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

Comprehensive and Detailed In-Depth Explanation:

1. Understanding MPLS Label Forwarding Information

The output of the display mpls lsp command shows the Label Forwarding Information Base (LFIB) used by the router to forward MPLS packets.

Each entry consists of:

FEC (Forwarding Equivalence Class): The destination IP prefix.

In Label / Out Label: Incoming label (received) and outgoing label (sent).

In IF / Out IF: Incoming and outgoing interfaces.

2. Analyzing the Given LSP Table

FEC (Destination IP)

In Label

Out Label

Outgoing Interface

1.1.1.1/32

3

NULL

-

2.2.2.2/32

NULL

3

GE0/0/1

2.2.2.2/32

1026

3

GE0/0/1

3.3.3.3/32

NULL

1027

GE0/0/1

3.3.3.3/32

1027

1027

GE0/0/1

Key Observations:

NULL means the label is removed (Penultimate Hop Popping - PHP).

Label 3 is the implicit null label, meaning the label is removed before sending to the next router.

For destination 3.3.3.3, the router adds label 1027 before forwarding it to GE0/0/1.

3. Evaluating Each Answer Option

Option A: "Before forwarding a packet destined for 2.2.2.2, the device adds label 3 to the packet." → Incorrect.

The output shows that packets destined for 2.2.2.2 have the NULL (label 3) entry, which means the label is removed before forwarding, not added.

Option B: "Before forwarding a packet destined for 1.1.1.1, the device adds label 3 to the packet." → Incorrect.

The entry 1.1.1.1 → 3/NULL means the label is removed (PHP), so the device does not add label 3.

Option C: "Before forwarding a packet destined for any IP address, the device removes labels from the packet." → Incorrect.

The device removes labels only for some destinations (e.g., 1.1.1.1 and 2.2.2.2), but it adds label 1027 for 3.3.3.3.

Not all IPs have their labels removed.

Option D: "Before forwarding a packet destined for 3.3.3.3, the device adds label 1027 to the packet." → Correct.

The entry 3.3.3.3 → NULL/1027 means that the router adds label 1027 before forwarding packets to GE0/0/1.

Final Answer:

Answer: D (Before forwarding a packet destined for 3.3.3.3, the device adds label 1027 to the packet).

HCIP-Datacom-Advanced Routing & Switching Technology References:

Understanding MPLS LDP (Label Distribution Protocol) and LFIB

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

MPLS Label Swapping and Forwarding Process

LSAs and Their Origin Router Associations:

Link-LSA: Origin Router = 10.0.1.1

Intra-Area-Prefix-LSA: Origin Router = 10.0.4.4

Network-LSA: Origin Router = 10.0.3.3

Inter-Area-Prefix-LSA: Origin Router = 10.0.1.1

Comprehensive and Detailed In-Depth Explanation:

In IS-IS (Intermediate System to Intermediate System), each LSP (Link-State PDU) is identified by an LSP ID.

The LSP Number field is 4 bits long, meaning a single IS-IS process can generate up to 16 LSP fragments per system ID.

Since each fragment carries a limited amount of information, a router may exceed this limit if it needs to advertise a large number of routes.

Solution: By adding additional system IDs, an IS-IS router can generate more LSP fragments, thereby overcoming the limitation.

This method is commonly used in large-scale IS-IS networks to ensure scalability and efficient routing information exchange.

???? Reference: HCIP-Datacom Advanced Routing & Switching Technology – IS-IS LSP Fragmentation Mechanism

Comprehensive and Detailed In-Depth Explanation:

1. Understanding the DIS (Designated Intermediate System) in IS-IS

In IS-IS, the DIS (Designated Intermediate System) is similar to the DR (Designated Router) in OSPF.

Unlike OSPF, IS-IS does not use an election based on priority; instead, the router with the highest priority becomes the DIS.

If there is a tie in priority, the router with the highest MAC address on the interface becomes the DIS.

The DIS is responsible for generating additional LSPs (pseudonode LSPs) for the link and synchronizing the database between routers.

2. How to Identify If the Router Is the DIS from the LSP

In the given LSP output, there are NO pseudonode LSPs (LSPs ending with .01).

The DIS is responsible for creating pseudonode LSPs, which represent a multi-access network in the IS-IS topology.

If the router were the DIS, it would generate both its own LSP (ending in .00) and a pseudonode LSP (ending in .01).

Since we only see an LSP ending in .00, this confirms that the router is NOT the DIS.

3. Evaluating the Answer Choices

Option A (TRUE) – Correct:

Since no pseudonode LSP is present, the router is not the DIS.

This confirms that the statement is TRUE.

