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Question 41.

In OSPF, which LSA type is generated by an ABR to advertise routes from one area into another area?

1. Type 3 Summary LSA
2. Type 1 Router LSA
3. Type 2 Network LSA
4. Type 5 AS External LSA

Correct Answer: 1. Type 3 Summary LSA

Explanation:

An Area Border Router generates Type 3 Summary LSAs to advertise networks from one OSPF area into another. This allows routers in different areas to learn inter-area destinations without maintaining the full topology database of every area. Type 1 LSAs describe an individual router’s links within an area, while Type 2 LSAs are generated for multi-access networks by the designated router. Type 5 LSAs describe external routes redistributed into OSPF by an ASBR. Type 3 LSAs are therefore fundamental to OSPF hierarchical inter-area routing.

Question 42.

Which OSPF router originates Type 5 LSAs for external routes?

1. Designated Router
2. Autonomous System Boundary Router
3. Backup Designated Router
4. Internal router only

Correct Answer: 2. Autonomous System Boundary Router

Explanation:

An Autonomous System Boundary Router, or ASBR, introduces routes from outside the OSPF routing domain and advertises them using Type 5 AS External LSAs in standard OSPF areas. These external routes may originate from BGP, static routing, or another routing protocol. An ABR connects OSPF areas but does not necessarily redistribute external routes. The DR and BDR serve functions on multi-access segments and do not automatically originate external LSAs. Understanding LSA origin is important when troubleshooting route redistribution into OSPF.

Question 43.

What is the primary characteristic of an OSPF stub area?

1. It must contain every external route
2. It cannot connect to Area 0
3. It reduces external routing information received by routers in the area
4. It disables inter-area routing

Correct Answer: 3. It reduces external routing information received by routers in the area

Explanation:

A stub area is designed to reduce routing information by limiting certain external LSAs from entering the area. Instead of maintaining detailed external routes, routers can often use a default route provided by the ABR to reach destinations outside the OSPF domain. This can reduce link-state database size and processing requirements on routers with limited resources. Stub areas can still communicate with other OSPF areas through an ABR. Their purpose is optimization, not isolation from all inter-area traffic.

Question 44.

What is the main purpose of an OSPF NSSA?

1. Provide only Layer 2 switching
2. Disable all external routing
3. Replace Area 0
4. Allow limited external route redistribution while retaining stub-like characteristics**

Correct Answer: 4. Allow limited external route redistribution while retaining stub-like characteristics

Explanation:

A Not-So-Stubby Area, or NSSA, combines reduced external route exposure with the ability to redistribute selected external routes from within the area. An ASBR inside the NSSA can introduce external routes using NSSA-specific LSAs, which can then be translated by an ABR for propagation into the wider OSPF domain. This makes NSSA useful when a remote area needs local redistribution but administrators still want to limit external routing information entering that area. NSSA does not replace the backbone area.

Question 45.

Which OSPF neighbor state indicates that two routers have fully synchronized their link-state databases?

1. Full
2. Init
3. ExStart
4. Exchange

Correct Answer: 1. Full

Explanation:

The Full state indicates that OSPF neighbors have completed adjacency formation and synchronized the relevant portions of their link-state databases. Earlier states such as Init, ExStart, and Exchange represent intermediate steps in neighbor discovery and database synchronization. When troubleshooting an expected full adjacency, administrators commonly verify parameters such as area IDs, authentication, timers, network types, and MTU-related issues. Reaching Full normally indicates that the OSPF adjacency is operational and capable of exchanging routing information correctly.

Question 46.

Two OSPF routers are connected directly but never form a neighbor relationship. Which configuration should be compared FIRST?

1. BGP Local Preference
2. OSPF area, timers, authentication, and interface parameters
3. MPLS Route Targets
4. STP bridge priorities

Correct Answer: 2. OSPF area, timers, authentication, and interface parameters

Explanation:

OSPF neighbors must agree on several important parameters before adjacency can form. Administrators should compare area IDs, Hello and Dead timers, authentication settings, subnet information, and relevant interface or network-type settings. If these parameters do not match, Hello packets may be received but the neighbor relationship may fail to progress. BGP attributes, MPLS VPN settings, and spanning-tree priorities are unrelated to basic OSPF neighbor formation. A systematic comparison of OSPF interface parameters is therefore the best starting point.

Question 47.

What is the purpose of changing OSPF interface cost manually?

1. Encrypt OSPF packets
2. Assign router IDs
3. Influence which path OSPF prefers
4. Change the OSPF area type automatically

Correct Answer: 3. Influence which path OSPF prefers

Explanation:

OSPF selects routes based on accumulated path cost. By changing the cost assigned to an interface, an administrator can influence the calculated total cost and therefore which path OSPF prefers. This is commonly used for traffic engineering when bandwidth-based defaults do not reflect desired routing policy. Manual cost changes should be applied carefully because they affect topology calculations and failover behavior. Interface cost does not assign router IDs, encrypt routing messages, or change an area’s type.

