Huawei H12-821_V1.0 Practice Test - Questions Answers

The question indicates that a router performs a lookup in its FIB table for a packet and determines that the tunnel ID in the matching entry is 0, suggesting that the packet needs to be forwarded through a tunnel such as an MPLS tunnel. However, this is a misunderstanding of the FIB functionality.

FIB Table Overview

The Forwarding Information Base (FIB) is used to make packet-forwarding decisions. Entries in the FIB include next-hop information, which can be directly linked to an interface or a tunnel.

If the Tunnel ID is 0, it indicates that the packet is forwarded via a normal routing path and not through a tunnel.

For MPLS or other tunnels, the Tunnel ID would have a non-zero value pointing to the associated tunnel.

MPLS Tunnel Operation

When a router forwards packets through an MPLS tunnel, a label-switched path (LSP) is set up. The FIB would reflect specific tunnel identifiers for packets that need such encapsulation.

HCIP-Datacom-Core Reference

Routing Principles and MPLSexplain the forwarding mechanisms clearly, stating that if a packet is routed normally, the tunnel ID remains 0.

The section on MPLSclarifies the encapsulation process and the role of tunnel identifiers.

Hence, the claim in the question is incorrect. A Tunnel ID of 0 implies no tunneling is required, and normal IP forwarding occurs

The scenario describes a mismatch in OSPF network types between two connected routers: one set to Point-to-Point (P2P) and the other set to Point-to-Multipoint (P2MP). While aligning Hello intervals may seem sufficient for establishing an OSPF neighbor relationship, the fundamental mismatch in network types introduces issues.

OSPF Network Types

P2P: Assumes a direct connection with a single neighbor, uses faster convergence and simpler LSDB synchronization.

P2MP: Supports multiple neighbors on a single interface, requiring different handling for DR/BDR roles and LSDB updates.

Impact of Network Type Mismatch

If Hello intervals are aligned, adjacency establishment might occur. However, mismatched network types affect neighbor role assignment and LSDB synchronization.

P2P expects a direct link and would handle updates differently than P2MP, which assumes multiple neighbors. This leads to inconsistencies in route calculation and forwarding.

HCIP-Datacom-Core Reference

OSPF Basics and Configurationclearly outlines the criticality of consistent network type configuration for stable OSPF operation.

Lab examples in the HCIP Datacom Lab Guidefurther demonstrate the consequences of such mismatches, including unstable neighbor states and incomplete LSDB synchronization.

Hence, the statement that neighbor relationships and LSDB synchronization remain unaffected is incorrect. Proper OSPF operation requires matching network types in addition to aligned Hello intervals.

When OSPF routers have interfaces on different network segments with different subnet masks, the network type can be adjusted to establish adjacency. A point-to-point (P2P) network type eliminates the requirement for matching subnet masks by treating the link as directly connected without intermediate devices.

P2P Network Characteristics

OSPF treats the link as a direct connection between two routers.

No DR/BDR election occurs, simplifying adjacency establishment.

Subnet mask differences do not hinder neighbor relationships as the link is viewed as a logical tunnel.

HCIP-Datacom-Core Reference

The OSPF configuration section explicitly mentions P2P as a suitable network type for resolving adjacency issues caused by mismatched subnet masks.

OSPF Area Design and Loop Prevention:

OSPF uses a hierarchical structure with areas to improve scalability and efficiency. Area 0, the backbone area, plays a crucial role in ensuring loop-free route distribution between areas. The following mechanisms are key to preventing routing loops:

Strict adherence to hierarchical area design.

Prohibition of direct inter-area route exchanges between non-backbone areas.

Analysis of Each Statement:

A . Inter-area routes cannot be directly transmitted between non-backbone areas.

This statement is TRUE. OSPF mandates that all inter-area routing must pass through Area 0. Direct inter-area route exchanges between two non-backbone areas are not allowed to prevent loops.

B . All non-backbone areas must be directly connected to area 0.

This statement is TRUE. OSPF requires every non-backbone area to connect directly to Area 0 to facilitate loop-free inter-area routing. Virtual links may be configured in exceptional cases where direct connection is not possible.

C . Inter-area routes need to be forwarded through area 0.

