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CCNA MPLS and Segment Routing Questions

43 of 193 questions · Page 3/3 · MPLS and Segment Routing · Answers revealed

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151
Multi-Selectmedium

A network engineer is designing a Segment Routing over IPv6 (SRv6) deployment. Which TWO statements are true about the SRv6 SID format and behaviors? (Choose two.)

Select 2 answers
A.End.X behavior is a default behavior for any SID that is not explicitly defined.
B.End.DT6 behavior is used for IPv6 L3VPN services.
C.End.DT4 behavior is used for EVPN L2VPN services over SRv6.
D.The SRv6 SID is 128 bits long and consists of a Locator, Function, and optional Arguments.
E.The SRH (Segment Routing Header) is an extension header that replaces the IPv6 header when SR is used.
AnswersB, D

Correct. End.DT6 decapsulates and forwards to IPv6 VPN.

Why this answer

SRv6 SID is 128 bits and consists of locator, function, and optional argument. End.DT6 is used for IPv6 L3VPN services. End.DT4 is for IPv4 L3VPN, not for L2VPN.

End.X is not the default behavior; the default is End. The SRH is an extension header that does not replace the IPv6 header. Therefore, the correct answers are B and D.

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152
MCQmedium

Which LDP label distribution mode is characterized by a router advertising labels for all prefixes in its routing table to all neighbors, regardless of whether they are needed?

A.Downstream on demand with conservative label retention
B.Downstream on demand with liberal label retention
C.Downstream unsolicited with conservative label retention
D.Downstream unsolicited with liberal label retention
AnswerD

Correct. This is the default mode for LDP.

Why this answer

Downstream unsolicited means the router sends label bindings without being asked; liberal label retention means it keeps all bindings even if not from the next hop.

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153
MCQmedium

During an MPLS network migration from LDP to Segment Routing, an engineer notices that some routers are not advertising Prefix-SIDs for certain loopbacks. The IGP is OSPF. What configuration is required on these routers to advertise Prefix-SIDs?

A.Enable 'mpls ldp autoconfig' on the loopback interface.
B.Enable 'segment-routing mpls' globally and configure 'prefix-sid index' under the loopback interface.
C.Configure 'segment-routing mpls set-adjacency-sid' on the loopback.
D.Configure 'segment-routing mpls' globally and assign a SID index under the OSPF router process for the loopback.
AnswerD

This enables SR globally and assigns the Prefix-SID under OSPF.

Why this answer

In OSPF, Prefix-SIDs for loopbacks are advertised by configuring 'segment-routing mpls' globally and then assigning a SID index under the OSPF router process using the 'prefix-sid index' command for the specific loopback network. This ties the SID to the OSPF prefix advertisement, enabling SR-MPLS forwarding without LDP.

Exam trap

Cisco often tests the distinction between where Prefix-SID configuration is applied (under the IGP process) versus interface-level commands, leading candidates to mistakenly configure 'prefix-sid index' directly on the loopback interface.

How to eliminate wrong answers

Option A is wrong because 'mpls ldp autoconfig' enables LDP on the interface, which is not used for Segment Routing and would not advertise Prefix-SIDs. Option B is wrong because 'prefix-sid index' is configured under the OSPF router process, not directly under the loopback interface; the interface-level command does not exist for OSPF. Option C is wrong because 'segment-routing mpls set-adjacency-sid' is used to assign Adjacency-SIDs on interfaces, not Prefix-SIDs for loopbacks.

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154
Multi-Selectmedium

Which two actions are performed by an MPLS label edge router (LER) when it receives an unlabeled IP packet destined for a prefix with an LSP? (Choose two.)

Select 2 answers
A.It performs a recursive lookup in the LFIB.
B.It performs a label swap operation.
C.It forwards the packet based on the MPLS label.
D.It pops the top label and forwards based on the inner header.
E.It performs a label push operation to add an MPLS label.
AnswersC, E

Correct: After pushing, the packet is forwarded using the label.

Why this answer

The LER (or ingress LSR) imposes a label stack (push operation) on the packet, then forwards it based on the label. It does not swap labels (that's done by LSRs), nor does it pop the label (egress LER does).

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155
MCQhard

Which SRv6 SID behavior is used to support EVPN-IRB (Integrated Routing and Bridging) for L2VPN with bridging?

A.End.X
B.End.DT2U
C.End.DT4
D.End.DT6
AnswerB

Correct: End.DT2U decapsulates and bridges to an Ethernet segment.

