Huawei H12-831_V1.0 Dumps Questions Answers

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:

Understanding the BGP and OSPF Network Topology in the Question:

External Route Importation

R1, R2, and R3 import 192.168.1.0/24 into BGP from an external source.

This means R1, R2, and R3 will advertise this route to their EBGP and iBGP peers.

BGP Route Selection Criteria

BGP follows specific rules to determine the best path for a route. The most relevant factors here are:

Highest Local Preference (LOCAL_PREF) → Preferred

Shortest AS Path → Preferred

Lowest Origin Type (IGP < EGP < Incomplete) → Preferred

Lowest MED (Multi-Exit Discriminator) → Preferred

Oldest eBGP route (if paths are equal) → Preferred

Route Selection Analysis for Each Statement:

A. For 192.168.1.0/24, R5 preferentially selects the route received from R2. ✅ (Correct)

R5 is in AS 65456 and peers with R6 using iBGP.

Since R2 is a central router with direct connectivity to R5, it is the most preferred path compared to indirect paths from R1 or R3.

By default, R5 will prefer the path from R2 over others unless explicitly influenced by BGP attributes.

✅ Statement A is correct.

B. For 192.168.1.0/24, R4 preferentially selects the route received from R1. ❌ (Incorrect)

R4 and R5 use Loopback0 to establish iBGP with R6, meaning they receive BGP routes from multiple sources.

BGP prefers the shortest AS path, and there is no specific information indicating that R1’s path is the shortest for R4.

R4 may actually prefer another router's advertisement (e.g., R2 or R3) depending on AS path length and other attributes.

❌ Statement B is incorrect.

C. For 192.168.1.0/24, if the default local-preference 150 command is configured on R3, R5 preferentially selects the route received from R3. ✅ (Correct)

By default, BGP assigns a local-preference of 100.

If R3 sets local-preference to 150, it is higher than other routes (R1, R2).

BGP prefers the highest local-preference value, so R5 will prefer the route received from R3 if this attribute is set.

✅ Statement C is correct.

D. For 192.168.1.0/24, R6 preferentially selects the route received from R4. ❌ (Incorrect)

R6 is an iBGP peer of both R4 and R5.

BGP best-path selection depends on attributes like local-preference, AS-path, and MED.

There is no explicit information suggesting that R4’s advertisement is preferred over R5’s.

R6 may prefer R5’s route depending on attributes.

❌ Statement D is incorrect.

Final Conclusion:

✅ A. R5 preferentially selects the route received from R2.❌ B. R4 does not necessarily prefer R1’s route.✅ C. If R3 sets local-preference 150, R5 will prefer the route from R3.❌ D. R6 does not necessarily prefer R4’s route.

Thus, the correct answers are: A, C.