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Question 141.

In OSPF, what is the main purpose of route filtering on an ABR?

1. Control which inter-area routes are advertised between OSPF areas
2. Change Ethernet duplex settings
3. Create MPLS labels
4. Elect the VRRP master

Correct Answer: 1. Control which inter-area routes are advertised between OSPF areas

Explanation:

Route filtering on an OSPF ABR can be used to control which inter-area prefixes are advertised between areas. This helps administrators limit unnecessary routing information, enforce network design boundaries, and reduce the risk of unwanted route propagation. Filtering must be designed carefully because overly restrictive policies can make valid destinations unreachable. OSPF filtering is a routing-control function and does not affect Ethernet duplex, MPLS label creation, or VRRP master election.

Question 142.

Two OSPF routers can ping each other but do not become neighbors. Which information should be checked FIRST?

1. BGP MED values
2. OSPF area, Hello/Dead timers, authentication, and network type
3. MPLS Route Targets
4. VRRP priority

Correct Answer: 2. OSPF area, Hello/Dead timers, authentication, and network type

Explanation:

IP reachability alone does not guarantee that an OSPF adjacency will form. Neighboring routers must have compatible OSPF parameters, including the area ID, Hello and Dead intervals, authentication settings, and appropriate network-type behavior. Addressing and MTU-related issues may also affect adjacency formation. Administrators should compare the OSPF interface settings on both routers before investigating unrelated protocols. BGP, MPLS VPN, and VRRP parameters do not determine basic OSPF neighbor establishment.

Question 143.

What is the main purpose of the OSPF SPF calculation?

1. Assign addresses to hosts
2. Create a BGP AS_Path
3. Calculate the shortest paths from the local router using the link-state database
4. Build Ethernet trunks

Correct Answer: 3. Calculate the shortest paths from the local router using the link-state database

Explanation:

OSPF maintains a link-state database describing the topology of an area. The router runs the Shortest Path First algorithm against this database to calculate the lowest-cost paths to reachable destinations. The resulting paths are then considered for installation in the routing table. When topology changes occur, relevant LSAs are flooded and SPF may run again. SPF is central to OSPF route calculation and is unrelated to address assignment or Layer 2 aggregation.

Question 144.

What is the purpose of OSPF LSA aging?

1. Increase interface bandwidth
2. Select a BGP route reflector
3. Configure multicast groups
4. Prevent stale link-state information from remaining indefinitely in the database**

Correct Answer: 4. Prevent stale link-state information from remaining indefinitely in the database

Explanation:

OSPF LSAs include age information so obsolete topology entries do not remain permanently in the link-state database. As LSAs age, they are refreshed by their originators when still valid. If an LSA reaches its maximum age without being refreshed, it can be removed from the database. This mechanism helps keep OSPF topology information current and prevents routers from indefinitely relying on stale network state. LSA aging is a control-plane maintenance function rather than a bandwidth or multicast feature.

Question 145.

Which BGP attribute is most commonly used inside an autonomous system to prefer one internet exit over another?

1. Local Preference
2. Originator ID
3. Cluster List
4. Router ID only

Correct Answer: 1. Local Preference

Explanation:

Local Preference is distributed to iBGP peers and is commonly used to influence outbound traffic. Higher Local Preference values are generally preferred, allowing administrators to designate one provider or external exit as the preferred path. For example, routes learned from a primary ISP can receive a higher Local Preference than routes learned from a backup ISP. Other attributes can still influence selection depending on the decision process, but Local Preference is one of the most direct tools for controlling outbound BGP routing.

Question 146.

A company advertises the same prefix through two eBGP links and wants one link to be less attractive to external networks. Which technique can help?

1. Reduce the VLAN ID
2. AS_Path prepending
3. Increase VRRP priority
4. Change OSPF router ID

Correct Answer: 2. AS_Path prepending

Explanation:

AS_Path prepending makes a selected BGP advertisement appear to traverse more autonomous systems by adding repeated instances of the local AS number. External routers commonly prefer shorter AS paths when more influential attributes are equal, so the prepended route can become less attractive. This is a common inbound traffic-engineering technique. However, external networks may use policies that override AS_Path length, so prepending influences rather than guarantees remote path selection.

Question 147.

What is the main purpose of a BGP route reflector cluster?

1. Provide DHCP redundancy
2. Create an MSTP region
3. Support scalable iBGP route reflection and help prevent reflection loops
4. Replace an autonomous system number

Correct Answer: 3. Support scalable iBGP route reflection and help prevent reflection loops

Explanation:

Route reflectors reduce the need for a full mesh of iBGP sessions. In more complex designs, cluster-related information helps route reflectors identify reflected routes and avoid routing loops caused by repeated reflection. Route reflector clusters can also support redundant route reflectors serving the same client group. This mechanism belongs specifically to iBGP scaling and does not provide DHCP or spanning-tree functionality.