Why this answer

End.DT2U is used for L2VPN bridging (EVPN VPWS) and EVPN-IRB uses End.DT2U for bridge domain.

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156
MCQhard

In SRv6, what is the function of the 'End.DT4' behavior?

A.Decapsulation and IPv4 lookup in a VRF table
B.Decapsulation and IPv6 lookup in a VRF table
C.Decapsulation and Ethernet switching
D.Encapsulation and forwarding to an IPv6 destination
AnswerA

Correct: End.DT4 is for L3VPNv4.

Why this answer

End.DT4 is a SRv6 endpoint behavior that decapsulates an IPv6 packet with an SRH and performs a lookup in the IPv4 VRF table for L3VPN.

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157
MCQmedium

Which RSVP message is used to tear down an MPLS-TE LSP by being sent from the ingress toward the egress?

A.PATH Error
B.PATH Tear
C.RESV Confirm
D.RESV Tear
AnswerB

Correct.

Why this answer

RSVP-TE uses PATH messages to establish state and RESV to reserve. To tear down, a PathTear message is sent from the ingress to the egress, or a ResvTear from egress to ingress.

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158
MCQmedium

An engineer configures an SRv6 SID with the behavior End.DT6. What is the primary function of this SID?

A.Perform IPv4 decapsulation and forward using a VPN IPv4 table.
B.Perform IPv6 decapsulation and forward using a VPN IPv6 routing table.
C.Perform IPv6 encapsulation and forward.
D.Perform IPv6 decapsulation and forward using the global IPv6 routing table.
AnswerB

Correct.

Why this answer

End.DT6 (Endpoint with Decapsulation and IPv6 table lookup) is used for L3VPN over SRv6. It decapsulates the outer IPv6 header and forwards based on the inner IPv6 destination using a specific VRF table.

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159
Multi-Selecthard

Which TWO commands are most effective to verify that an SR-TE policy is active and forwarding traffic? (Choose two.)

Select 2 answers
A.show segment-routing traffic-eng policy
B.show isis database verbose
C.show mpls forwarding-table labels
D.show segment-routing traffic-eng segment-list
E.show bgp labels
AnswersA, C

Shows the SR-TE policies, their state (active/inactive), and candidate paths.

Why this answer

The 'show segment-routing traffic-eng policy' command displays the operational state of SR-TE policies, including whether the policy is 'up' and actively forwarding traffic. It shows the candidate path, binding SID, and forwarding status, making it the primary verification command for SR-TE policy activity.

Exam trap

Cisco often tests the distinction between configuration verification (e.g., segment-lists) and operational/forwarding verification, leading candidates to choose 'show segment-routing traffic-eng segment-list' (which only shows configured lists) instead of the policy state or forwarding table commands.

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160
MCQeasy

A service provider is deploying MPLS in their core network and wants to ensure that all routers in the MPLS domain can dynamically exchange label bindings. Which protocol should be enabled on all routers to meet this requirement?

A.LDP
B.OSPF
C.iBGP
D.RSVP-TE
AnswerA

LDP dynamically distributes labels for all prefixes in the IGP.

Why this answer

LDP (Label Distribution Protocol) is the correct choice because it is the standard protocol used in MPLS networks to dynamically distribute label bindings for all routes in the routing table. It operates by establishing LDP sessions between directly connected LSRs and exchanging label mappings for each prefix, ensuring every router in the MPLS domain has the necessary labels to forward labeled packets.

Exam trap

Cisco often tests the distinction between protocols that distribute labels for all routes (LDP) versus protocols that only distribute labels for specific routes (e.g., iBGP for BGP routes) or are used for traffic engineering (RSVP-TE), leading candidates to mistakenly choose iBGP or RSVP-TE when the question asks for dynamic label exchange across the entire MPLS domain.

How to eliminate wrong answers

Option B (OSPF) is wrong because OSPF is an interior gateway protocol (IGP) used for IP routing, not for distributing MPLS label bindings; it does not carry label information. Option C (iBGP) is wrong because iBGP is used to exchange BGP routes and their associated labels (via the 'label' or 'vpnv4' address family) but only for BGP-learned prefixes, not for all routes in the MPLS domain, and it requires LDP or another label distribution protocol for IGP routes. Option D (RSVP-TE) is wrong because RSVP-TE is a signaling protocol for traffic engineering and MPLS-TE tunnels, not for general dynamic label exchange for all prefixes; it is used for explicit path setup and resource reservation, not for distributing label bindings for all IGP routes.

