Pass the Huawei HCIP-Datacom H12-831_V1.0 Questions and answers with ValidTests

Comprehensive and Detailed In-Depth Explanation:

To determine which devices perform MPLS label operations, we need to analyze MPLS forwarding behavior based on the roles of each router in the MPLS domain.

1. Understanding MPLS Label Operations

MPLS label switching involves three main operations:

Label Push (Ingress LSR - Label Imposition): The ingress router (R1) assigns an MPLS label to the packet.

Label Swap (Transit LSR - Label Forwarding): The transit routers (R2, R3) swap the label for the next-hop router.

Label Pop (Egress LSR - Label Disposition): The egress router (R4) removes the MPLS label before forwarding the packet to its final destination.

2. Examining MPLS Roles of Each Router

???? R1 (Ingress LSR) → Performs Label PUSH

R1 adds an MPLS label (1025) to the IP packet and forwards it to R2.

Operation: Push MPLS Label (Label Imposition).

✅ R1 performs MPLS label operations.

???? R2 (Transit LSR) → Performs Label SWAP

R2 receives a labeled packet (1025) from R1, swaps it with label 1026, and forwards it to R3.

Operation: Swap MPLS Label (Transit Label Forwarding).

✅ R2 performs MPLS label operations.

???? R3 (Transit LSR) → Performs Label SWAP

R3 receives label 1026 and swaps it with label 3, then forwards it to R4.

Operation: Swap MPLS Label (Transit Label Forwarding).

✅ R3 performs MPLS label operations.

???? R4 (Egress LSR) → Does NOT Perform MPLS Operations

R4 is the egress router and does not forward the packet further using MPLS.

The last router removes the MPLS label (Pop) and forwards the plain IP packet.

R4 is NOT performing MPLS label switching (swap or push), only popping the label before delivering the packet.

❌ R4 does NOT perform MPLS label operations.

Comprehensive and Detailed In-Depth Explanation:

Understanding the LSA Table in the Figure

The LSA Type displayed in the LSDB of R3 is an Intra-Area-Prefix-LSA (Type 9), with the following attributes:

LS Type: Intra-Area-Prefix-LSA (Type 9)

Link State ID: 0.0.0.1

Originating Router: 10.0.2.2 (R2)

Referenced LSA Type: 0x2001 (Router-LSA, Type 1)

Referenced Link State ID: 0.0.0.0

Referenced Originating Router: 10.0.2.2 (R2)

Prefix: 2000:2::/128

Analysis of the Statement

What is an Intra-Area-Prefix-LSA (Type 9)?

Used in OSPFv3 to advertise IPv6 prefixes associated with a Router-LSA (Type 1) or Network-LSA (Type 2).

Each router generates its own Router-LSA (Type 1) and references it in an Intra-Area-Prefix-LSA (Type 9).

How does this apply to R3's LSDB?

The LSA is generated by R2 (10.0.2.2), meaning R2 originated the prefix advertisement.

The referenced LSA is a Router-LSA, indicating that the IPv6 prefix is associated with R2's router advertisement.

Since R3 sees this LSA in its LSDB, it means R2 has advertised its IPv6 prefix (2000:2::/128) into the OSPFv3 domain.

✅ Since the LSA describes an IPv6 prefix associated with R2’s Router-LSA, the statement is TRUE.

Final Conclusion:

✅ The LSA is generated by R2 and describes the IPv6 prefix associated with the Router-LSA.✅ Thus, the correct answer is: A. TRUE.

Comprehensive and Detailed In-Depth Explanation:To determine whether the statement is true or false, we need to analyze the OSPFv3 topology, the role of area types, and the behavior of Inter-Area-Prefix-LSAs. Let’s break it down step by step:

Understanding the OSPFv3 Topology and Area Types:

The figure shows three routers (R1, R2, and R3) connected in an OSPFv3 network. R1 and R2 are in Area 0 (the backbone area), while R2 and R3 are in Area 1 (a common area, also known as a standard or regular area).

R2 is an Area Border Router (ABR) because it connects Area 0 and Area 1. ABRs are responsible for summarizing and advertising routes between areas.

