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CCNA Networking Questions

68 of 218 questions · Page 3/3 · Networking · Answers revealed

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151
MCQmedium

A service provider uses BGP route reflectors to scale iBGP. A route reflected from a route reflector arrives at a client with the originator-id set. What is the purpose of the originator-id attribute?

A.To identify the route reflector that reflected the route
B.To indicate the cluster-id of the route reflector
C.To prevent routing loops by ensuring the route is not accepted if the originator-id matches the local router-id
D.To carry the next-hop information for IPv6 prefixes
AnswerC

Correct; if a router receives a route with its own router-id as originator-id, it discards the route.

Why this answer

The originator-id is set by the route reflector to the router-id of the originating router in the AS. It prevents routing loops by allowing a route to be ignored if the originator-id matches the receiving router's ID.

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152
MCQeasy

A client reports intermittent connectivity issues when accessing a server across a provider MPLS network. The issue occurs only during peak hours. Which component should be checked first?

A.BGP peering status between CE and PE.
B.CPU utilization of the server.
C.Interface errors and discards on the CE router.
D.MPLS labels in the core.
AnswerC

Peak traffic can cause output discards or CRC errors, leading to connectivity drops.

Why this answer

Intermittent connectivity during peak hours strongly suggests a bandwidth or queuing issue at the edge of the MPLS network. Interface errors (e.g., CRC, runts) and discards on the CE router indicate congestion or Layer 1/2 problems, which are the most common cause of such time-dependent symptoms. Checking this first aligns with the standard troubleshooting methodology of verifying the physical and data-link layers before moving to higher-layer protocols.

Exam trap

Cisco often tests the principle that intermittent issues during peak hours are almost always due to congestion or interface errors at the edge, not control-plane or core problems, leading candidates to incorrectly focus on BGP or MPLS labels.

How to eliminate wrong answers

Option A is wrong because BGP peering between CE and PE is a control-plane function; if it were flapping or down, connectivity would be lost entirely or consistently, not just intermittently during peak hours. Option B is wrong because server CPU utilization is an endpoint issue unrelated to the MPLS network; while high CPU could cause slow responses, it would not cause intermittent connectivity across the provider network. Option D is wrong because MPLS labels in the core are typically stable and not affected by peak-hour traffic patterns; label switching is deterministic and congestion in the core would manifest as drops or discards at the CE/PE edge, not as label failures.

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153
Multi-Selecthard

Which THREE of the following are valid IS-IS TLV types used to support traffic engineering and segment routing? (Choose three.)

Select 3 answers
A.TLV 22 (Extended IS Reachability)
B.TLV 2 (IS Neighbors)
C.TLV 36 (Prefix SID)
D.TLV 135 (Extended IP Reachability)
E.TLV 128 (IP Internal Reachability)
AnswersA, C, D

Correct. Used for TE and wide metrics.

Why this answer

IS-IS TE extensions use various TLVs. TLV 22 (Extended IS Reachability) carries wide metrics and TE information. TLV 135 (Extended IP Reachability) carries IP prefixes with wide metrics.

TLV 36 (Prefix SID) is used for segment routing to assign SIDs to prefixes.

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154
MCQmedium

Which MPLS VPN technology allows a service provider to offer overlapping IP addresses to different customers while using a single routing table per VPN?

A.Any Transport over MPLS (AToM)
B.VPLS
C.MPLS Traffic Engineering (TE)
D.MPLS Layer 3 VPN (BGP/MPLS IP VPN)
AnswerD

Uses VRFs to isolate routing per VPN.

Why this answer

D is correct because MPLS Layer 3 VPN (BGP/MPLS IP VPN) uses per-VPN routing tables (VRFs) to maintain separate forwarding instances for each customer. This allows overlapping IP addresses across different customers while keeping each VPN's routes isolated. The provider edge routers exchange VPNv4 routes using MP-BGP with route distinguishers (RDs) to make overlapping prefixes unique, and route targets (RTs) control which VRFs import/export those routes.

Exam trap

Cisco often tests the distinction between Layer 2 VPN technologies (AToM, VPLS) and Layer 3 VPNs (MPLS L3VPN), so the trap here is that candidates confuse 'overlapping IP addresses' with 'overlapping MAC addresses' or assume any MPLS-based service can handle IP overlap, but only per-VPN routing tables (VRFs) in L3VPN provide that isolation.

How to eliminate wrong answers

Option A is wrong because Any Transport over MPLS (AToM) is a Layer 2 VPN technology that transports Layer 2 frames (e.g., Ethernet, ATM) over an MPLS backbone, not a Layer 3 VPN solution, and it does not provide per-VPN routing tables or handle overlapping IP addresses. Option B is wrong because VPLS (Virtual Private LAN Service) is a Layer 2 VPN technology that emulates an Ethernet LAN across an MPLS network, using MAC address learning and flooding, not IP routing tables, so it cannot isolate overlapping IP addresses per VPN. Option C is wrong because MPLS Traffic Engineering (TE) is a mechanism for optimizing traffic paths across the MPLS core using RSVP-TE or CR-LDP, not a VPN technology; it does not create per-VPN routing tables or address overlapping IPs.

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155
MCQhard

A service provider has recently deployed MPLS L3VPN to provide IP connectivity to multiple enterprise customers. One customer reports that they cannot reach a remote site that is connected to a different PE router. The engineer checks the BGP VPNv4 table on the customer's PE and sees the route for the remote site, but the next-hop is unreachable. The interface between the PE and P routers is up/up, and IGP reachability to the PE's loopback is fine. What is the most likely cause? Consider that the network uses LDP for label distribution and OSPF as the IGP.

A.MPLS LDP is not enabled on the interface connecting to the P router
B.BGP next-hop-self is not configured on the PE
C.The VRF route-target import/export is misconfigured
D.The PE router does not have an LSP to the remote PE
AnswerA

LDP must be enabled on the interface to exchange labels for the loopback route.

Why this answer

Since the interface between the PE and P routers is up/up and IGP reachability to the PE's loopback is fine, the issue is that MPLS LDP is not enabled on that interface. Without LDP, the PE cannot advertise a label for its loopback to the P router, so the P router cannot push the correct label for packets destined to the remote PE. This makes the BGP VPNv4 next-hop unreachable even though the route itself is present in the table.

Exam trap

The trap here is that candidates often focus on BGP configuration (like next-hop-self) or VRF import/export when the route is present but unreachable, missing the fundamental MPLS label distribution issue that LDP must be enabled on all transit interfaces for end-to-end LSPs.

How to eliminate wrong answers

Option B is wrong because BGP next-hop-self is used to change the next-hop of VPNv4 routes to the local PE's loopback, but the problem states the route is present with an unreachable next-hop, not that the next-hop is incorrect. Option C is wrong because VRF route-target import/export misconfiguration would cause the route to be missing from the VRF table entirely, not to appear with an unreachable next-hop. Option D is wrong because an LSP to the remote PE is exactly what is missing, but the root cause is that LDP is not enabled on the interface, which prevents label distribution and thus the LSP from being built.

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156
Multi-Selecthard

A service provider is deploying L3VPN with inter-AS option B (ASBR-to-ASBR). Which TWO statements are true about this design?

Select 2 answers
A.The VPN label is removed by the ASBR before forwarding to the neighbor AS.
B.ASBRs peer using eBGP and exchange labeled VPN-IPv4 prefixes.
C.Route reflectors are required to propagate VPN routes between ASes.
D.ASBRs perform label swap for VPN labels when forwarding traffic.
E.ASBRs exchange unlabeled IPv4 routes and use MP-BGP to carry VPNv4 routes.
AnswersB, D

ASBRs eBGP peer and exchange VPNv4 prefixes with labels, allowing end-to-end MPLS.

Why this answer

In inter-AS Option B, ASBRs peer using eBGP and exchange labeled VPN-IPv4 prefixes (AFI 1, SAFI 128). This allows the VPNv4 routes to be carried across AS boundaries without requiring a full mesh of MP-IBGP between PEs, as the ASBRs re-advertise the routes with a new next-hop and perform label allocation for the VPN labels.

Exam trap

Cisco often tests the misconception that Option B requires route reflectors or that the VPN label is removed at the ASBR, when in fact the ASBR performs a label swap and directly exchanges VPNv4 prefixes via eBGP without needing route reflectors.

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157
MCQeasy

An ISP is designing an MPLS core network and needs to choose an IGP that supports fast convergence. Which IGP meets this requirement and is most commonly used in MPLS core networks?

A.IS-IS
B.OSPFv3
C.EIGRP
D.RIPng
AnswerA

IS-IS provides fast convergence and is the predominant IGP in service provider MPLS cores.

Why this answer

IS-IS is the correct choice because it is a link-state IGP that inherently supports fast convergence through mechanisms like incremental SPF (iSPF) and prefix-independent convergence (PIC). It is widely deployed in MPLS core networks due to its scalability, extensibility via TLVs, and native support for MPLS Traffic Engineering (MPLS-TE) without requiring additional protocol extensions like OSPF's opaque LSA.

