Huawei Updated H12-831_V1.0 Exam Questions and Answers by sana

Comprehensive and Detailed In-Depth Explanation:

Key OSPF Concepts to Consider:

Area 1 is an NSSA (Not-So-Stubby Area):

NSSAs allow external routes but advertise them as Type-7 LSAs.

The ABR (R2) converts these Type-7 LSAs into Type-5 LSAs for the backbone (Area 0).

Area 2 is a Stub Area:

Stub areas do not allow external routes (Type-5 LSAs).

This means that R5 (inside Area 2) will NOT receive external routes like 10.0.4.4/32 or 10.0.4.0/24.

Effect of asbr-summary 10.0.4.0 255.255.255.0 on R1 and R3:

This command ensures that R1 and R3 will see a summarized route 10.0.4.0/24 instead of specific /32 routes.

Routing Table Analysis:

R1 (ABR for NSSA, with ASBR summary configured)

Connected to Area 1 (NSSA) and Area 0 (Backbone)

Since asbr-summary 10.0.4.0 255.255.255.0 is configured on R1, it contains both 10.0.4.4/32 (from R4) and the summarized 10.0.4.0/24.

✅ Matches Option 4: The routing table contains both 10.0.4.4/32 and 10.0.4.0/24.

R5 (Inside Area 2 - Stub Area)

Stub areas do not receive external routes (Type-5 LSAs).

Since R5 is inside a stub area, it does not contain 10.0.4.4/32 or 10.0.4.0/24.

✅ Matches Option 3: The routing table does not contain 10.0.4.4/32 or 10.0.4.0/24.

R2 (ABR between NSSA and Backbone, but no ASBR summary configured)

R2 converts Type-7 LSAs from R4 into Type-5 LSAs and advertises them into Area 0.

Since no ASBR summary is configured on R2, it only sees the specific external route 10.0.4.4/32 but not the summarized 10.0.4.0/24.

✅ Matches Option 1: The routing table contains 10.0.4.4/32 only.

R3 (Another ABR with ASBR summary configured)

R3 has asbr-summary 10.0.4.0 255.255.255.0 configured, which means it will summarize the external routes.

Instead of 10.0.4.4/32, R3 will only have the summarized 10.0.4.0/24 route.

✅ Matches Option 2: The routing table contains 10.0.4.0/24 only.

References:

HCIP-Datacom-Advanced Routing & Switching Technology V1.0 – OSPF Areas (Stub, NSSA, and Backbone)

OSPF External Route Advertisement and Summarization

OSPF NSSA Behavior and Type-7 to Type-5 Conversion

Comprehensive and Detailed In-Depth Explanation:

1. Understanding MPLS Label Stack and the S (Bottom of Stack) Bit

MPLS (Multiprotocol Label Switching) packets can have multiple labels stacked on top of each other.

Each label has the following fields:

Label (20 bits): Identifies the forwarding path.

EXP (3 bits): Used for QoS/priority marking.

TTL (8 bits): Time-to-live, similar to IP TTL.

S (1 bit): Bottom of Stack (BoS) indicator:

S = 0: This is not the last label in the stack.

S = 1: This is the last label in the stack, meaning the next header is the IP header.

2. Analyzing the MPLS Packet Structure in the Figure

The diagram shows three MPLS headers (labels).

The last MPLS header before the IP header must have S = 1, indicating the bottom of the stack.

All other MPLS headers above it must have S = 0, meaning they are part of the stacked labels.

3. Evaluating the Answer Choices

Option

S1

S2

S3

Correct?

Reasoning

A

3

2

1

❌ Incorrect

S field cannot have values greater than 1.

B

2

1

0

❌ Incorrect

S field must be 0 for non-bottom labels.

C

1

1

0

❌ Incorrect

There should be only one label with S = 1 (BoS).

D

0

0

1

✅ Correct

Only the last label should have S = 1.

✅ Correct Answer: D. S1 = 0, S2 = 0, S3 = 1

Final Conclusion:

Only the last MPLS label (closest to the IP header) should have S = 1.

All previous MPLS labels should have S = 0.

Thus, the correct answer is: D. S1 = 0, S2 = 0, S3 = 1

Comprehensive and Detailed In-Depth Explanation:

Step 1: Analyzing the Primary Path

The given network uses IS-IS Auto FRR, which ensures a backup route in case of failure.

The shortest path from R1 to 10.0.3.3/32 is calculated based on the OSPF link costs:

Path 1 (Primary Path): R1 → R2 → R3

Cost: 10 (R1→R2) + 5 (R2→R3) = 15 ✅

Step 2: Checking the Backup Path

The statement claims that the backup path is R1 → R4 → R2 → R3. Let's calculate the cost:

Path 2 (Backup Path via R4): R1 → R4 → R2 → R3

Cost: 10 (R1→R4) + 50 (R4→R2) + 5 (R2→R3) = 65 ❌ (Too high)

The backup path should be an alternative shortest path that is as close as possible to the primary path in cost.

R1 → R4 → R2 → R3 is not a feasible backup path because the cost (65) is significantly higher than the primary path (15).

Step 3: Correct Backup Path Selection

If IS-IS Auto FRR is enabled, a feasible backup path should have a cost close to the primary path.

In this scenario, no viable alternative path meets the Auto FRR requirements efficiently.

The given backup path is incorrect.

Final Answer: ✅ FALSE (Option B)

References:

HCIP-Datacom-Advanced Routing & Switching Technology V1.0 – IS-IS Auto FRR Mechanism

OSPF Route Selection and Fast Reroute Principles

OSPF and IS-IS Path Cost Calculation