Pass the Huawei HCIE-Datacom H12-891_V1.0 Questions and answers with ExamsMirror

Understanding BFD (Bidirectional Forwarding Detection)

BFDis ahigh-speed fault detection protocolused to monitor link failures inmilliseconds.

It operates atLayer 2 (Data Link Layer) and Layer 3 (Network Layer)to providefast convergence.

Why Can BFD Detect Failures in Milliseconds?

Useslightweight control packetsthat are exchanged between routers.

Works withOSPF, IS-IS, BGP, and MPLSto trigger fast convergence.

Detection time = (Transmit Interval) × (Detect Multiplier)

Example: If the interval is3msand the multiplier is3, failure detection takes9ms.

Why is the Answer TRUE?

✅BFD achieves millisecond-level detectionby sending rapid probe packets.

✅Used in high-availability networkswhere link failures must be detected inreal-time.

Real-World Application

Service Provider Networks:BFD improves network resilience by detecting link failuresfaster than traditional routing timers.

Enterprise WANs:Used inSD-WANsto switch paths dynamically.

MPLS Fast Reroute (FRR):Works withMPLS Traffic Engineeringfor near-instantaneous failover.

✅Reference:Huawei HCIE-Datacom Guide – BFD for Fast Link Failure Detection

OSPFv3 Configuration Principles:

OSPFv3, the OSPF version supporting IPv6, differs from OSPFv2 in several aspects:

Interface-based Configuration:

OSPFv3 is configured on interfaces directly, unlike OSPFv2 which uses the network command.

Router ID:

The Router ID is still a 32-bit number, typically represented as an IPv4 address.

It must be explicitly specified since OSPFv3 requires a Router ID for operation, even though it supports IPv6.

Analysis:

A. [R1-GigabitEthernet0/0/1] ospfv3 1 area 0:

Correct. This command enables OSPFv3 process 1 on the GE0/0/1 interface in Area 0.

B. [R1-ospfv3-1] router-id 10.1.1.1:

Correct. This correctly sets the Router ID for OSPFv3 process 1.

C. [R1-ospfv3-1-area-0.0.0.0] network 2001:DB8:2345:12::1 ::

Incorrect. OSPFv3 does not use the network command as in OSPFv2. Instead, OSPFv3 is directly configured on interfaces.

D. [R1] router id 10.1.1.1:

Incorrect. The Router ID should be set within the OSPFv3 process view, not the global configuration view.

Therefore, the correct answers are A and B.

Comprehensive and Detailed In-Depth Explanation:

WhenIS-ISis used as theIGP (Interior Gateway Protocol)to advertiseSegment Routing (SR) SIDs, theLevel-2 (L2) routeradvertisesnode SIDs globally across the entire IS-IS domain, not just within a single area.

Level-1 (L1) routersonly advertise within the local area, butL2 routersdistribute information across multiple areas, enabling inter-area reachability.

Therefore, the statement isFALSE.

Comprehensive and Detailed In-Depth Explanation:

In theinter-AS MPLS VPN Option Bsolution:

Option Bestablishes amulti-hop eBGPconnection between the ASBRs (ASBR-PE1 and ASBR-PE2) to exchangeVPNv4 routes.

In this setup, theASBRs exchange labeled VPNv4 routesviaMP-BGP(Multiprotocol BGP).

The command:

[ASBR-PE1-bgp-af-vpnv4] undo peer 10.0.34.4 enable

is used todisable VPNv4 route exchangebetween theASBR-PE1andASBR-PE2.

If this command is executed,ASBR-PE1willnot transmit VPNv4 routestoASBR-PE2, breaking theinter-AS VPN connectivity.

Therefore, the statement isFALSEbecauseASBR-PE1andASBR-PE2mustexchange VPNv4 routes to maintain connectivity between the VPN sites across different ASes.

Comprehensive and Detailed In-Depth Explanation:

Thetelemetry protocol stackis structured as follows:

Communication layer (A):Managesdata transmissionbetween the telemetry client (device) and the server.

Transport layer (B):Uses protocols such asgRPC, HTTP/2, or TCPto transmit telemetry data.

Data encoding layer (C):Encodes data using formats likeJSON, Protobuf, or XML.

TheData analysis layer (D)isnot part of the telemetry protocol stackitself.

Data analysis typically occursafter the data has been collected and receivedon the server or collector.

Telemetry focuses ondata collection, encoding, transport, and transmission, not on analysis.

Therefore, the correct answer isD.

InSRv6 (Segment Routing over IPv6),flavorsare extensions to standard SRv6 instructions that define how theSegment Routing Header (SRH)is handled at specific points:

PSP (Penultimate Segment Pop): Instructs thepenultimate nodeto remove the SRH before reaching the last node →✅

USD (Ultimate Segment Decapsulation): Instructs thefinal segmenttoremove the SRH and IPv6 header, exposing the payload →✅

PSD (Penultimate Segment Decapsulation): Instructs thepenultimate nodeto remove the SRH and deliver the payload →✅

USP (Ultimate Segment Pop)isnot a standard flavorin Huawei SRv6 implementations →❌

Correct answers: A, B, D

Understanding OSPF Neighbor States

OSPF (Open Shortest Path First) establishes neighbor relationships through several states:

1️⃣Down:No Hello packets received.