Exam trap

Cisco often tests the misconception that OSPF is the default IGP for all networks, but in MPLS core environments, IS-IS is the preferred choice due to its native TE support and hierarchical scalability, making OSPF a distractor despite its fast convergence capabilities.

How to eliminate wrong answers

Option B (OSPFv3) is wrong because while OSPFv3 supports IPv6 and fast convergence, it is less commonly used in MPLS core networks compared to IS-IS due to its reliance on opaque LSAs for MPLS-TE, which adds complexity; IS-IS is the dominant IGP in service provider cores. Option C (EIGRP) is wrong because EIGRP is a Cisco-proprietary distance-vector protocol that does not natively support MPLS-TE and is not designed for the hierarchical, scalable architecture required in MPLS core networks. Option D (RIPng) is wrong because RIPng is a distance-vector protocol with slow convergence (based on hop count) and is unsuitable for any modern MPLS core network due to its lack of fast convergence, scalability, and MPLS-TE support.

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158
MCQmedium

An engineer configures the 'set-overload-bit' command on an IS-IS router. What is the immediate effect on the network?

A.The router stops participating in SPF calculations.
B.The router's LSPs are no longer advertised.
C.The router's neighbors drop adjacency.
D.The router is not used as a transit path but still has routes.
AnswerD

Correct. Overload bit prevents transit traffic while still being reachable.

Why this answer

The 'set-overload-bit' command causes the IS-IS router to set the overload bit in its Link State PDUs (LSPs). This signals to other routers that the advertising router should not be used for transit traffic, but it still participates in SPF calculations and can be reached as a leaf or end node. Therefore, the router retains its routes and adjacency, but other routers will not forward traffic through it.

Exam trap

Cisco often tests the misconception that setting the overload bit stops all routing functionality or drops adjacencies, when in fact it only prevents transit traffic while preserving reachability and neighbor relationships.

How to eliminate wrong answers

Option A is wrong because the router continues to participate in SPF calculations; the overload bit only affects transit path selection, not the router's own SPF computation. Option B is wrong because the router's LSPs are still advertised; the overload bit is a flag within the LSP, not a suppression of LSP advertisement. Option C is wrong because the overload bit does not cause neighbors to drop adjacency; adjacencies remain established and the router can still be reached directly.

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159
MCQmedium

A service provider is deploying OSPFv3 for IPv6 in its core. The network uses multiple areas. Which LSA type is used by OSPFv3 to advertise prefix information for inter-area routes?

A.Network LSA (Type 0x2002)
B.Intra-Area Prefix LSA (Type 0x2009)
C.Router LSA (Type 0x2001)
D.Inter-Area Prefix LSA (Type 0x2003)
AnswerD

Correct LSA for inter-area prefixes.

Why this answer

In OSPFv3, Inter-Area Prefix LSAs (Type 0x2003) are used to advertise prefixes from other areas.

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160
MCQeasy

Which QoS mechanism marks packets at the edge of the network to classify traffic into different service classes?

A.Queuing
B.Shaping
C.Policing
D.Marking
AnswerD

Marking sets the QoS bits in the packet header.

Why this answer

Marking is the QoS mechanism that sets the Differentiated Services Code Point (DSCP) or IP Precedence bits in the packet header at the ingress edge of the network. This classification allows downstream devices to apply per-hop behaviors (PHBs) based on the marked value, as defined in RFC 2474 and RFC 2597. Unlike queuing, shaping, or policing, marking does not delay or drop packets; it only modifies the packet header to indicate the desired service class.

Exam trap

Cisco often tests the distinction between marking and policing, where candidates mistakenly think policing is used for classification because it can re-mark packets, but the question specifically asks for the mechanism that marks at the edge to classify traffic, which is marking itself, not policing.

How to eliminate wrong answers

Option A is wrong because queuing is a congestion management mechanism that determines the order in which packets are transmitted out of an interface, not a mechanism for marking packets at the edge. Option B is wrong because shaping smooths traffic bursts by buffering and delaying packets to conform to a configured rate, but it does not mark packets with classification information. Option C is wrong because policing enforces a traffic rate by dropping or re-marking packets that exceed a threshold, but its primary function is rate limiting, not initial marking at the edge for classification.

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161
MCQmedium

A service provider is migrating from OSPF to IS-IS in the core network. They need to support Traffic Engineering and Segment Routing. Which IS-IS TLV must be supported to carry TE information?

A.TLV 130
B.TLV 22
C.TLV 135
D.TLV 128
AnswerB

TLV 22 (Extended IS reachability) carries TE information including wide metrics.

Why this answer

IS-IS TLV 22 (Extended IS reachability) carries TE metrics and other attributes for traffic engineering. TLVs 128 and 130 are IP reachability TLVs; TLV 135 is for extended IP reachability.

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162
MCQhard

Refer to the exhibit. Which statement is correct about the LDP bindings for prefix 10.0.0.0/24?

A.There are two remote label bindings for this prefix from different LSRs.
B.Both remote bindings are from the same LSR.
C.The local label for this prefix is 18.
D.The router is performing PHP for this prefix.
AnswerA

Correct. Two remote bindings exist.

Why this answer

The exhibit shows two remote label bindings for prefix 10.0.0.0/24, one with an incoming label of 18 from LSR 192.168.1.2 and another with an incoming label of 19 from LSR 192.168.1.3. This indicates that two different LSRs have advertised label bindings for the same prefix, which is typical in an MPLS network with multiple paths. Therefore, option A is correct.

Exam trap

Cisco often tests the distinction between local and remote label bindings, and the trap here is that candidates may mistakenly interpret the labels 18 and 19 as local labels or assume both remote bindings come from the same LSR, leading them to choose option B or C.

How to eliminate wrong answers

Option B is wrong because the two remote bindings originate from different LSRs (192.168.1.2 and 192.168.1.3), not the same LSR. Option C is wrong because the local label for this prefix is not shown in the exhibit; the labels 18 and 19 are remote labels advertised by other LSRs, not the local label of the router itself. Option D is wrong because PHP (Penultimate Hop Popping) is indicated by an implicit null label (label 3) being advertised, but the exhibit shows explicit label values (18 and 19), so PHP is not being performed for this prefix.

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163
MCQmedium

Based on the exhibit, which prefix is NOT reachable via MPLS forwarding?

A.10.1.1.1/32
B.10.3.3.3/32
C.10.2.2.2/32
D.10.5.5.5/32
AnswerD

The outgoing label is Untagged, so MPLS forwarding is not applied, likely causing packet drop.

Why this answer

(10.5.5.5/32) is correct because the exhibit shows that the LDP label bindings are only present for prefixes 10.1.1.1/32, 10.2.2.2/32, and 10.3.3.3/32. The prefix 10.5.5.5/32 is not in the LDP label information base (LIB), meaning no MPLS label has been assigned to it, so it cannot be forwarded via MPLS and must be forwarded using standard IP routing instead.

Exam trap

Cisco often tests the distinction between prefixes that have LDP label bindings versus those that are simply present in the routing table, trapping candidates who assume all IGP routes are automatically MPLS-switched without verifying the label bindings.

How to eliminate wrong answers

Option A is wrong because 10.1.1.1/32 has an LDP label binding (label 16) as shown in the exhibit, making it reachable via MPLS forwarding. Option B is wrong because 10.3.3.3/32 has an LDP label binding (label 18) as shown in the exhibit, making it reachable via MPLS forwarding. Option C is wrong because 10.2.2.2/32 has an LDP label binding (label 17) as shown in the exhibit, making it reachable via MPLS forwarding.

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164
MCQmedium

In OSPFv3, which statement correctly describes how OSPFv3 differs from OSPFv2 regarding router IDs and adjacencies?

A.OSPFv3 uses a 32-bit Router ID independent of IPv6 addresses.
B.OSPFv3 uses the same LSA types as OSPFv2.
C.OSPFv3 requires both IPv4 and IPv6 addresses on interfaces.
D.OSPFv3 uses IPv6 addresses to form adjacencies.
AnswerA

Correct. Router ID is 32-bit and must be configured or derived from an IPv4 address.

Why this answer

OSPFv3 uses a 32-bit Router ID independent of IPv6 addresses. Adjacencies are formed with Router IDs, not link-local addresses. OSPFv3 runs per-link rather than per-subnet.

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165
MCQmedium

In BGP route selection, when comparing two routes with the same LOCAL_PREF, which attribute is considered next?

A.Locally originated routes (prefer routes originated by the local router)
B.MED
C.AS_PATH length
D.eBGP over iBGP
AnswerA

Correct; routes originated locally are preferred over received routes.

Why this answer

After LOCAL_PREF, the next step in BGP best path selection is whether the route was originated locally (via network or aggregate command). Then AS_PATH length is considered.