What is a Common Area in OSPFv3?:

A "common area" in OSPFv3 refers to a standard or regular OSPF area, as opposed to special areas like Stub, Totally Stubby, Not-So-Stubby Area (NSSA), or Totally NSSA. Common areas allow all types of LSAs, including inter-area, intra-area, and external LSAs, to be flooded within the area, except for restrictions in special area types.

Since Area 1 is described as a "common area," it behaves like a standard OSPF area, allowing the full exchange of LSAs, including Inter-Area-Prefix-LSAs.

Role of Inter-Area-Prefix-LSA (Type 3 LSA in OSPFv3):

Inter-Area-Prefix-LSAs (Type 3 LSAs in OSPFv3) are used by ABRs to advertise prefixes from one area to another. These LSAs are critical for inter-area routing, allowing routers in one area to learn about networks in other areas.

In this case, R2, as an ABR, generates Inter-Area-Prefix-LSAs to describe routes (network segments) in Area 1 and advertises them into Area 0. Conversely, R2 also receives Inter-Area-Prefix-LSAs from Area 0 (via R1) and advertises them into Area 1.

Does the Inter-Area-Prefix-LSA Exist in Both Area 0 and Area 1?:

Yes, Inter-Area-Prefix-LSAs exist in both areas because R2, as an ABR, floods these LSAs into both Area 0 and Area 1. Specifically:

R2 generates Inter-Area-Prefix-LSAs for network segments in Area 1 and advertises them into Area 0, where R1 (in Area 0) can receive and process them.

Similarly, R2 receives Inter-Area-Prefix-LSAs for network segments in Area 0 (from R1) and advertises them into Area 1, where R3 (in Area 1) can receive and process them.

Since Area 1 is a common area (not a stub or NSSA), it allows Inter-Area-Prefix-LSAs to be flooded within the area, ensuring that routers like R3 can learn about routes in Area 0, and routers like R1 can learn about routes in Area 1.

Verifying the Statement:

The statement claims that R2 generates an Inter-Area-Prefix-LSA to describe the routes of a network segment in Area 1, and such an LSA exists in both Area 0 and Area 1. This is accurate because:

R2, as an ABR, generates Inter-Area-Prefix-LSAs for Area 1’s network segments and floods them into Area 0.

R2 also receives and floods Inter-Area-Prefix-LSAs from Area 0 into Area 1.

Therefore, Inter-Area-Prefix-LSAs describing routes in Area 1 exist in both Area 0 and Area 1, making the statement true.

Conclusion:

The statement is true because Inter-Area-Prefix-LSAs generated by R2 for Area 1’s network segments are flooded into both Area 0 and Area 1, and Area 1, being a common area, allows these LSAs to exist within it.

References to HCIP-Datacom-Advanced Routing & Switching Technology Documents:

HCIP-Datacom-Advanced Routing & Switching Technology V1.0, Section on OSPFv3: OSPF Area Types, Inter-Area Routing, and LSA Types (specifically Inter-Area-Prefix-LSA, Type 3 LSA).

HCIP-Datacom-Advanced Routing & Switching Technology V1.0, Chapter on OSPF Routing Protocols: ABR Behavior, Common Areas, and LSA Flooding Mechanisms.

Comprehensive and Detailed In-Depth Explanation:

1. Understanding MPLS Label Switching

MPLS (Multiprotocol Label Switching) operates by adding labels to packets to enable fast switching across an MPLS domain.

Labels are swapped at each router (LSR - Label Switch Router) based on the LFIB (Label Forwarding Information Base).

When an MPLS packet reaches a router, it checks the incoming label and swaps it with an outgoing label as per its label forwarding table.

The label value "3" is the implicit null label, which is used for PHP (Penultimate Hop Popping).

2. Analyzing the MPLS Label Flow in the Figure

At PE1: The packet enters the MPLS domain and is labeled with 1033.

At P1: P1 forwards the packet based on label 1033.

At P2:

P2 receives label 1033 and swaps it with label 3 (as per the figure).

Label 3 (implicit null) means that the label is removed before reaching PE2 (PHP - Penultimate Hop Popping).

This ensures that PE2 receives a pure IP packet without an MPLS label.

3. Evaluating Each Answer Option

Option A: "The label value is 3." → Incorrect.