Question 148.

What is the purpose of the BGP Cluster_List attribute?

1. Identify VLAN membership
2. Set OSPF cost
3. Assign MPLS labels
4. Record route reflector cluster information to help prevent reflection loops**

Correct Answer: 4. Record route reflector cluster information to help prevent reflection loops

Explanation:

When a BGP route is reflected, route reflector cluster information can be carried in the Cluster_List attribute. A route reflector can examine this information to determine whether the route has already passed through its own cluster and reject it if necessary. This helps prevent routing loops in route-reflector topologies. The attribute is part of BGP’s route-reflection mechanism and is unrelated to VLAN configuration, OSPF metrics, or MPLS forwarding labels.

Question 149.

What is the main purpose of BGP route dampening where it is used?

1. Reduce instability caused by repeatedly flapping routes
2. Increase interface speed
3. Create VRRP groups
4. Replace route filtering

Correct Answer: 1. Reduce instability caused by repeatedly flapping routes

Explanation:

Route dampening is intended to reduce the impact of unstable prefixes that repeatedly appear and disappear. A route that flaps frequently can accumulate a penalty, and if the penalty exceeds a threshold, the route may be temporarily suppressed. As the penalty decays over time, the route can become usable again. Dampening must be applied carefully because overly aggressive parameters can delay recovery of legitimate routes. It is a stability mechanism rather than a substitute for route filtering.

Question 150.

Why is an IGP commonly used to provide reachability between iBGP loopback addresses?

1. To create Layer 2 VLANs
2. To maintain stable reachability for BGP sessions across redundant internal paths
3. To assign BGP communities
4. To eliminate all routing tables

Correct Answer: 2. To maintain stable reachability for BGP sessions across redundant internal paths

Explanation:

iBGP sessions are often established using loopback addresses because loopbacks are not tied to a single physical interface. An IGP such as OSPF or IS-IS advertises those loopbacks and provides reachability through whatever internal path is currently available. If one physical link fails, the IGP can converge to another path while the loopback address remains unchanged. This separation improves resilience and makes the BGP control plane less dependent on individual physical links.

Question 151.

Which IS-IS PDU is used to discover and maintain neighbor adjacencies?

1. IS-IS Hello PDU
2. BGP Update
3. OSPF Database Description packet
4. MPLS Echo Request only

Correct Answer: 1. IS-IS Hello PDU

Explanation:

IS-IS Hello PDUs are exchanged between neighboring IS-IS routers to discover peers and maintain adjacency state. Different forms are used depending on whether the connection is broadcast or point-to-point and whether Level-1 or Level-2 operation is involved. If expected Hellos stop arriving, the adjacency can time out and the topology can be recalculated. Hello PDUs therefore perform a fundamental neighbor-maintenance role in IS-IS.

Question 152.

In IS-IS, what is the primary purpose of the Link State Database?

1. Store DHCP leases
2. Maintain topology information learned through IS-IS LSPs
3. Hold VRRP virtual IP addresses
4. Store only BGP communities

Correct Answer: 2. Maintain topology information learned through IS-IS LSPs

Explanation:

IS-IS routers build a Link State Database from the LSPs they originate and receive. The database represents the network topology for the relevant routing level and includes information such as links, metrics, and reachable prefixes. Routers run SPF against this database to calculate best paths. Because IS-IS is a link-state protocol, consistency of the LSDB among routers in the same level is essential for predictable routing.

Question 153.

What is the purpose of MPLS penultimate hop popping?

1. Add an additional service label
2. Increase the BGP AS_Path
3. Remove the outer transport label one hop before the egress router when appropriate
4. Disable label switching

Correct Answer: 3. Remove the outer transport label one hop before the egress router when appropriate

Explanation:

Penultimate hop popping allows the router immediately before the egress LSR to remove the outer MPLS transport label. This reduces the amount of label processing required at the egress router for that outer label. In MPLS VPN services, an inner service label may still remain so the egress PE can identify the appropriate VPN context. PHP is therefore an MPLS forwarding optimization rather than a method for disabling label switching.

Question 154.

In an MPLS Layer 3 VPN, which component keeps routes from different customers in separate forwarding tables on a PE router?