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166
MCQeasy

A service provider's network core runs IS-IS as the IGP. After adding a new router, some routers have incomplete LSP databases. The new router's interfaces are up, and IS-IS adjacency is up with neighbors. What is the cause? The network has a mix of interface MTUs, with some links having MTU 1500 and others 4470.

A.MTU mismatch on the interface
B.IS-IS overload bit set on the new router
C.IS-IS LSP flooding is inhibited on the new router
D.The new router's system-id is a duplicate
AnswerA

Smaller MTU can cause LSP fragmentation/drop, leading to incomplete databases.

Why this answer

An MTU mismatch on the interface causes incomplete LSP databases. IS-IS uses a maximum LSP size derived from the interface MTU minus the IS-IS header overhead (typically 3 bytes for the LSP header). When a router with a smaller MTU (e.g., 1500) receives an LSP that was generated on a larger MTU link (e.g., 4470), the LSP may be too large to be stored or processed, leading to fragmentation or rejection.

This results in an incomplete LSP database on some routers, even though adjacencies are up.

Exam trap

Cisco often tests the misconception that MTU mismatch only affects adjacency formation, but in IS-IS, adjacencies can form even with MTU mismatch, and the real impact is on LSP database synchronization due to LSP size constraints.

How to eliminate wrong answers

Option B is wrong because the IS-IS overload bit prevents the router from being used for transit traffic but does not affect LSP database completeness; it only sets the overload flag in the router's LSP, signaling other routers to avoid using it for transit. Option C is wrong because if LSP flooding were inhibited on the new router, it would not send its own LSPs, but the problem states that adjacencies are up and some routers have incomplete databases, which points to a size mismatch rather than a flooding suppression. Option D is wrong because a duplicate system-id would cause adjacency failures or routing loops, not incomplete LSP databases; IS-IS would detect the duplicate via LSP sequence numbers and reject the newer LSP, but adjacencies would still form.

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167
MCQeasy

Which IS-IS metric type supports values up to 2^24-1 and is used for traffic engineering?

A.Extended metrics
B.Narrow metrics
C.TE metrics
D.Wide metrics
AnswerD

Wide metrics use 24 bits and support larger values for TE.

Why this answer

IS-IS wide metrics use a 24-bit field, allowing values up to 16,777,215, and are required for TE.

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168
MCQeasy

In IS-IS, which type of router is responsible for exchanging routing information between different areas?

A.L1/L2 router
B.L1 router
C.Attached router
D.L2 router
AnswerD

L2 routers exchange information between areas.

Why this answer

L2 routers handle inter-area routing in IS-IS, while L1 routers handle intra-area routing. L1/L2 routers act as both.

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169
MCQhard

A service provider is deploying BGP PIC (Prefix Independent Convergence) for edge routers. What is the primary benefit of BGP PIC?

A.It provides fast convergence by pre-installing backup paths in the FIB
B.It reduces the number of BGP updates during a flap
C.It reduces the memory usage on the router
D.It eliminates the need for BFD
AnswerA

Correct; backup paths are precomputed and installed.

Why this answer

BGP PIC provides fast failover by precomputing backup paths in the forwarding table, enabling sub-second convergence upon failure, independent of the number of prefixes.

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170
MCQeasy

Which BGP attribute is used to influence inbound traffic to a multi-homed AS by indicating the preferred entry point?

A.AS_PATH
B.COMMUNITY
C.MED
D.LOCAL_PREF
AnswerC

MED influences inbound traffic.

Why this answer

MED (Multi-Exit Discriminator) is used to influence inbound traffic from neighboring ASes by suggesting the preferred path into the AS.

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171
MCQmedium

A customer reports intermittent packet loss on a MPLS L3VPN connection. The PE router shows 'show mpls forwarding' entries for the CE prefix, but ping from the PE to the CE fails intermittently. Which action should be taken to isolate the issue?

A.Check 'show ip route vrf CUSTOMER' and 'show bgp vpnv4 unicast vrf CUSTOMER' to confirm the VRF routes.
B.Use 'show mpls lsp' to verify the LSP to the CE's PE.
C.Examine the 'show mpls forwarding vrf CUSTOMER' output to see label operations.
D.Review the 'show bgp vpnv4 unicast all' output to verify route advertisement.
AnswerA

This verifies that the VRF has the correct routes and that BGP VPNv4 routes are properly imported.

Why this answer

The intermittent packet loss suggests a control-plane issue rather than a data-plane problem. By checking 'show ip route vrf CUSTOMER' and 'show bgp vpnv4 unicast vrf CUSTOMER', you can verify that the VRF route for the CE prefix is present in the routing table and that BGP is advertising the correct VPNv4 route with the proper next-hop and label. This isolates whether the failure is due to missing or incorrect route propagation, which is a common cause of intermittent reachability in MPLS L3VPN.

Exam trap

Cisco often tests the distinction between control-plane verification (routing table, BGP) and data-plane verification (MPLS forwarding, LSP) in MPLS L3VPN troubleshooting, leading candidates to mistakenly focus on label operations or LSPs when the root cause is a missing or unstable route.

How to eliminate wrong answers

Option B is wrong because 'show mpls lsp' verifies the label-switched path (LSP) between PEs, but the issue is between the PE and the CE, which is a Layer 3 adjacency (often a direct link or static route) and does not involve an LSP. Option C is wrong because 'show mpls forwarding vrf CUSTOMER' shows label operations for packets entering the VRF, but since the ping from PE to CE fails intermittently, the problem is likely in the VRF route presence or BGP advertisement, not in the MPLS forwarding table. Option D is wrong because 'show bgp vpnv4 unicast all' displays all VPNv4 routes from all VRFs, which is too broad and may obscure the specific VRF route; the VRF-specific command is more targeted and efficient for isolating the CE prefix issue.

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172
MCQhard

In an IS-IS network, a service provider wants to achieve fast convergence using Partial Route Computation (PRC). Which of the following events triggers a PRC instead of a full SPF?

A.A router's system ID changes
B.An LSP with a new fragment arrives
C.A new adjacency is formed between two routers
D.An IP prefix is withdrawn from a directly connected network
AnswerD

Correct. A prefix change without topology change triggers PRC.

Why this answer

PRC is triggered when an IP prefix changes (addition, removal, or metric change) without a change in the IS-IS topology (i.e., no change in adjacency or reachable IS-IS neighbors). PRC only recalculates the affected prefixes, avoiding full SPF.

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173
MCQhard

A network engineer is troubleshooting OSPFv3 on a service provider's IPv6 network. The router shows that OSPFv3 adjacency never reaches FULL, says 'Init'. The neighbor is directly connected over a point-to-point link. What is the most likely cause?

A.LSA throttling prevents exchanges
B.Mismatched OSPF router-id
C.Mismatched area IDs
D.Missing 'ipv6 ospf' interface command on one side
AnswerD

OSPFv3 requires explicit interface configuration to activate adjacency.

Why this answer

The 'Init' state in OSPFv3 indicates that the router has received a Hello packet from the neighbor but the neighbor has not received a Hello packet back. On a point-to-point link, the most common cause is that the 'ipv6 ospf <process-id> area <area-id>' interface command is missing on one side, which prevents OSPFv3 from sending Hellos on that interface. Without this command, the interface is not enabled for OSPFv3, so the neighbor never sees a Hello and the adjacency cannot progress to FULL.

Exam trap

Cisco often tests the distinction between OSPFv2 and OSPFv3 interface activation methods, trapping candidates who assume that OSPFv3 uses a similar 'network' command or that a global OSPF process automatically enables all interfaces.

How to eliminate wrong answers

Option A is wrong because LSA throttling controls the rate of LSA generation and flooding, not the formation of adjacencies; it would not cause the adjacency to stall in Init. Option B is wrong because mismatched OSPF router-IDs do not prevent adjacency formation in OSPFv3; the router-ID is used for router identification but is not checked during the Hello exchange for adjacency compatibility. Option C is wrong because mismatched area IDs would cause the adjacency to stall in the ExStart or Exchange state, not in Init; the Init state indicates that the Hello packet was received but not reciprocated, which is unrelated to area ID mismatch.

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174
MCQeasy

Which BGP attribute is used to indicate the preference of a route within an AS and is propagated to iBGP peers but not to eBGP peers by default?

A.WEIGHT
B.LOCAL_PREF
C.MED
D.AS_PATH
AnswerB

Correct. LOCAL_PREF is used for route preference within an AS.

Why this answer

LOCAL_PREF is a well-known discretionary attribute that indicates the degree of preference for a route within an AS. It is sent to iBGP peers but not to eBGP peers by default.

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175
Multi-Selectmedium

Which three BGP attributes are considered during the route selection process before comparing AS_PATH length? (Choose three.)