Label 3 (implicit null) is not actually sent to PE2.

Instead, P2 removes the label before sending the packet to PE2.

Option B: "There is no label." → Correct.

Since P2 performs PHP (Penultimate Hop Popping), the label is removed, and PE2 receives only an IP packet.

Option C: "The label value is 1033." → Incorrect.

Label 1033 was used earlier in the MPLS path but was swapped out before reaching PE2.

Option D: "The label values are 3 and 1033." → Incorrect.

Only one label is present at a time.

Label 1033 was swapped for label 3, but label 3 was removed before reaching PE2.

Final Answer:

Answer: B (There is no label).

HCIP-Datacom-Advanced Routing & Switching Technology References:

MPLS Label Forwarding Mechanism

Penultimate Hop Popping (PHP) and Implicit Null Label (3)

MPLS Label Swapping and Label Forwarding Table (LFIB)

Comprehensive and Detailed In-Depth Explanation:

1. Understanding the Issue of OSPF Type 5 LSAs in a Dual-Homed CE Scenario

OSPF Type 5 LSAs (External LSAs) are used in OSPF to advertise external routes, such as those learned via BGP or redistributed from another protocol.

In a dual-homed CE setup, like the one in the figure, CE2 connects to both PE2 and PE3 using OSPF.

PE2 and PE3 both redistribute BGP routes into OSPF and advertise them to CE2.

CE2 then forwards these LSAs between PE2 and PE3, potentially causing a routing loop because each PE may re-learn its own previously advertised routes.

2. How Route Tags Prevent Loops in OSPF-BGP Redistribution

The route-tag command assigns a tag to redistributed routes.

When a router receives an external OSPF LSA (Type 5), it checks the route tag.

If the route tag matches the locally configured tag, the router ignores the route to prevent loops.

Example Configuration on PE Routers:

plaintext

CopyEdit

PE2:

router ospf 1

import-route bgp 100 route-tag 65001

PE3:

router ospf 1

import-route bgp 100 route-tag 65001

Both PE2 and PE3 tag their redistributed BGP routes with 65001.

If PE3 receives an LSA from CE2 with the tag 65001, it ignores it (because it knows it originally advertised it).

✅ This prevents OSPF from incorrectly preferring a route that was originally sourced from itself, avoiding routing loops.

3. Evaluating the Answer Choices

✅ TRUE: If the route tag of the received route matches the locally configured tag, the route is ignored, preventing loops.❌ FALSE: The statement is correct, so False is incorrect.

Thus, the correct answer is: A. TRUE

Final Conclusion:

✅ The statement is TRUE because route tagging is a valid method to prevent OSPF Type 5 LSA loops in dual-homed scenarios.

Comprehensive and Detailed In-Depth Explanation:

1. Understanding OSPFv3 and DR/BDR Election

In OSPFv3, on broadcast and non-broadcast networks, routers elect a Designated Router (DR) and a Backup Designated Router (BDR) to reduce LSA flooding.

The DR is responsible for generating Network LSAs (Type 2 LSAs) for the subnet.

The BDR takes over if the DR fails.

Other routers (DROther) only form adjacencies with the DR and BDR but still receive LSAs.

2. Understanding the LSA Output in the Figure

The LSA shown in the figure is a Type 1 (Router LSA).

The originating router is R2 (Router ID: 10.0.2.2).

The LSA lists two "Transit Network" links connected to R2.

The Neighbor Router IDs listed are 10.0.1.1 (R1) and 10.0.2.2 (R2).

3. Why the Statement is FALSE

A transit network (broadcast network) should have only ONE DR.

R1 and R2 both being DRs is incorrect, as only one DR per subnet should exist.

The presence of two "Transit Network" entries in the LSA suggests that there are two separate OSPF broadcast networks, each with its own DR.

This means R1 and R2 are each DRs on separate networks, but not both DRs on the same network.

Final Answer:

Answer: B (FALSE)

HCIP-Datacom-Advanced Routing & Switching Technology References:

OSPFv3 DR/BDR Election Process

OSPFv3 LSA Types and Their Roles

OSPFv3 Network Topology and LSA Interpretation

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