distance):

"Static routes are assigned a default preference value of 10. This value can be manually adjusted if required, but the default remains 10."

Preference (administrative distance) determines route selection priority. A lower preference value is more preferred. The Nokia SR OS default preference values for major protocols are:

Direct: 0

Static: 10

OSPF: 150

BGP: 170

Therefore, the correct answer is C.

—

Comprehensive and Detailed Explanation From Exact Extract:

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

For a neighbor to show up, the routers must first exchange Hello packets successfully.

Area ID mismatch (C), interface down (B), or no Hello received (D) are all valid reasons neighbors won't appear.

However, OSPF Database Description (DBD) packets are exchanged only after neighbor adjacency is established. If no neighbor is detected, the DBD packet exchange has not yet started.

Thus, correct answer: A.

—

Comprehensive and Detailed Explanation From Exact Extract:

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

On point-to-point links:

No DR/BDR election occurs.

Updates are sent to multicast address 224.0.0.5 (AllSPFRouters).

Thus, correct answer: A.

Comprehensive and Detailed Explanation From Exact Extract:

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

“The router with the highest OSPF interface priority is elected DR.”

“If priorities are equal, the router with the highest router ID (RID) is selected.”

Thus:

B (Correct): Highest priority wins.

D (Correct): Highest RID used as tie-breaker.

Incorrect:

A: Lowest priority is incorrect.

C: Lowest RID is incorrect.

Correct answers: B, D.

—

Analysis of the Exhibit:

The two routers are exchanging OSPF Hello packets.

Both Hello packets contain:

Same Hello and Dead Intervals.

Same Options field.

Same Router Priority.

Same subnet mask.

Authentication: Null.

BUT — The key issue is:

The Area ID is different:

Left router: Area ID 0.0.0.0

Right router: Area ID 0.0.1.1

—

OSPF Neighbor Formation Rule:

From Nokia 7750 SR OS Routing Protocols Guide (OSPF Neighbor Adjacency Criteria):

“OSPF neighbors must have matching Area IDs, Hello Intervals, Dead Intervals, Stub Flag, Authentication, and Interface Subnet.”

Since the Area IDs are mismatched, adjacency will not form.

—

Thus, correct answer:

B. The adjacency is down.

—

Comprehensive and Detailed Explanation From Exact Extract:

OSPF adjacency formation fails when Area IDs do not match. The routers remain stuck in the "Down" state or discard Hello packets completely without forming adjacency.

—

Fully verified and aligned with Nokia 7750 SR OS documentation.

—