Select 3 answers
A.LOCAL_PREF
B.Weight
C.NEXT_HOP
D.Locally originated routes
E.MED
AnswersA, B, D

LOCAL_PREF is compared second.

Why this answer

Weight, LOCAL_PREF, and locally originated routes are compared before AS_PATH length in the BGP best path selection algorithm.

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176
MCQmedium

Segment Routing with TI-LFA (Topology Independent Loop-Free Alternate) provides fast convergence. Which statement accurately describes TI-LFA?

A.TI-LFA only protects against link failures, not node failures.
B.TI-LFA uses a pre-computed backup tunnel signaled via RSVP-TE.
C.TI-LFA computes a backup path that is guaranteed to be loop-free and topology independent.
D.TI-LFA requires BFD to detect failures.
AnswerC

TI-LFA uses post-convergence path and ensures loop avoidance.

Why this answer

TI-LFA (Topology Independent Loop-Free Alternate) computes a backup path that is guaranteed to be loop-free regardless of the network topology, using segment lists (label stacks) to steer traffic around a failure. It does not rely on the presence of a natural loop-free alternate in the IGP topology, making it topology independent. This ensures fast convergence (typically sub-50ms) without requiring signaling protocols like RSVP-TE.

Exam trap

Cisco often tests the distinction between TI-LFA and traditional LFA or RSVP-TE; the trap here is assuming TI-LFA requires a signaling protocol (like RSVP-TE) or that it only protects link failures, when in fact it is topology independent and protects both link and node failures using segment routing.

How to eliminate wrong answers

Option A is wrong because TI-LFA protects against both link and node failures, as it can compute a backup path that avoids a specific failed node by using adjacency segments or prefix segments. Option B is wrong because TI-LFA does not use RSVP-TE; it is a segment routing mechanism that uses pre-computed MPLS label stacks (segment lists) derived from the IGP (IS-IS or OSPF) without any signaling. Option D is wrong because TI-LFA does not require BFD for failure detection; it relies on IGP fast convergence mechanisms such as link-state advertisements (LSA) or hello timers, though BFD can be optionally used to accelerate detection.

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177
MCQmedium

When implementing MPLS TE tunnels in a service provider core, what is the purpose of the 'affinity' attribute?

A.To set the color of the tunnel
B.To adjust the cost of TE tunnels
C.To define administrative groups for link inclusion/exclusion
D.To bind tunnels to specific interfaces
AnswerC

Affinity allows tunnels to restrict links based on administrative group membership.

Why this answer

The 'affinity' attribute in MPLS TE is used to define administrative groups (also known as link colors) that allow you to include or exclude specific links from a TE tunnel path based on user-defined properties. This enables traffic engineering policies such as forcing traffic to avoid certain links or preferring links with specific characteristics, without modifying the underlying IGP metric.

Exam trap

Cisco often tests the confusion between 'affinity' (administrative groups for link inclusion/exclusion) and 'color' (a separate attribute used in Segment Routing or for visual identification), leading candidates to mistakenly choose Option A.

How to eliminate wrong answers

Option A is wrong because 'affinity' does not set the color of the tunnel; it uses color-like bitmask values to represent administrative groups on links, not to assign a visual or logical color to the tunnel itself. Option B is wrong because adjusting the cost of TE tunnels is done via the 'metric' or 'cost' command under the tunnel interface, not through the affinity attribute. Option D is wrong because binding tunnels to specific interfaces is achieved using the 'mpls traffic-eng tunnels' command on the interface or explicit path definitions, not via affinity.

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178
Multi-Selectmedium

A service provider is implementing MAP-T (Mapping of Address and Port using Translation) to transition customers from IPv4 to IPv6. Which two characteristics of MAP-T are true? (Choose two.)

Select 2 answers
A.MAP-T is primarily used for IPv6-to-IPv4 communication only
B.MAP-T requires a dedicated IPv6 prefix for each customer
C.MAP-T employs a Border Router that performs translation between IPv4 and IPv6
D.MAP-T uses encapsulation to transport IPv4 packets over an IPv6 network
E.MAP-T can operate in stateless mode
AnswersC, E

Correct. The BR performs IPv4-IPv6 translation.

Why this answer

MAP-T uses stateful or stateless translation between IPv4 and IPv6, and it uses a Border Router (BR) that performs translation. MAP-T does not use encapsulation; it uses translation (NAT). MAP-T also uses a Domain Name System (DNS) but that is not unique.

The key characteristics: it uses a BR for translation and can be stateless.

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179
Multi-Selectmedium

An engineer is configuring BGP route reflectors to scale iBGP. Which TWO attributes are used to prevent routing loops in a route reflector cluster? (Choose two.)

Select 2 answers
A.ORIGINATOR_ID
B.NEXT_HOP
C.LOCAL_PREF
D.AS_PATH
E.CLUSTER_LIST
AnswersA, E

ORIGINATOR_ID prevents loops within a cluster.

Why this answer

CLUSTER_LIST and ORIGINATOR_ID are used to prevent loops. CLUSTER_LIST is a sequence of cluster IDs; ORIGINATOR_ID identifies the originator.

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180
MCQmedium

An SP network uses OSPF with RSVP-TE for traffic engineering. Which OSPF extension is required to advertise TE link parameters such as maximum reservable bandwidth?

A.OSPF Type 7 LSAs
B.OSPF Type 11 Opaque LSAs
C.OSPF Type 10 Opaque LSAs
D.OSPF Type 5 LSAs
AnswerC

Correct. Type 10 Opaque LSAs carry TE information.

Why this answer

OSPF TE extensions are defined in RFC 3630, which introduces the Opaque LSA (Type 9, 10, 11). Specifically, Type 10 Opaque LSAs carry TE information. The TE LSA is carried in an Opaque LSA of area scope (Type 10).

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181
MCQhard

An engineer is troubleshooting a BGP route reflector setup. Clients are not receiving all routes. The 'show bgp neighbors' output shows a state of 'Active'. What is the most likely cause?

A.The route reflector does not have a full mesh with clients
B.The route reflector is detecting an AS_PATH loop
C.Next-hop reachability issue
D.Incorrect BGP neighbor statement on the route reflector or client
AnswerD

Active state indicates TCP session failure, often due to misconfiguration.

Why this answer

The 'Active' state in BGP indicates that the router is trying to establish a TCP connection with the neighbor but has not yet succeeded. This is most commonly caused by a misconfiguration in the BGP neighbor statement, such as an incorrect IP address, wrong AS number, or a missing neighbor statement on one side. Since the route reflector is not forming a BGP session, clients cannot receive any routes, which explains the symptom.

Exam trap

Cisco often tests the distinction between BGP session states and route advertisement issues, so candidates mistakenly attribute 'Active' to routing problems like next-hop unreachability or loops, when it actually points to TCP connection or neighbor configuration errors.

How to eliminate wrong answers

Option A is wrong because a full mesh is not required with a route reflector; the route reflector design intentionally avoids a full mesh by having clients connect only to the reflector. Option B is wrong because an AS_PATH loop would cause routes to be withdrawn or not advertised, but the BGP session itself would still reach an 'Established' state, not 'Active'. Option C is wrong because next-hop reachability issues affect route installation in the RIB, not the BGP session state; the session would still be 'Established' even if next hops are unreachable.

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182
MCQmedium

A service provider is troubleshooting BGP route selection between two eBGP peers. The router receives a prefix from Peer A with local preference 150 and AS path length 3. From Peer B, the same prefix has local preference 100 and AS path length 2. Which route will be preferred?

A.The route from Peer A because it has a higher weight.
B.The route from Peer A because local preference is higher.
C.Both routes are equally preferred and will be used for load balancing.
D.The route from Peer B because AS path is shorter.
AnswerB

Local preference is the first tiebreaker after weight; higher value wins.

Why this answer

B is correct because BGP's route selection process evaluates local preference before AS path length. The route from Peer A has a local preference of 150, which is higher than Peer B's 100, so it is preferred regardless of the shorter AS path from Peer B. Local preference is the second BGP attribute in the decision process (after weight) and is used to influence outbound traffic from an AS.

Exam trap

Cisco often tests the order of BGP path selection attributes, and the trap here is that candidates mistakenly prioritize AS path length over local preference because they remember 'shortest AS path' but forget it comes after local preference in the decision process.

How to eliminate wrong answers

Option A is wrong because weight is a Cisco-proprietary attribute that is not mentioned in the question; local preference is the relevant attribute here, and weight would only be considered first if it were set. Option C is wrong because the routes have different local preference values (150 vs. 100), so they are not equally preferred; BGP does not load-balance between routes with different local preferences. Option D is wrong because although AS path length is considered in BGP path selection, it is evaluated after local preference; since local preference differs, the AS path length is not compared.

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183
MCQhard

A service provider uses OSPFv3 for IPv6 routing in the core. Which statement correctly describes OSPFv3 adjacency formation on a broadcast multiaccess network?

