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

Comprehensive and Detailed In-Depth Explanation:

1. Understanding MPLS TTL Processing

TTL (Time to Live) is an IP field that prevents routing loops by decrementing at each hop.

In an MPLS network, the TTL field can be processed in two modes:

Uniform Mode (Default Mode)

Pipe Mode (TTL is preserved in the MPLS domain)

2. TTL Processing in Pipe Mode

Pipe mode is used to prevent the MPLS backbone from affecting the TTL of an IP packet.

When an IP packet enters the MPLS domain, the original TTL is copied into the MPLS header.

Inside the MPLS network:

MPLS labels use their own TTL values, and the IP TTL is not decremented.

Each P router (transit router) decrements only the MPLS TTL, not the IP TTL.

When the packet leaves the MPLS domain (at PE2):

The original IP TTL is restored but decremented by 1 as it exits to CE2.

3. Analyzing the TTL Behavior in the Given Topology

Initial IP TTL from CE1 = 255

At PE1:

IP TTL remains 255 as it is copied into the MPLS label.

MPLS TTL starts at 255 (same as IP TTL).

At P Router:

MPLS TTL is decremented (255 → 254).

IP TTL remains unchanged (255) because of Pipe mode.

At PE2 (Egress Router):

IP TTL is restored and decremented by 1 (255 → 254).

Thus, the correct sequence is:✅ IP TTL: 255 → 255 → 254

4. Evaluating the Answer Choices

Option

TTL Values

Correct?

Reasoning

A

255 → 254 → 252

❌ Incorrect

IP TTL should remain 255 inside MPLS

B

254 → 253 → 252

❌ Incorrect

IP TTL starts at 255, not 254

C

255 → 255 → 254

✅ Correct

IP TTL is preserved in pipe mode inside MPLS and decremented only at egress.

D

254 → 254 → 253

❌ Incorrect

Initial IP TTL should be 255, not 254

✅ Correct Answer: C. 255 → 255 → 254

Final Conclusion:

Pipe mode ensures that the MPLS domain does not affect the IP TTL while the packet is inside the MPLS network.

Only the egress PE (PE2) decrements the IP TTL before forwarding the packet to CE2.

Thus, the correct TTL sequence is 255 → 255 → 254.

Comprehensive and Detailed In-Depth Explanation:

To analyze the correct answer, we need to break down the topology and protocol behaviors step by step:

Understanding the IS-IS and OSPFv3 Network Setup:

IS-IS IPv6 is running between R2, R6, and R3.

OSPFv3 is running on other links.

Area 1 is a stub area, which means it does not accept external routes but can have a default route (::/0).

Area 2 is an NSSA (Not-So-Stubby Area), meaning it allows external routes as NSSA external LSAs.

Analysis of Each Answer Option:

A. "The routing table of R6 contains the route ::/0." → FALSE

R6 only participates in IS-IS (Level-1) and does not receive a default route (::/0) from Level-2 routers.

Since R6 is in IS-IS Level-1 and does not have a connection to a Level-2 router that provides a default route, it will not have a ::/0 route in its routing table.

Comprehensive and Detailed In-Depth Explanation:

Understanding DHCP Snooping and its Configuration

1. What is DHCP Snooping?

DHCP Snooping is a security feature that helps prevent rogue DHCP servers from providing false IP configurations to clients.

It classifies switch ports as either:

Trusted (allowing DHCP replies from legitimate DHCP servers or relay agents).

Untrusted (blocking unauthorized DHCP responses from clients or attackers).

Analysis of the Commands

✅ A. [SW3-GigabitEthernet0/0/2] dhcp snooping enable (Required)

Interface GigabitEthernet0/0/2 is connected to a DHCP client (PC2).

DHCP snooping must be enabled on this interface to filter DHCP messages and prevent rogue DHCP servers.

Since PC2 is a client, this port should remain UNTRUSTED.

✅ This command is required.

❌ B. [SW3] dhcp snooping enable ipv4 (Not Required - Correct Answer)

The correct command to enable DHCP Snooping globally on Huawei switches is:

dhcp snooping enable

There is no need to explicitly specify ipv4 since DHCP is inherently an IPv4-based protocol.

❌ This command is unnecessary, making it the correct answer.

✅ C. [SW3-GigabitEthernet0/0/3] dhcp snooping trusted (Required)

GigabitEthernet0/0/3 is connected to the DHCP relay agent, which forwards DHCP messages between clients and the DHCP server.

This port must be configured as "trusted" to allow DHCP responses from the relay or the DHCP server.

✅ This command is required.

✅ D. [SW3-GigabitEthernet0/0/1] dhcp snooping enable (Required)

GigabitEthernet0/0/1 is connected to a DHCP client (PC1).

Similar to Port 0/0/2, DHCP snooping must be enabled on this interface to filter DHCP messages and prevent rogue DHCP responses.

✅ This command is required.

Final Conclusion:

✅ A. Required (PC2’s port needs DHCP snooping enabled).❌ B. Not required (Incorrect command syntax, ipv4 is unnecessary).✅ C. Required (Port connected to DHCP relay must be trusted).✅ D. Required (PC1’s port needs DHCP snooping enabled).

Thus, the correct answer is: B. [SW3] dhcp snooping enable ipv4.