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

Comprehensive and Detailed In-Depth Explanation:

Understanding OSPFv3 Areas and Routing Behavior

OSPFv3 Area Types in the Network:

Area 1 (Stub Area): Does not accept external LSAs (Type 5) but can receive Inter-Area-Prefix-LSAs (Type 3).

Area 2 (Common Area): Fully functional area that allows both Type 3 (Inter-Area) and Type 5 (External) LSAs.

Area 3 (NSSA - Not-So-Stubby Area): Does not accept Type 5 (External) LSAs but allows ASBRs to inject external routes using Type 7 LSAs, which are later converted to Type 5 in other areas.

Effect of stub no-summary Command on Area 1 (Configured on R2):

This command prevents Type 3 LSAs from entering Area 1, meaning R4 will not learn about 2000::6/128 from other areas.

R4 only has a default route (::/0) provided by R2 but not individual routes like 2000::6/128.

Route and Packet Flow Analysis

A. "The LSDB of R4 contains the Inter-Area-Prefix-LSA that is generated by R1 and describes 2000::6/128." ❌ (False)

R4 is in Area 1 (a totally stub area), which does not receive Type 3 LSAs.

Since 2000::6/128 is an external route from Area 2, it will not be present in R4’s LSDB.

Thus, R4 does NOT have the Inter-Area-Prefix-LSA for 2000::6/128.

❌ Statement A is incorrect.

B. "The path for data packets from R4 to 2000::6 is R4 -> R1 -> R5 -> R3 -> R6." ❌ (False)

R4 is in a totally stub area (Area 1), and it does not have a specific route for 2000::6/128.

It only has a default route (::/0) pointing to R2.

If R4 forwards traffic, it will send it to R2, NOT via R1, R5, and R3.

❌ Statement B is incorrect.

C. "The routing table of R4 does not contain the route 2000::6/128." ✅ (True - Correct Answer)

R4 is in a totally stub area (Area 1), meaning it only receives a default route from R2 (::/0).

It does NOT receive specific routes like 2000::6/128.

✅ Statement C is correct.

D. "The path for data packets from R4 to 2000::6 is R4 -> R2 -> R6." ❌ (False)

While R4 does forward all unknown traffic to R2 (due to the default route), R2 will NOT directly forward to R6.

R2 is an ABR (Area Border Router), and the actual path to 2000::6 must traverse Area 2, likely through other routers.

❌ Statement D is incorrect.

Final Conclusion:

✅ C. The routing table of R4 does not contain the route 2000::6/128.

Thus, the correct answer is: C. The routing table of R4 does not contain the route 2000::6/128.

Comprehensive and Detailed In-Depth Explanation:

1. Understanding MPLS LDP (Label Distribution Protocol)

LDP (Label Distribution Protocol) runs over TCP port 646.

LDP uses the transport address (Loopback0 in this case) to establish TCP sessions between neighbors.

For an LDP session to be established, both routers must:

Enable LDP on the interface (GE0/0/1).

Allow TCP traffic on port 646 (used for LDP adjacency formation).

2. Analyzing the Issue and Possible Causes

The figure shows a successful ping from R2 to R1’s Loopback0 (10.0.1.1/32).

This means basic IP connectivity between R1 and R2 is working.

OSPF is properly advertising Loopback0 addresses between the routers.

The problem must be related to LDP itself, not general IP reachability.

3. Evaluating Each Answer Option

Option A: "R1’s GE0/0/1 rejects packets with the destination IP address 10.0.2.2."

Incorrect.

The ping test in the figure shows successful replies from 10.0.1.1 to 10.0.2.2, meaning packets to 10.0.2.2 are not being rejected.

This is NOT the cause of the LDP failure.

Option B: "MPLS LDP is not enabled on R2’s GE0/0/1."

Correct.

If LDP is not enabled on R2’s GE0/0/1, then R1 and R2 cannot form an LDP session.

MPLS LDP must be enabled on both interfaces for label exchange.

This is a valid cause of the issue.

Option C: "R2’s GE0/0/1 rejects packets with TCP destination port 646."

Correct.

LDP operates over TCP port 646, so if R2’s GE0/0/1 has a firewall or ACL blocking TCP port 646, then LDP will fail.

Blocking TCP 646 on R2’s interface would prevent the session from establishing.

This is another valid cause of the issue.

Option D: "R1’s GE0/0/1 rejects packets with UDP destination port 646."

Incorrect.

LDP does not use UDP; it uses TCP port 646.

This is not a valid cause of the issue.

Final Answer:

✅ B and C are correct.

HCIP-Datacom-Advanced Routing & Switching Technology References:

MPLS LDP Session Establishment and TCP 646 Dependency

How OSPF Advertises Loopback0 for LDP Transport

Common LDP Debugging Techniques and ACL Issues