A.OSPFv3 does not use DR/BDR election; instead, it forms full mesh adjacencies.
B.All routers on the network form full adjacencies with every other router.
C.The DR is elected based on the highest router ID, with a tiebreaker of highest interface priority.
D.The Designated Router (DR) is elected to reduce the number of adjacencies and LSDB flooding.
AnswerD

DR/BDR reduce O(n^2) to O(n).

Why this answer

OSPFv3 operates similarly to OSPFv2 on broadcast networks, electing a DR and BDR using Hello protocol, and forming adjacencies with the DR.

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184
MCQeasy

A service provider is implementing MPLS L3VPN and needs to ensure that BGP route advertisement uses a specific next-hop. Which technique ensures BGP advertises the PE-CE next-hop instead of the PE-PE loopback?

A.Route-map with set next-hop
B.next-hop-unchanged
C.next-hop-self
D.Disable next-hop-check
AnswerC

Sets the next-hop to the router's own address for iBGP advertisements.

Why this answer

In MPLS L3VPN, the PE-CE next-hop (the PE interface facing the CE) must be advertised to the remote PE so that the remote PE knows to forward traffic directly to the local PE's CE-facing interface. The `next-hop-self` command on the PE forces BGP to set the next-hop to the PE's own IP address (typically the loopback or the interface used for BGP peering), which overrides the default behavior of preserving the original next-hop. This ensures that the remote PE uses the correct next-hop for VPN traffic.

Exam trap

Cisco often tests the distinction between `next-hop-self` (used to force the PE's own address as the next-hop) and `next-hop-unchanged` (used to preserve the original next-hop in inter-AS scenarios), and candidates confuse these two commands.

How to eliminate wrong answers

Option A is wrong because a route-map with `set next-hop` can manually override the next-hop, but it is not the standard or most efficient technique for this specific requirement; it requires additional configuration and may not be as reliable as `next-hop-self` in all scenarios. Option B is wrong because `next-hop-unchanged` is used in MPLS VPN inter-AS scenarios (option B) to preserve the original next-hop across AS boundaries, which is the opposite of what is needed here. Option D is wrong because disabling next-hop-check (`no bgp next-hop-check`) is used in BGP confederation or route reflector scenarios to allow routes with unreachable next-hops to be accepted, not to change the next-hop value.

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185
MCQmedium

In IS-IS, which metric type supports traffic engineering (TE) and is required for segment routing?

A.Default metric (TLV 2)
B.Narrow metrics (TLV 128/130)
C.TE metric (TLV 138)
D.Wide metrics (TLV 135/236/22)
AnswerD

Wide metrics use 24-bit fields and support TE.

Why this answer

IS-IS wide metrics (TLV 135/236/22) support TE and segment routing; narrow metrics (TLV 128/130) are limited to 6-bit metrics.

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186
MCQhard

A service provider wants to use BGP Prefix Independent Convergence (PIC) for faster failover. Which BGP feature must be enabled to support PIC?

A.BGP additional paths
B.BGP multipath
C.BGP route reflectors
D.BGP fast external fallover
AnswerA

Additional paths allow backup path installation.

Why this answer

BGP PIC requires BGP additional paths (add-path) or BGP PIC edge, which installs backup paths in the FIB independent of the prefix count.

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187
Matchingmedium

Match each QoS mechanism to its primary function.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts
Matches

Identifying packets based on specified criteria

Setting QoS bits (e.g., DSCP, CoS) in packet headers

Enforcing traffic rate limits by dropping or remarking excess packets

Buffering excess traffic to smooth output rate

Congestion avoidance by selectively dropping packets based on queue depth

Why these pairings

In QoS, Classification identifies traffic, Marking sets priority, Policing drops/excess, Shaping buffers, and WRED drops probabilistically. Queuing handles packet scheduling, not rate enforcement.

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188
MCQmedium

A service provider is deploying MPLS L3VPN over an OSPF backbone. The PE routers are configured with OSPF as the IGP. The CE router of customer A is connected to two PEs for redundancy. Which configuration is required on the PE routers to ensure that the CE router can load-balance traffic across both PEs without loops?

A.Use OSPF sham-links between the two PEs.
B.Use the BGP cost community to adjust the path selection on the CE.
C.Configure OSPF with the capability vrf-lite and enable the down-bit on the PE-CE link.
D.Disable the DN-bit on the PE-CE OSPF interface.
AnswerC

The down-bit prevents the CE from re-advertising routes learned from one PE to the other PE, avoiding loops.

Why this answer

Configuring OSPF with the capability vrf-lite and enabling the down-bit on the PE-CE link prevents routing loops in a multi-homed CE scenario. The down-bit is set by the PE when redistributing routes into OSPF, ensuring that the CE does not re-advertise those routes back to another PE, which would cause a loop. The vrf-lite capability allows the CE to understand the down-bit without requiring full MPLS/VPN functionality, enabling load-balancing across both PEs safely.

Exam trap

Cisco often tests the distinction between the down-bit (DN-bit) used in OSPF PE-CE scenarios and the sham-link concept used for OSPF area 0 extension, leading candidates to mistakenly choose sham-links for loop prevention in multi-homed CE designs.

How to eliminate wrong answers

Option A is wrong because OSPF sham-links are used to connect two PE routers in different OSPF areas within an MPLS L3VPN to maintain OSPF adjacency across the backbone, not to prevent loops or enable load-balancing for a multi-homed CE. Option B is wrong because the BGP cost community influences path selection on the PE side for inter-AS or multi-homing scenarios, but it does not affect the CE's OSPF routing decisions or prevent loops in the PE-CE OSPF domain. Option D is wrong because disabling the DN-bit (down-bit) on the PE-CE OSPF interface would allow the CE to re-advertise routes learned from one PE back to the other PE, creating a routing loop; the DN-bit must be enabled to prevent this.

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189
Multi-Selectmedium

Which TWO of the following are characteristics of MPLS LDP?

Select 2 answers
A.Distributes labels for BGP routes
B.Label distribution based on IP routing table
C.Requires an IGP like OSPF for session reachability
D.UDP for discovery, TCP for session
E.TCP for both discovery and session
AnswersB, D

By default, LDP distributes labels for all IGP routes.

Why this answer

B is correct because MPLS LDP distributes labels for routes found in the IP routing table, not for BGP routes. LDP peers exchange label bindings for each prefix in the IGP routing table, enabling label-switched paths (LSPs) for those destinations. This is fundamental to LDP's operation as defined in RFC 5036.

Exam trap

Cisco often tests the misconception that LDP requires an IGP for session reachability, but the trap is that LDP uses its own UDP discovery mechanism and can form sessions over any IP reachable path, though an IGP is commonly used for loopback reachability in practice.

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190
MCQeasy

In IS-IS, what is the purpose of the Partial Route Computation (PRC)?

A.To compute the entire SPF tree after a topology change
B.To recompute only the routes that are affected by a change in IP reachability
C.To calculate the shortest path to all destinations in the L2 backbone
D.To repair IS-IS adjacency after link failure
AnswerB

PRC is triggered by IP prefix changes.

Why this answer

PRC recomputes only affected routes when a leaf changes, without full SPF, enabling faster convergence.

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191
MCQeasy

Which IS-IS router type maintains a full Layer 2 routing table of all areas and can act as a gateway between areas?

A.Level 2
B.Level 0
C.Level 1/2
D.Level 1
AnswerA

Correct. Level 2 routers maintain the full L2 routing table and route between areas.

Why this answer

In IS-IS, Level 2 routers route between areas and maintain a complete L2 database. Level 1 routers route within an area, and Level 1/2 routers act as both, but the question asks for the type that maintains a full L2 routing table of all areas. Level 2 routers specifically perform inter-area routing.

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192
MCQeasy

In IS-IS, what is the purpose of the NET (Network Entity Title) address field SEL (Selector) byte?

A.Identifies the application layer protocol
B.Indicates that this is a router (always 0x00)
C.Identifies the area
D.Identifies the system ID
AnswerB

SEL=0x00 indicates the network entity.

Why this answer

The SEL byte is always 0x00 for routers, indicating that the address refers to the router itself rather than a specific service.

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193
MCQmedium

An SP engineer is troubleshooting IS-IS routing. The network uses wide metrics, and some links have been configured with TE metrics. Which statement correctly describes the relationship between TE metrics and narrow metrics in IS-IS?

A.Narrow metrics are used only for L1 routes, while TE metrics are used for L2.
B.Wide metrics allow values up to 2^24 - 1, while narrow metrics allow up to 63.
C.TE metrics override narrow metrics for SPF calculations.
D.TE metrics are used by default when metric-style wide is configured.
AnswerB

Wide metrics use 24 bits (max 16777215), narrow use 6 bits (max 63).

Why this answer

IS-IS supports both narrow (6-bit) and wide (24-bit) style metrics. TE metrics are separate TLVs that carry additional information for MPLS-TE and do not replace the standard metric used for SPF computation.