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161
Multi-Selecthard

A network engineer is troubleshooting an LDP session failure between two directly connected routers. The routers are configured with the 'mpls ldp' command under the interface. The 'show mpls ldp neighbor' command shows no neighbors. Which two additional pieces of information should the engineer verify? (Choose two.)

Select 2 answers
A.Verify that the interface IP addresses are in the same subnet.
B.Verify that TCP port 646 is open on the interface ACL.
C.Verify that 'mpls ip' is enabled globally.
D.Verify that the LDP router IDs are reachable via routing.
E.Verify that OSPF is configured on the interface.
AnswersA, D

LDP hello messages are sent to the all-routers multicast address; mismatched subnets prevent discovery.

Why this answer

LDP sessions are established only between directly connected LSRs when using the default 'mpls ldp' interface configuration. The LDP Hello messages are sent as UDP packets to the multicast address 224.0.0.2, and if the interface IP addresses are not in the same subnet, the Hello messages will not be received, preventing neighbor discovery.

Exam trap

Cisco often tests the distinction between LDP Hello (UDP multicast) and LDP session (TCP unicast), and candidates mistakenly focus on TCP ACLs or global MPLS commands instead of verifying subnet adjacency and router ID reachability.

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162
MCQmedium

A service provider is using Segment Routing with TI-LFA for fast convergence. During a link failure, the router performing the local repair must compute a backup path that avoids the failed link. Which type of Adjacency Segment Identifier (Adj-SID) is required for the backup path to be loop-free?

A.Anycast Adj-SID
B.Unprotected Adj-SID
C.Protected Adj-SID
D.Backup Adj-SID
AnswerC

Protected Adj-SID enables fast reroute protection in SR networks.

Why this answer

In Segment Routing with TI-LFA, the router performing local repair must compute a backup path that is guaranteed loop-free. A Protected Adj-SID (Adj-SID with the B-flag set) is required because it indicates that the adjacency segment is protected by a fast-reroute mechanism, allowing the router to use a precomputed backup path that avoids the failed link while ensuring no forwarding loops occur.

Exam trap

Cisco often tests the misconception that any Adj-SID can be used for TI-LFA backup, but only the Protected Adj-SID (B-flag set) guarantees loop-free behavior, while Unprotected Adj-SID leads to potential loops or blackholing.

How to eliminate wrong answers

Option A is wrong because an Anycast Adj-SID identifies a set of routers sharing the same anycast prefix, not a specific adjacency, and it does not provide per-link protection or loop-free backup path computation. Option B is wrong because an Unprotected Adj-SID (B-flag not set) explicitly indicates no fast-reroute protection is available, so it cannot be used for a TI-LFA backup path. Option D is wrong because there is no standard 'Backup Adj-SID' in Segment Routing; the correct term is Protected Adj-SID, and the backup path is derived from the protected adjacency SID.

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163
MCQmedium

In RSVP-TE, which message is sent by the headend router along the explicit route to reserve resources and establish an LSP?

A.RESV
B.PATH
C.RSVP Hello
D.RSVP Tear
AnswerB

Correct. PATH carries the ERO and requests resources.

Why this answer

In RSVP-TE, the headend router sends a PATH message along the explicit route to signal the desired LSP. This message carries the label request, traffic parameters, and the explicit route object (ERO) to each downstream router, which then forwards it hop-by-hop toward the destination. Only after the PATH message reaches the tailend does the tailend respond with a RESV message to actually reserve resources and establish the LSP.

Exam trap

Cisco often tests the distinction between the direction of PATH (headend to tailend) and RESV (tailend to headend), trapping candidates who confuse which message initiates the reservation versus which message actually reserves resources.

How to eliminate wrong answers

Option A is wrong because the RESV message is sent by the tailend router back toward the headend to reserve resources and confirm the LSP, not by the headend. Option C is wrong because the RSVP Hello message is used for neighbor liveness detection and graceful restart, not for LSP establishment. Option D is wrong because the RSVP Tear message is used to tear down an existing LSP, not to establish one.