This statement is TRUE. All inter-area traffic must traverse Area 0 to ensure hierarchical routing and loop prevention. This rule is a core design principle of OSPF.

D . An ABR cannot inject Type 3 LSAs that describe routes to a network segment in an area back to the same area.

This statement is TRUE. OSPF explicitly prohibits an ABR from injecting Type 3 LSAs describing a route into the same area where the route originates. This mechanism prevents routing loops within an area.

Conclusion:

All options (A, B, C, D) are correct. OSPF enforces a robust loop prevention mechanism through hierarchical routing, mandatory traversal via Area 0, and strict rules on LSA propagation by ABRs. This ensures reliable and loop-free inter-area routing in OSPF networks.

OSPF Authentication Overview

The Auth Type field in the OSPF packet header determines the authentication mode. If the Auth Type is 1, plaintext authentication is used.

Plaintext authentication involves transmitting the password in an easily readable format, which is less secure compared to MD5.

HCIP-Datacom-Core Reference

Authentication mechanisms, including plaintext authentication, are detailed in the OSPF security configuration chapter, confirming that Auth Type = 1 corresponds to plaintext.

OSPF LSA Flooding Mechanism

If a router receives an LSA identical to one already in its LSDB, it does not flood the LSA again unless the LSA has changed (i.e., the sequence number or content has been updated).

OSPF ensures efficient use of bandwidth by avoiding redundant flooding of unchanged LSAs.

HCIP-Datacom-Core Reference

The OSPF LSDB synchronization process explains that unchanged LSAs are not reflooded, ensuring stability and resource optimization.

OSPF Authentication Overview

OSPF supports three authentication modes:

Null Authentication: No authentication (default).

Plaintext Authentication: Uses clear-text passwords.

MD5 Authentication: Secure cryptographic authentication.

Interface-Level Priority

When both interface-level and area-level authentication are configured, OSPF prioritizes interface-level authentication. This ensures that interface-specific security overrides area-wide configurations for greater granularity and security.

HCIP-Datacom-Core Reference

OSPF authentication hierarchy and configurations are detailed in the OSPF security configuration chapter.

Silent Interface Explanation

The silent-interface command is used to prevent OSPF from sending or receiving OSPF packets on the specified interface (GE 0/0/1). This disables OSPF adjacency establishment and stops route advertisement for that interface.

Network Observations

Statement A: R2 can access the server.

This is correct, as the silent interface does not impact data traffic, only OSPF-related communication.

Statement B: GE 0/0/1 of R1 cannot send OSPF packets.

Correct due to the silent-interface configuration.

Statement C: The network segment to which GE 0/0/1 of R1 belongs cannot be advertised.

This is correct, as the silent interface prevents route advertisement.

Statement D: GE 0/0/1 of R1 cannot accept OSPF packets.

Correct, as the silent interface configuration blocks packet reception.

HCIP-Datacom-Core Reference

OSPF interface command behavior is outlined in the configuration and lab examples sections.

Understanding Router ID and Its Role in OSPF:

In OSPF, the Router ID uniquely identifies a router within the OSPF domain. If two routers are configured with the same Router ID, it can lead to issues such as LSA conflicts and flapping. This is because the Router ID is used as a key in OSPF operations, including LSA generation and database synchronization.

Scenario Details:

Different Areas: Even if the two routers belong to different areas, the Router ID remains globally significant in the OSPF domain. This means that any duplication of Router IDs will confuse OSPF mechanisms.

ASBR (Autonomous System Boundary Router): When one of the routers is an ASBR, it generates Type 4 and Type 5 LSAs to describe external routes. These LSAs use the Router ID as an identifier. If another router in the network has the same Router ID, conflicts occur during LSDB synchronization.

Impact of Router ID Duplication:

LSA Flapping: The OSPF process receives conflicting LSAs from routers with the same Router ID. This results in continuous updates and withdrawals of these LSAs, causing flapping.

Routing Instability: LSA flapping leads to frequent recalculations of the OSPF shortest path tree (SPT), affecting overall network stability.

Conclusion:

The statement is TRUE. LSA flapping occurs when two routers in an OSPF network have the same Router ID, even if they are in different areas and one is an ASBR. This is due to the global significance of Router IDs in OSPF and the role they play in LSA generation and propagation.