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194
MCQhard

In BGP, an SP router receives multiple routes to the same prefix. The first route has a weight of 100, LOCAL_PREF of 200, AS_PATH length 3, and is learned via eBGP. The second route has weight 50, LOCAL_PREF 150, AS_PATH length 2, and is learned via iBGP. According to BGP route selection, which route will be preferred?

A.The second route (AS_PATH length 2)
B.The first route (eBGP learned)
C.The second route (LOCAL_PREF 150)
D.The first route (weight 100)
AnswerD

Correct. Weight is checked first; higher weight wins.

Why this answer

BGP selection order: highest weight first. Route 1 has weight 100, Route 2 has weight 50, so Route 1 is preferred regardless of other attributes.

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195
MCQeasy

Which BGP community is used to prevent a route from being advertised to any eBGP peers?

A.NO_EXPORT
B.INTERNET
C.LOCAL_AS
D.NO_ADVERTISE
AnswerA

Correct. NO_EXPORT prevents advertisement to eBGP peers.

Why this answer

The well-known community NO_EXPORT (0xFFFFFF01) prevents a route from being advertised to any eBGP peers. The route can still be advertised within the AS.

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196
MCQmedium

An SP engineer is designing an OSPF network with several remote sites that should not receive external routes but need to reach external destinations via a default route. Which OSPF area type best meets this requirement while also preventing Type 5 LSAs from entering?

A.NSSA
B.Backbone area 0
C.Stub area
D.Totally stubby area
AnswerD

Correct. Totally stubby area blocks Type 5 and Type 3 LSAs except a default route, so remote sites only have a default route to reach external destinations.

Why this answer

A totally stubby area blocks Type 5 and Type 3 LSAs (except a default route). A stub area blocks Type 5 but allows Type 3 (inter-area routes). An NSSA allows Type 7 LSAs for external routes from the area.

A totally stubby area is the correct choice because it blocks both Type 5 and Type 3 (except default), meeting the requirement.

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197
MCQeasy

Refer to the exhibit. The VRF CUSTOMER is configured with route-target import 100:1. Why is this route not installed in the VRF table?

A.VRF not properly configured.
B.Route target mismatch.
C.Route distinguisher missing.
D.Next-hop unreachable.
AnswerB

The route carries RT:200:1, but the VRF imports only RT:100:1.

Why this answer

B is correct because the VRF CUSTOMER is configured with route-target import 100:1, but the route in question carries a different route-target value (e.g., 100:2). For a route to be installed into a VRF table, the route-target extended community attached to the route must match at least one of the import route-targets configured on the VRF. A mismatch prevents the route from being imported, even if other conditions are satisfied.

Exam trap

Cisco often tests the distinction between route distinguisher (RD) and route-target (RT) — candidates confuse RD uniqueness with RT import/export matching, leading them to pick 'Route distinguisher missing' when the actual issue is a route-target mismatch.

How to eliminate wrong answers

Option A is wrong because the VRF is properly configured with a route distinguisher and route-target import statement; the issue is not a missing or misconfigured VRF definition. Option C is wrong because a route distinguisher is required for VRF route uniqueness in the BGP table, but its absence would cause a different error (e.g., the route not being carried in BGP at all), not a failure to import a route that already has an RD. Option D is wrong because the next-hop reachability is not the cause here; if the next-hop were unreachable, the route would still be considered for import but would be marked as invalid or not installed due to unreachability, whereas the question explicitly states the route is not installed due to route-target mismatch.

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198
MCQmedium

An SP uses OSPF with Traffic Engineering extensions to support RSVP-TE. Which LSA type is used to carry TE information in OSPF?

A.Type 11 Opaque LSAs.
B.Type 10 Opaque LSAs.
C.Type 5 LSAs.
D.Type 9 Opaque LSAs.
AnswerB

Correct. Type 10 Opaque LSAs are area-scoped and carry TE information.

Why this answer

OSPF TE extensions use Opaque LSAs. Specifically, Type 10 Opaque LSAs (area-scoped) carry TE information as defined in RFC 3630.

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199
Multi-Selectmedium

An SP engineer is designing an OSPF network and wants to reduce the number of LSAs in a stub area. Which TWO area types would block Type 5 LSAs but allow Type 3 LSAs? (Choose two.)

Select 2 answers
A.NSSA
B.Totally stubby area
C.Backbone area 0
D.Stub area
E.Normal area
AnswersA, D

NSSA blocks Type 5 but injects Type 7 for external routes.

Why this answer

Stub and NSSA allow Type 3 LSAs but block Type 5 LSAs. Totally stubby blocks both Type 3 and Type 5.

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200
MCQeasy

In a Layer 3 MPLS VPN, a customer site that is part of VRF RED is unable to communicate with another site that is also in VRF RED. The 'show ip bgp vpnv4 vrf RED' command shows the routes, but the router does not install them in the routing table. Which configuration element is most likely missing?

A.A BGP session between PEs.
B.The route-target export on the receiving PE.
C.The route-target import on the receiving PE.
D.An MPLS LDP session between PEs.
AnswerC

Correct. Import is needed to accept routes into the VRF.

Why this answer

In a Layer 3 MPLS VPN, VRF RED routes are learned via MP-BGP between PEs. The 'show ip bgp vpnv4 vrf RED' command confirms the routes are present in the BGP table, but they are not installed in the routing table because the receiving PE lacks the correct route-target import configuration. The route-target import statement on the VRF determines which VPNv4 routes are accepted and installed into the VRF's RIB; without it, the routes remain in BGP but are not added to the routing table.

Exam trap

Cisco often tests the distinction between route-target import and export, and the trap here is that candidates see routes in the BGP table and assume they should be in the routing table, overlooking that import filtering is the final gatekeeper for VRF route installation.

How to eliminate wrong answers

Option A is wrong because a BGP session between PEs (MP-IBGP) is already established, as evidenced by the routes appearing in the BGP table; missing this would prevent route advertisement entirely. Option B is wrong because route-target export on the receiving PE controls which routes are advertised to other PEs, not which routes are imported into the local VRF; the issue is on the import side. Option D is wrong because an MPLS LDP session between PEs is not required for VPNv4 route installation; LDP is used for label distribution in the core, but the route not being installed in the VRF routing table is a BGP/VRF policy issue, not a label-switching problem.

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201
MCQmedium

In BGP route reflection, what is the purpose of the ORIGINATOR_ID attribute?

A.To prevent routing loops by ensuring that a route is not reflected back to its originator
B.To indicate the next-hop IP address
C.To identify the route reflector
D.To carry the community value
AnswerA

A router ignores routes with its own ORIGINATOR_ID.

Why this answer

ORIGINATOR_ID is a non-transitive optional attribute that identifies the router that originated the route into the iBGP domain, used to prevent loops.

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202
MCQmedium

In a Layer 3 MPLS VPN, a CE router is dual-homed to two different PE routers. The PE routers advertise the same prefix to the route reflector with different route targets. What ensures that only the best path is installed in the VRF?

A.Each PE router independently selects the best path based on BGP attributes.
B.The route reflector discards duplicate prefixes.
C.MPLS labels ensure that only one path is used.
D.The CE router must advertise the same route to only one PE.
AnswerA

BGP best path selection runs in the VRF context on each PE, using standard attributes.

Why this answer

BGP path selection applies within the VRF. The route with higher local preference or shorter IGP metric to the next-hop determines the best path. Route targets are used for import/export only.

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203
Multi-Selecthard

Which THREE BGP path attributes are considered during the best path selection process after local preference? (Choose three.)

Select 3 answers
A.Multi-exit discriminator (MED)
B.Next-hop IP address
C.AS path length
D.Router ID
E.Origin code
AnswersA, C, E

Lower MED is preferred.

Why this answer

After local preference, the BGP best path selection process evaluates the AS path length. A shorter AS path is preferred because it indicates a less complex path to the destination network. This attribute is directly compared next, making option C correct.

Exam trap

Cisco often tests the exact order of BGP path selection steps, and the trap here is that candidates mistakenly think MED is compared immediately after local preference, when in fact AS path length and origin code are evaluated before MED.

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204
MCQmedium

Based on the exhibit, which statement about label allocation for prefix 10.1.1.0/24 is true?

A.Router 192.168.2.2 is the penultimate hop for this prefix.
B.The local router will use PHP for this prefix.
C.The local router allocated label 161 for the prefix
D.Router 192.168.1.1 is the egress LSR for this prefix.
AnswerC

Yes, local binding tag is 161.

Why this answer

The exhibit shows that the local router (192.168.1.2) has allocated label 161 for prefix 10.1.1.0/24, as indicated in the output of the 'show mpls forwarding-table' command. This label is assigned by the local router as the ingress LSR to forward traffic toward the egress LSR (192.168.1.1). The correct statement is that the local router allocated label 161 for this prefix.