Question 48.

What is the main purpose of an OSPF router ID?

1. Provide a VLAN number
2. Determine interface duplex
3. Encrypt LSAs
4. Uniquely identify the OSPF router within the routing domain**

Correct Answer: 4. Uniquely identify the OSPF router within the routing domain

Explanation:

The OSPF router ID is a 32-bit value used to uniquely identify a router in the OSPF domain. It appears in protocol operations such as LSAs and neighbor relationships and is also considered in certain elections. Router IDs should be unique and stable to avoid routing inconsistencies. Administrators often configure router IDs explicitly in important environments rather than relying entirely on automatic selection. The router ID is not an interface VLAN number, encryption key, or physical interface parameter.

Question 49.

Which BGP attribute is normally preferred when it has the highest value and is used across iBGP peers to influence outbound traffic?

1. Local Preference
2. MED
3. Origin code
4. Router ID

Correct Answer: 1. Local Preference

Explanation:

Local Preference is used within an autonomous system to influence which external path internal BGP routers prefer. A higher Local Preference value is generally preferred. Administrators commonly modify it through routing policy to direct outbound traffic toward a preferred provider or exit point. MED is usually interpreted in the opposite direction, with lower values normally preferred, and is commonly used to suggest preferred inbound entry points. Local Preference is therefore one of the most important BGP traffic-engineering attributes for outbound path control.

Question 50.

A BGP router receives two routes with equal earlier attributes, but one route has a shorter AS_Path. Which route is generally preferred?

1. The route with the longer AS_Path
2. The route with the shorter AS_Path
3. Both routes must always be installed equally
4. The route with the highest MED regardless of other factors

Correct Answer: 2. The route with the shorter AS_Path

Explanation:

The AS_Path attribute records the autonomous systems a route has traversed. During normal BGP best-path selection, a shorter AS_Path is generally preferred when earlier selection criteria are equal. This often corresponds to a path that crosses fewer autonomous systems, although it does not necessarily mean lower latency or better bandwidth. Administrators can also manipulate AS_Path information for traffic-engineering purposes. Other attributes, such as Local Preference, may take precedence depending on the platform’s BGP decision process.

Question 51.

What is the primary purpose of BGP route aggregation?

1. Increase the number of prefixes exchanged
2. Disable BGP policy control
3. Advertise a summarized prefix instead of many more-specific routes
4. Replace AS_Path loop prevention

Correct Answer: 3. Advertise a summarized prefix instead of many more-specific routes

Explanation:

BGP aggregation allows multiple related prefixes to be represented by a broader summary route. This can reduce the number of routes advertised between routers and improve control-plane scalability. Aggregation works best when the IP addressing plan is structured to support contiguous summarization. Administrators should carefully consider whether more-specific routes must also remain available and how aggregation affects path selection. Aggregation does not replace BGP loop prevention or policy controls; it simply reduces routing detail where appropriate.

Question 52.

Why might an administrator prepend an AS number multiple times to a BGP route advertisement?

1. To increase route encryption
2. To create a VLAN
3. To lower OSPF cost
4. To make that BGP path appear less attractive to external routers**

Correct Answer: 4. To make that BGP path appear less attractive to external routers

Explanation:

AS_Path prepending adds additional occurrences of an autonomous system number to the path advertised to selected external peers. Because shorter AS paths are generally preferred when more important attributes are equal, prepending can make one inbound path less attractive than another. This is a common traffic-engineering technique for influencing how external networks reach an organization. It is not guaranteed to override every external policy because neighboring networks may use attributes that take precedence over AS_Path length.

Question 53.

What is the main benefit of using a BGP route reflector?

1. Reduce the number of required iBGP peerings
2. Replace eBGP
3. Eliminate all routing policy
4. Perform Ethernet loop prevention

Correct Answer: 1. Reduce the number of required iBGP peerings

Explanation:

Without route reflection, traditional iBGP design requires a full mesh of peerings among BGP speakers in the same autonomous system. The number of sessions grows rapidly as more routers are added. A route reflector allows client routers to exchange routes indirectly through the reflector, substantially reducing the number of required sessions. Route reflectors improve scalability but should be designed with redundancy and awareness of path-selection effects. They do not replace eBGP or Layer 2 loop-prevention mechanisms.

Question 54.

What does a BGP route policy commonly use a prefix list for?