1. OSPF DR
2. LACP
3. STP region
4. VPN instance or VRF**

Correct Answer: 4. VPN instance or VRF

Explanation:

A VPN instance, commonly referred to as a VRF, provides a separate routing and forwarding context for a customer or VPN. Each VRF can contain its own routes and associated interfaces, allowing multiple customers to use overlapping address spaces without conflict. Route Distinguishers make VPN prefixes unique in the provider control plane, while Route Targets control import and export policy. The VRF itself provides the local routing-table separation on the PE router.

Question 155.

Which MPLS Layer 3 VPN attribute makes identical customer prefixes unique when carried by MP-BGP?

1. Route Distinguisher
2. VRRP priority
3. OSPF cost
4. STP path cost

Correct Answer: 1. Route Distinguisher

Explanation:

Customers in different VPNs may use the same IPv4 prefixes. A Route Distinguisher is combined with each customer prefix to form a unique VPN route so overlapping addresses can coexist in the provider’s MP-BGP control plane. The Route Distinguisher does not decide which VRFs should import the route; Route Targets perform that policy function. Keeping these concepts separate is important when troubleshooting MPLS Layer 3 VPN routing.

Question 156.

What is the main purpose of a VRRP virtual IP address?

1. Identify a BGP route reflector
2. Provide hosts with a stable default gateway independent of which router is currently master
3. Create an MPLS label
4. Advertise an OSPF summary route

Correct Answer: 2. Provide hosts with a stable default gateway independent of which router is currently master

Explanation:

Hosts use the VRRP virtual IP address as their default gateway. The router currently acting as VRRP master handles traffic sent to that virtual gateway, while a backup can assume the role if the master fails. Because the host configuration does not need to change during failover, VRRP provides transparent first-hop redundancy. Tracking and preemption features can further improve the behavior of the gateway pair during upstream failures and recovery.

Question 157.

A network has an Eth-Trunk with several active member links. How is traffic commonly distributed across the members?

1. By a hashing algorithm based on selected packet fields
2. Every packet is copied onto every link
3. Only one link can ever forward traffic
4. By OSPF LSA type

Correct Answer: 1. By a hashing algorithm based on selected packet fields

Explanation:

Link aggregation usually distributes traffic across active member links using a hash calculated from selected fields such as source and destination MAC addresses, IP addresses, or transport information. This approach helps keep packets from the same flow on a consistent member and avoids unnecessary packet reordering. It also means one individual flow may not use the full aggregate bandwidth of all members. Eth-Trunk provides aggregate capacity across multiple flows rather than duplicating every packet.

Question 158.

What is the main purpose of QoS traffic classification?

1. Increase physical interface bandwidth
2. Identify traffic so different policies can be applied to different classes
3. Replace routing protocols
4. Elect an MPLS egress router

Correct Answer: 2. Identify traffic so different policies can be applied to different classes

Explanation:

Traffic classification determines which packets belong to specific QoS classes. Classification may use addresses, protocols, ports, DSCP values, or other packet characteristics. Once classified, traffic can be marked, policed, shaped, queued, or dropped according to policy. Classification does not create additional bandwidth, but it allows available resources to be allocated in a controlled way during congestion. A consistent classification design is essential for end-to-end QoS behavior.

Question 159.

Which technology can provide rapid forwarding-failure detection for OSPF, IS-IS, or BGP?

1. BFD
2. DHCP relay
3. Route Distinguisher
4. MSTP only

Correct Answer: 1. BFD

Explanation:

Bidirectional Forwarding Detection is designed to detect failures in the forwarding path rapidly. Routing protocols can use BFD status to react faster than they would using their normal Hello or keepalive timers alone. Once BFD reports a failure, OSPF, IS-IS, or BGP can withdraw affected routes or recalculate paths according to its own logic. BFD does not replace the routing protocol; it complements it by providing fast liveness information.

Question 160.

Which design BEST supports scalability and resiliency in a large Huawei enterprise or service-provider network?

1. Place all routing protocols into one uncontrolled redistribution domain
2. Disable redundant paths to simplify configuration
3. Advertise every specific prefix everywhere
4. Use hierarchical design, structured addressing, route policies, redundancy, summarization, fast failure detection, and continuous monitoring**

Correct Answer: 4. Use hierarchical design, structured addressing, route policies, redundancy, summarization, fast failure detection, and continuous monitoring

Explanation:

Large networks are most reliable when routing and services are designed deliberately. Hierarchical topology and structured addressing support summarization and smaller routing domains. Route policies control which prefixes are accepted or advertised, while redundancy and BFD improve resilience and convergence. Technologies such as OSPF, IS-IS, BGP, MPLS, VRRP, and QoS should each have clearly defined roles. Continuous monitoring helps operators identify failures and policy problems before they become widespread service disruptions.