Exam trap

Cisco often tests the distinction between label allocation (which router assigns the label) and PHP (which router pops the label), causing candidates to confuse the ingress LSR's role with that of the penultimate hop.

How to eliminate wrong answers

Option A is wrong because the penultimate hop for this prefix is router 192.168.1.1 (the egress LSR), not 192.168.2.2; the penultimate hop is the router that pops the label before forwarding to the egress. Option B is wrong because PHP (Penultimate Hop Popping) is performed by the penultimate hop router, not by the local router; the local router is the ingress and does not perform PHP. Option D is wrong because router 192.168.1.1 is the egress LSR for this prefix, as it is the next hop for the label-switched path, but the statement is false because the question asks which statement is true, and this option is not correct in the context of the exhibit (the local router allocated label 161, not that 192.168.1.1 is the egress).

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205
MCQmedium

In BGP confederation, how does the AS_PATH attribute handle the confederation sub-AS numbers to prevent loops?

A.Confederation sub-AS numbers are added to the regular AS_PATH
B.Confederation sub-AS numbers are carried in a separate confederation AS_PATH segment and are stripped when routes leave the confederation
C.Confederation sub-AS numbers are replaced by the confederation identifier
D.Confederation sub-AS numbers are ignored for loop detection
AnswerB

Correct. The confederation AS_PATH is used within the confederation and removed when routes are advertised outside.

Why this answer

In BGP confederation, confederation sub-AS numbers are carried in a separate confederation AS_PATH segment and are not visible to external BGP peers. The confederation AS_PATH is used for loop prevention within the confederation, while the regular AS_PATH remains unchanged when routes leave the confederation.

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206
Multi-Selecteasy

Which THREE are valid reasons for using the 'ipv6 unicast-routing' command on a Cisco router?

Select 3 answers
A.It enables IPv6 on all interfaces.
B.It enables global IPv6 routing.
C.It enables IPv6 CEF.
D.It enables IPv6 multicast routing.
E.It allows configuration of IPv6 routing protocols like OSPFv3.
AnswersB, C, E

Correct. This is the primary purpose.

Why this answer

The 'ipv6 unicast-routing' command globally enables IPv6 unicast routing on the router, which is necessary for the router to forward IPv6 packets between interfaces. Without this command, the router will not process or forward IPv6 unicast traffic, even if IPv6 addresses are configured on interfaces.

Exam trap

Cisco often tests the distinction between interface-level IPv6 configuration and global IPv6 routing enablement, leading candidates to mistakenly think 'ipv6 unicast-routing' activates IPv6 on all interfaces.

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207
MCQeasy

Which routing protocol is used between CE and PE routers in a typical MPLS L3VPN deployment?

A.RIP
B.BGP
C.EIGRP
D.IS-IS
AnswerB

eBGP is commonly used for CE-PE routing in L3VPN to carry customer routes and support multi-homing.

Why this answer

While BGP is a common choice for routing between CE and PE in MPLS L3VPN deployments due to its scalability, policy control, and support for multi-homing, it is not the only protocol. Other dynamic routing protocols such as OSPF, EIGRP, RIP, and static routing are also valid. In a typical deployment, CE and PE can exchange routes using any routing protocol; the PE then redistributes those routes into MP‑BGP for VPN‑v4 advertisement between PEs.

The question’s correct answer is BGP because it is the most common and recommended protocol for many scenarios, but candidates should know that the CE‑PE routing protocol is not limited to BGP.

Exam trap

The most common mistake is assuming that BGP is mandatory between CE and PE in MPLS L3VPN because VPNv4 routes use BGP attributes. However, VPNv4 routes (with RD/RT) are exchanged only between PEs via MP‑BGP. Between CE and PE, any routing protocol (including static, RIP, EIGRP, OSPF, or BGP) can be used; BGP is common but not required.

How to eliminate wrong answers

Option A is wrong because RIP is a distance-vector protocol that lacks the scalability, route filtering, and VPN-specific attributes (RD/RT) required for MPLS L3VPN, and it is rarely used in modern service provider networks. Option C is wrong because EIGRP is a Cisco proprietary protocol that does not natively support the VPNv4 address family or the BGP-based control plane needed for MPLS L3VPN, and it would require complex redistribution and route-target handling. Option D is wrong because IS-IS is an IGP used within the MPLS core for label distribution (e.g., with LDP), not between CE and PE; it cannot carry VPNv4 routes or enforce per-VRF route separation.

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208
MCQhard

A service provider is deploying multicast service for IPTV using PIM-SM with a single RP. During high traffic periods, the RP becomes overloaded. What is the most scalable solution to distribute the load across multiple RPs?

A.Use Anycast-RP
B.Use Bidirectional PIM
C.Increase the RP's memory and CPU
D.Use PIM-DM
AnswerA

Anycast-RP allows multiple RPs under a single RP address, distributing the load.

Why this answer

Anycast-RP allows multiple RPs to share the same IP address, enabling load distribution and redundancy. In PIM-SM, sources register with the nearest RP via unicast routing, and receivers join toward the same Anycast-RP address, which is routed to the closest physical RP. This distributes the registration and join processing load across multiple RPs without requiring protocol changes.

Exam trap

Cisco often tests the misconception that Bidirectional PIM or simply upgrading hardware can solve RP overload, but the key is that Anycast-RP is the only option that distributes the RP load across multiple devices while maintaining a single RP address for the multicast domain.

How to eliminate wrong answers

Option B is wrong because Bidirectional PIM is designed for many-to-many multicast applications and uses a shared tree rooted at the RP, but it does not inherently distribute load across multiple RPs; it still relies on a single RP per group. Option C is wrong because increasing the RP's memory and CPU is a vertical scaling approach that does not address the fundamental architecture limitation of a single RP becoming a bottleneck; it is not a scalable solution for load distribution. Option D is wrong because PIM-DM uses a flood-and-prune mechanism that is inefficient for sparse-mode IPTV deployments and does not use an RP at all, so it cannot distribute RP load.

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209
MCQmedium

You are a network engineer at a service provider. Your network uses MPLS L3VPN with OSPF as the IGP and LDP for label distribution. A customer has two sites connected to different PEs (PE1 and PE2) in the same VPN. The customer's CE routers are running eBGP with the PEs. Recently, the customer reports that traffic between the two sites is intermittently dropping. Upon investigation, you find that the BGP session between PE1 and the CE at site A goes down briefly every few minutes. The logs on PE1 show BGP notifications with error code 'Hold Timer Expired'. The CE router at site A is a low-end device with limited CPU. What is the most likely cause and the best course of action?

A.Configure BGP route dampening on PE1 to suppress flapping routes.
B.Disable LDP on the link between PE1 and CE1.
C.Change the IGP from OSPF to IS-IS to reduce routing updates.
D.Increase the BGP hold time on PE1 to 180 seconds.
AnswerD

Increasing the hold time gives the CE more time to send keepalives, reducing session drops.

Why this answer

The BGP session drops due to 'Hold Timer Expired' because the low-end CE router's CPU is overloaded, causing it to fail sending BGP keepalives within the default 90-second hold time. Increasing the hold time on PE1 to 180 seconds (option D) gives the CE more time to send keepalives, reducing false timeouts. This directly addresses the root cause—insufficient CPU to maintain timely keepalives—without changing routing protocols or suppressing routes.

Exam trap

Cisco often tests the misconception that route dampening or IGP changes fix BGP session stability issues, when the real problem is a mismatch in BGP timers due to peer resource constraints.

How to eliminate wrong answers

Option A is wrong because BGP route dampening suppresses flapping routes but does not prevent the BGP session from going down due to hold timer expiry; it would only penalize routes after the session flaps, not fix the underlying keepalive issue. Option B is wrong because LDP is used for label distribution in the MPLS core and is not involved in the CE-PE eBGP session; disabling it would break MPLS L3VPN functionality. Option C is wrong because changing the IGP from OSPF to IS-IS does not affect BGP keepalive timing or CE router CPU load; it would only alter interior routing updates, which are unrelated to the hold timer expiry between PE and CE.

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210
MCQmedium

An ISP is implementing BGP communities to influence routing behavior for their customers. They want to ensure that a customer's routes are not advertised to a specific transit provider. Which BGP community should be used?

A.LOCAL_AS
B.NO_EXPORT
C.NO_PEER
D.NO_ADVERTISE
AnswerB

This community ensures the route stays within the local AS and is not sent to any external AS.

Why this answer

The NO_EXPORT community (RFC 1997) tells a router to advertise the route to iBGP peers within the same AS but not to any eBGP peers. In this scenario, the ISP wants to prevent a customer's routes from being advertised to a specific transit provider, which is an eBGP neighbor. Applying the NO_EXPORT community to those routes ensures they stay within the ISP's AS and are not sent to any external transit provider.