1. Determine Ethernet speed
2. Match specific network prefixes for filtering or policy actions
3. Create MPLS labels
4. Elect an IS-IS DIS

Correct Answer: 2. Match specific network prefixes for filtering or policy actions

Explanation:

A prefix list provides a structured way to match selected IP prefixes, often including prefix-length conditions. Routing policies can reference those matches to permit, deny, or modify routes. For example, an administrator may allow only approved customer prefixes to be advertised to an upstream provider. Prefix filtering is an important routing-security and policy technique because it helps prevent accidental or unauthorized route propagation. Prefix lists do not control Ethernet speed or IS-IS elections.

Question 55.

In IS-IS, what is the main purpose of the SPF algorithm?

1. Assign IP addresses
2. Encrypt LSPs
3. Calculate shortest paths using the link-state database
4. Establish BGP sessions

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

Explanation:

IS-IS is a link-state routing protocol. Routers collect topology information in a link-state database and then run the Shortest Path First algorithm to calculate the best path to reachable destinations. The calculated results are used to build routing information for packet forwarding. When topology changes occur, updated LSPs are flooded and SPF may be run again. The algorithm does not assign addresses, encrypt protocol messages, or establish sessions for unrelated routing protocols.

Question 56.

What is the primary function of the IS-IS NET configured on a router?

1. Identify the default gateway
2. Configure a BGP community
3. Specify an MPLS label
4. Provide the NSAP-based identifier containing area and system information**

Correct Answer: 4. Provide the NSAP-based identifier containing area and system information

Explanation:

In IS-IS, the Network Entity Title, or NET, identifies the router within the IS-IS domain. It contains information including the area address, system ID, and an NSEL value. Routers use the area portion to determine Level-1 area membership, while the system ID uniquely identifies the device within the routing domain. Proper NET planning is therefore important for stable IS-IS operation. It is unrelated to BGP communities, MPLS labels, or host default gateways.

Question 57.

What is the main purpose of MPLS label swapping on a transit router?

1. Replace an incoming label with the appropriate outgoing label and forward the packet
2. Convert every packet to IPv6
3. Assign customer IP addresses
4. Encrypt the packet payload

Correct Answer: 1. Replace an incoming label with the appropriate outgoing label and forward the packet

Explanation:

A transit Label Switching Router examines the incoming MPLS label and uses its forwarding information to determine the next-hop action. In a typical swap operation, the router replaces the incoming label with a new outgoing label and forwards the packet toward the next hop in the label-switched path. Other possible MPLS operations include label push and pop. Label switching is a forwarding mechanism and does not provide payload encryption or host address assignment.

Question 58.

What is the purpose of penultimate hop popping in MPLS?

1. Add several extra labels before the destination
2. Remove the outer label on the router before the final egress router when appropriate
3. Disable MPLS forwarding
4. Replace BGP with OSPF

Correct Answer: 2. Remove the outer label on the router before the final egress router when appropriate

Explanation:

Penultimate hop popping allows the router immediately before the egress label-switching router to remove the outer transport label. The egress router then receives the packet without needing to perform that particular outer-label lookup. This can simplify forwarding processing at the egress. In networks with multiple labels, such as MPLS VPNs, an inner service label may still remain. Penultimate hop popping is an MPLS forwarding optimization and does not disable MPLS or replace routing protocols.

Question 59.

What is the primary purpose of BFD when associated with OSPF or BGP?

1. Encrypt routing updates
2. Replace route calculation
3. Detect link or forwarding failures faster than normal protocol timers may detect them
4. Assign routing metrics automatically

Correct Answer: 3. Detect link or forwarding failures faster than normal protocol timers may detect them

Explanation:

Bidirectional Forwarding Detection provides rapid liveness detection between network devices. When used with OSPF, BGP, or other routing protocols, BFD can identify a forwarding-path failure much faster than relying solely on normal Hello or keepalive timers. The associated routing protocol can then react and converge to an alternate path more quickly. BFD does not calculate routes itself; it provides failure information to other protocols. This makes it valuable in networks with strict convergence requirements.

Question 60.

Which design approach BEST improves scalability when combining OSPF, BGP, and MPLS technologies in a large network?

1. Advertise every specific route everywhere with no filtering
2. Use hierarchical routing, controlled route policies, summarization, redundancy, and clear protocol boundaries
3. Place all devices in one Layer 2 broadcast domain
4. Redistribute every routing protocol into every other protocol without restrictions

Correct Answer: 2. Use hierarchical routing, controlled route policies, summarization, redundancy, and clear protocol boundaries

Explanation:

Large networks require structured design to remain predictable and manageable. Hierarchical routing limits topology scope, while summarization reduces the number of routes that must be processed. BGP route policies provide precise control over route advertisements and path selection. MPLS can support scalable service separation, and redundant paths improve availability. Clear boundaries between protocols reduce the risk of redistribution loops and unintended route propagation. A flat topology with uncontrolled redistribution increases operational complexity and can make failures much harder to isolate.