2️⃣Init:Routerreceives Hello packetsbutits own Router ID is missingin the received packets.

3️⃣2-Way:Routerssee each other in Hello packets. (Stable state for non-DR/non-BDR routers).

4️⃣ExStart:Routersstart exchanging LSAsbut may faceMTU mismatches.

5️⃣Exchange:Routers sendDatabase Description (DBD) packetsto synchronize link-state databases.

6️⃣Loading:Routers exchangeLink-State Requests (LSRs) & Updates (LSUs)to fully sync.

7️⃣Full:Routers are fully synchronized.

Analysis of the Answer Choices

✅A. For non-DR and non-BDR routers on an Ethernet link, the 2-Way state is acceptable.

Correct:Non-DR/Non-BDR routers remain in2-Way statesince they don’t need to form full adjacency.

✅B. The Init state indicates that the router has received Hello packets from its neighbors, but the received Hello packets do not contain the OSPF RID of the receiving router.

Correct:If an OSPF router receives a Hello butdoes not see its own Router ID, it stays in theInit state.

✅C. The ExStart state indicates that two neighbor routers have inconsistent MTUs or the same OSPF Router ID (RID).

Correct:If MTUs mismatch or RIDs conflict, theDBD exchange fails, causing routers to remain inExStart state.

❌D. The Attempt state indicates that the router does not send unicast Hello packets to configured neighbors.

Incorrect:

InAttempt state, routerssend unicast Hello packetsto manually configured neighbors (common inNBMA networks).

If no Hello responses are received, they remain inAttempt state, but theyDO send Hellos.

Real-World Application

OSPF Troubleshooting:Identifying routers stuck inInit or ExStart statescan point toMTU mismatches or neighbor misconfigurations.

NBMA Networks (Frame Relay, DMVPN):Manually configured neighbors may stay inAttempt state if Hello packets are lost.

✅Reference:Huawei HCIE-Datacom Study Guide – OSPF Neighbor States & Troubleshooting

VRRP (Virtual Router Redundancy Protocol) and Flapping

VRRP ensuresgateway redundancyby allowing multiple routers to work together as avirtual router.Flappingoccurs when themaster and backup routers frequently switch roles.

Causes of VRRP Flapping:

✅A. Too small interval between Advertisement packet transmissions–Can cause flapping

If theAdvertisement interval is too short, frequent VRRP state changes can occur.

✅C. Packet loss on an interface of the backup device–Can cause flapping

If thebackup router loses VRRP packets, it might assume themaster is downand initiate an election.

✅D. Flapping on the link transmitting VRRP Advertisement packets–Can cause flapping

If the link used for VRRPexperiences instability, packets may be lost, triggering VRRP role changes.

Incorrect Answer Explanation:

❌B. Inconsistent preemption delay settings – Does NOT cause VRRP flapping

Preemption delay only affectswhen a router takes over as masterafter recovering from a failure.

It doesnot directly cause flappingbecause the role change happens onlyafter a delay, preventing frequent transitions.

Reference from Huawei HCIE-Datacom Documentation:

Huawei VRRP Configuration Guide – Preventing VRRP Flapping

HCIE-Datacom Study Material – VRRP Timers and Stability

Understanding MPLS Label Retention Modes

In MPLS (Multiprotocol Label Switching), aLabel Switching Router (LSR)can operate in one of two label retention modes:

1️⃣Liberal Label Retention Mode (LLRM)

Stores all labels receivedfrom neighbors, even if they are not immediately used.

Advantage:Fast convergenceduring link failures.

Disadvantage:Consumes more memory & label space.

2️⃣Conservative Label Retention Mode (CLRM)

Only keeps labels for the best next-hop path.

Advantage:Saves memory & label space.

Disadvantage:Slower convergence when topology changes.

Why are C and D Correct?

✅C. An LSR reserves all labels distributed by its neighbor.

Correct:InLiberal Mode, the routeraccepts and stores all labelsreceived, even for non-best paths.

✅D. The liberal mode requires more memory and label space.

Correct:Since the routerkeeps all labels, it requireshigher memory & storage.

Why are A and B Incorrect?

❌A. An LSR retains labels from a neighboring LSR only when the neighbor is its next hop.

Incorrect:This describesConservative Mode, not Liberal Mode.

❌B. This label retention mode saves memory and label space.

Incorrect:Liberal Mode consumes more memory, not less.

Real-World Application

Liberal Mode is used in large-scale MPLS networks where fast failover is critical.

Conservative Mode is preferred in memory-constrained environments.

✅Reference:Huawei HCIE-Datacom Study Guide – MPLS Label Retention Modes