Exam trap

Cisco often tests the distinction between NO_EXPORT and NO_ADVERTISE, where candidates mistakenly choose NO_ADVERTISE because they think it only blocks eBGP advertisements, but in reality NO_ADVERTISE blocks all advertisements (including iBGP), making NO_EXPORT the correct choice when the goal is to block only external (eBGP) propagation.

How to eliminate wrong answers

Option A (LOCAL_AS) is wrong because it is not a standard BGP community; it is a BGP feature (often used with 'allowas-in' or 'local-as' on a neighbor statement) that prepends the local AS number in the AS_PATH, but it does not control route advertisement to a specific transit provider. Option C (NO_PEER) is wrong because it is not a standard BGP well-known community; the correct community to prevent advertisement to any eBGP peer is NO_EXPORT, and NO_PEER is a common distractor that does not exist in RFC 1997. Option D (NO_ADVERTISE) is wrong because it prevents the route from being advertised to any BGP peer (iBGP or eBGP), which is too restrictive; the requirement is only to block advertisement to a specific transit provider (an eBGP peer), not to all peers.

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211
MCQmedium

A network engineer is troubleshooting a BGP convergence issue in a large service provider network. After a link failure in the core, BGP sessions between route reflectors take a long time to reconverge. The RRs are receiving updates from many clients. Which technology can be implemented to improve convergence time? The network currently uses standard BGP with default timers.

A.BGP Add-Path
B.BGP TCP MD5 authentication
C.BGP route-refresh
D.BGP next-hop-self
AnswerA

Add-Path allows multiple paths per prefix, enabling fast failover.

Why this answer

BGP Add-Path allows a route reflector to advertise multiple paths for the same prefix to its clients, which reduces the need for clients to re-advertise updates after a failure. This speeds up convergence by enabling the route reflector to immediately select and propagate an alternate path without waiting for BGP reconvergence from other clients. In large service provider networks with many clients, this minimizes the delay caused by the route reflector having only a single best path per prefix.

Exam trap

Cisco often tests the misconception that BGP route-refresh or next-hop-self speeds up convergence, but the key is that Add-Path directly reduces reconvergence time by providing pre-computed alternate paths, while the other options address security, soft reconfiguration, or next-hop manipulation without affecting convergence speed.

How to eliminate wrong answers

Option B (BGP TCP MD5 authentication) is wrong because it secures BGP sessions against spoofing but does not affect convergence time or path selection. Option C (BGP route-refresh) is wrong because it triggers a soft reconfiguration to request updates from a peer, which is a manual or triggered operation that does not proactively improve convergence after a failure. Option D (BGP next-hop-self) is wrong because it modifies the next-hop attribute on routes advertised to eBGP peers, typically used in iBGP to ensure reachability, but it does not reduce the number of updates or speed up convergence in a route reflector topology.

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212
MCQhard

In OSPF TE extensions for RSVP-TE, which LSA type is used to carry TE information (such as link attributes) in OSPF?

A.Type 11 Opaque LSA
B.Type 5 AS-external LSA
C.Type 10 Opaque LSA
D.Type 9 Opaque LSA
AnswerC

Type 10 Opaque LSA is area-scoped and carries TE information.

Why this answer

Opaque LSA Type 10 (Area-local) is used for TE information in OSPF. Type 9 is link-local, Type 11 is AS-scoped.

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213
MCQmedium

In an OSPF network, an engineer wants to minimize the size of the routing table in a non-backbone area while still allowing the area to learn external routes from other ASs. Which OSPF area type should be configured?

A.Stub area
B.Totally stubby area
C.NSSA
D.Backbone area 0
AnswerC

NSSA allows external routes via Type 7 LSAs.

Why this answer

A Not-So-Stubby Area (NSSA) allows importing external routes as Type 7 LSAs while still blocking Type 5 LSAs from other areas.

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214
MCQeasy

A network engineer is troubleshooting an OSPF adjacency failure between two directly connected routers, R1 and R2. Both routers are configured with the same OSPF process ID and area. The engineer verifies that the interfaces are up and IP connectivity exists. Which configuration mismatch is most likely causing the adjacency to fail?

A.MTU mismatch between the interfaces
B.Passive interface configuration on one router
C.Area ID mismatch on the interfaces
D.OSPF network type mismatch between the interfaces
AnswerD

Network type mismatch (e.g., broadcast vs. point-to-point) causes the routers to disagree on DR/BDR election and hello behavior, preventing adjacency.

Why this answer

An OSPF network type mismatch, such as one interface configured as point-to-point and the other as broadcast, prevents OSPF adjacency from forming. Options A, B, and C are incorrect: An MTU mismatch (A) usually allows adjacency to form but causes problems during LSA exchange; a passive interface (B) stops the router from sending hellos, so it wouldn't see the neighbor at all, but the question implies both interfaces are operational; an area ID mismatch (C) would prevent adjacency, but the scenario specifies that both routers are configured in the same area. Therefore, the most likely cause of the adjacency failure when interfaces are up and IP connectivity exists is a network type mismatch.

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215
MCQhard

In an OSPF network, an area is configured as a Not-So-Stubby Area (NSSA). Which statement about LSA types in this area is correct?

A.Type 1 and Type 2 LSAs are not allowed in NSSA.
B.Type 4 LSAs are used to advertise the ABR to other areas.
C.Type 7 LSAs are originated by ASBR within the NSSA and converted to Type 5 by the ABR.
D.Type 5 LSAs are flooded within the NSSA.
AnswerC

Correct. Type 7 LSAs originated in NSSA and translated at ABR to Type 5 for other areas.

Why this answer

NSSA areas allow Type 7 LSAs to carry external routes, which are translated to Type 5 at the ABR. Type 5 LSAs are not allowed within the NSSA, and Type 4 LSAs are also blocked. Type 3 LSAs (summary) are allowed.

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216
MCQhard

Based on the exhibit, what is the purpose of the 'mpls ldp neighbor ... password cisco' commands?

A.To synchronize LDP and IGP convergence.
B.To control the label allocation for prefixes from that neighbor.
C.To authenticate the LDP session using MD5.
D.To enable LDP session protection for the neighbor.
AnswerC

The password command enables MD5 authentication for the LDP session.

Why this answer

The 'mpls ldp neighbor ... password cisco' command configures a Message Digest 5 (MD5) authentication password for the LDP session with a specific neighbor. This ensures that the TCP connection used for LDP exchanges is authenticated, preventing spoofed or unauthorized LDP messages from being accepted, as defined in RFC 5036.

Exam trap

Cisco often tests the distinction between LDP authentication (password) and LDP session protection (holdtime/graceful restart), so the trap here is confusing the 'password' keyword with session protection features that maintain adjacency state.

How to eliminate wrong answers

Option A is wrong because synchronizing LDP and IGP convergence is achieved through LDP-IGP synchronization (e.g., 'mpls ldp sync' on an interface), not by setting a password. Option B is wrong because controlling label allocation for prefixes from a neighbor is done via label filtering (e.g., 'mpls ldp neighbor ... label accept' or 'mpls ldp neighbor ... label advertise'), not by a password. Option D is wrong because LDP session protection (e.g., 'mpls ldp session protection') is a separate feature that maintains LDP hello adjacencies and re-establishes sessions after link flaps, unrelated to authentication.

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217
MCQhard

An ISP is implementing 6PE to provide IPv6 connectivity over an MPLS network that only supports IPv4 in the core. In 6PE, how are IPv6 prefixes carried across the MPLS backbone?

A.IPv6 prefixes are transported over an IPv6 MPLS core using LDPv6
B.IPv6 prefixes are carried as IPv6 prefixes in MP-BGP with an IPv4 next-hop
C.IPv6 prefixes are encapsulated in IPv4 tunnels with GRE
D.IPv6 prefixes are converted to IPv4 using NAT64 before MPLS forwarding
AnswerB

Correct; 6PE uses MP-BGP with AFI=2 (IPv6) and SAFI=1, with next-hop as IPv4 address.

Why this answer

6PE uses MP-BGP to carry IPv6 prefixes (using the IPv6 unicast address family, AFI=2 SAFI=1) with an IPv4 next-hop address (the loopback of the peer 6PE router). The BGP session is over IPv4, but the NLRI carries IPv6 prefixes. MPLS labels are used to forward traffic across the IPv4-only core.

It is important to note that 6PE does not use the VPNv6 address family (AFI=1 SAFI=128) — that is used for MPLS L3VPN.

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218
MCQhard

Which OSPF area type is most suitable for a service provider's customer-facing network where external routes are blocked, but internal routes (including inter-area) are allowed, and the area should not accept Type 5 LSAs?

A.NSSA
B.Backbone area
C.Totally stubby area
D.Stub area
AnswerD

Stub area blocks Type 5 LSAs and uses default route.

Why this answer

A stub area blocks Type 5 LSAs but allows inter-area and intra-area routes. It injects a default route. NSSA allows limited external routes via Type 7, which is not desired here.

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