Option B (FALSE) – Incorrect:

If the router were the DIS, it would generate pseudonode LSPs, but they are missing from the output.

Therefore, the statement is NOT false.

Final Answer:

Answer: A (TRUE)

HCIP-Datacom-Advanced Routing & Switching Technology References:

IS-IS Designated Intermediate System (DIS) Selection Process

Pseudonode LSP Generation in Multi-Access Networks

IS-IS LSP Structure and Identification of DIS

Comprehensive and Detailed In-Depth Explanation:

LSR ID (Label Switch Router ID) uniquely identifies an MPLS router in the network.

By default, the LSR ID is the IP address of a physical interface on the router.

Best Practice: Manually configuring the LSR ID to use a loopback interface address improves stability and reachability in MPLS networks.

The loopback interface is always up, ensuring that the LSR ID remains accessible even if physical interfaces go down.

???? Reference: HCIP-Datacom Advanced Routing & Switching Technology – MPLS LSR ID Configuration

Comprehensive and Detailed In-Depth Explanation:

1. Understanding the Hub-Spoke BGP Design

Hub-Spoke topology is commonly used in MPLS Layer 3 VPNs, where:

Hub-PE acts as a central router.

Spoke-PEs connect to the hub but do not communicate directly with each other.

eBGP (External BGP) is used between the Hub-PE and Spoke-PE routers.

2. The Role of AS Numbers in BGP Route Transmission

By default, BGP prevents routes from being sent back to the same AS (Autonomous System) to avoid loops (via the AS_PATH loop prevention mechanism).

However, in a Hub-Spoke MPLS VPN, the Hub-PE must be allowed to re-advertise routes learned from one spoke to another spoke.

This requires allowing repetitive AS numbers within the same AS to avoid BGP rejecting valid routes.

3. Solution: Using "Allow-AS Loop" on the Hub-PE

The allowas-in or allow-as-loop command must be used on Hub-PE to allow repetitive local AS numbers.

This ensures that routes from Spoke-PE1 (AS65002) are advertised to Spoke-PE2 (AS65003) via Hub-PE, even if AS numbers are repeated.

Example Configuration on Hub-PE:

plaintext

CopyEdit

bgp 65001

peer Spoke-PE1 allow-as-loop 2

peer Spoke-PE2 allow-as-loop 2

This allows Hub-PE to accept and propagate routes even if the AS number appears more than once.

4. Evaluating the Answer Choices

Option

Correct?

Reasoning

A. TRUE

✅ Correct

Hub-PE can be configured to allow repetitive AS numbers using the allow-as-loop feature.

B. FALSE

❌ Incorrect

BGP by default prevents repeated AS numbers, but this can be overridden.

✅ Correct Answer: A. TRUE

Final Conclusion:

By default, BGP prevents AS loopbacks.

Hub-PE needs to allow repetitive AS numbers for proper route transmission in the hub-spoke model.

The "allow-as-loop" command enables this functionality.

Thus, the correct answer is: TRUE.

Comprehensive and Detailed In-Depth Explanation:

1. Understanding the Network Topology

R1 and R2 are OSPF-enabled routers connected via the 10.0.12.0/24 subnet (Area 0).

PC1 (192.168.1.2/30) is connected to R1 via GE0/0/1.

PC2 (192.168.1.22/30) is connected to R2 via GE0/0/1.

OSPF ensures that R1 and R2 know about each other's directly connected networks.

2. Analyzing Connectivity

Can R1 ping 192.168.1.22 (PC2)?

Yes, if routing is properly configured.

OSPF ensures that R1 learns about 192.168.1.22 from R2.

Since R1 has a route to 192.168.1.22 via R2, it can ping PC2 successfully.

✅ Option A is correct.

Can R1 ping 192.168.1.21 (R2's interface)?

Yes, since R1 and R2 are OSPF neighbors.

R1 learns 192.168.1.21 via OSPF and can reach it directly.

✅ Option B is correct.

Can R2 ping 192.168.1.2 (PC1)?

Yes, since OSPF ensures that R2 learns about 192.168.1.2 from R1.

Since R2 has a route to 192.168.1.2 via R1, it can ping PC1 successfully.

✅ Option C is correct.

Can PC1 and PC2 ping each other?

Yes, if default gateways and routing are configured correctly.

If R1 and R2 can ping each other and forward packets, PC1 and PC2 should be able to communicate.

❌ Option D is incorrect because PC1 and PC2 should be able to ping each other.

Final Answer:

✅ A, B, and C are correct.

HCIP-Datacom-Advanced Routing & Switching Technology References:

OSPF Inter-Router Communication and Route Advertisement

Default Gateway and Routing in Multi-Router Networks

Troubleshooting End-to-End Connectivity in Routed Networks