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164
MCQeasy

A service provider has deployed segment routing with OSPF as the IGP in its core network. The network consists of 100 routers in a single area. The operations team reports that after a link failure between Router X and Router Y, traffic from Router A to Router B is taking a suboptimal path even though IGP convergence is complete and all routers have updated their LSDB. Router A and Router B are both segment routing capable. The team verifies that no SR-TE policies are configured and that all routers are using the default SPF algorithm. The expected shortest path from A to B should go through the newly restored link, but instead it still traverses an alternate path. Which action should resolve the issue?

A.Remove and re-add the adjacency SID configuration on the restored link.
B.Execute 'clear ip ospf process' on all routers along the expected path.
C.Configure an SR-TE policy from A to B with an explicit path using the restored link.
D.Issue 'clear mpls forwarding labels' on Router A to rebuild the label table.
AnswerB

Correct. 'clear ip ospf process' forces a fresh LSA flood and SPF computation, updating the routing table to use the restored link.

Why this answer

The suboptimal path persists despite LSDB convergence due to OSPF not performing a full SPF recalculation after the link restoration. 'clear ip ospf process' forces a complete SPF computation and reflooding of LSAs, which corrects the routing table. Option A is incorrect because adjacency SIDs are dynamically allocated and removing/re-adding them does not address the root cause. Option C is incorrect because SR-TE policies are not required for shortest-path forwarding and may unnecessarily override the IGP.

Option D is incorrect because clearing MPLS forwarding labels does not affect the IGP routing decision; the issue is at Layer 3, not label allocation.

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165
MCQhard

A service provider is migrating from LDP to Segment Routing in an IS-IS network. After enabling IS-IS with segment-routing on all routers, they observe that some prefixes still receive labels from LDP instead of from SR. Which configuration is most likely missing on these routers?

A.Missing 'segment-routing mpls' command under IS-IS
B.OSPF segment-routing is still configured
C.TI-LFA is not enabled
D.SRGB range conflicts with MPLS label range
AnswerA

This command enables SR label allocation in IS-IS.

Why this answer

When migrating from LDP to Segment Routing in an IS-IS network, the 'segment-routing mpls' command must be explicitly enabled under the IS-IS routing process. Without this command, IS-IS will not allocate MPLS labels for prefixes using the Segment Routing (SR) extension, causing the router to fall back to LDP for label distribution. This is the most common missing configuration when SR labels are not being assigned.

Exam trap

Cisco often tests the distinction between enabling segment-routing globally versus enabling it under the IGP process; the trap here is that candidates assume 'segment-routing mpls' is automatically applied when SR is enabled globally, but it must be explicitly configured under IS-IS or OSPF.

How to eliminate wrong answers

Option B is wrong because OSPF segment-routing configuration is irrelevant in an IS-IS network; the question explicitly states IS-IS is used, so OSPF settings have no effect on IS-IS SR behavior. Option C is wrong because TI-LFA (Topology Independent Loop-Free Alternate) is a fast-reroute mechanism that relies on SR, but its absence does not prevent SR from assigning labels; it only affects protection. Option D is wrong because an SRGB range conflict with the MPLS label range would cause label allocation failures or errors, not a fallback to LDP; LDP would still be used only if SR is not properly enabled.

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166
MCQmedium

In an EVPN-VPWS (Virtual Private Wire Service) deployment, which identifier is used to uniquely identify an Ethernet Segment for multi-homing?

A.Ethernet Tag ID
B.MAC address of the CE
C.Ethernet Segment Identifier (ESI)
D.Route Distinguisher (RD)
AnswerC

Correct identifier.

Why this answer

In EVPN-VPWS, the Ethernet Segment Identifier (ESI) uniquely identifies an Ethernet Segment across all multi-homed Provider Edge (PE) nodes. This 10-byte value is used in Designated Forwarder (DF) election and split-horizon filtering to prevent loops when a Customer Edge (CE) is multi-homed to multiple PEs. The ESI is carried in EVPN routes (Type 1 and Type 4) to signal multi-homing membership.

Exam trap

Cisco often tests the distinction between the Ethernet Tag ID (used for service identification) and the ESI (used for multi-homing segment identification), causing candidates to confuse the two identifiers in EVPN-VPWS contexts.

How to eliminate wrong answers

Option A is wrong because the Ethernet Tag ID identifies a specific service instance (e.g., VLAN or VPWS service) within an EVPN instance, not the multi-homed segment. Option B is wrong because the MAC address of the CE is not used for segment identification; EVPN-VPWS relies on the ESI for multi-homing, and MAC addresses are irrelevant in point-to-point VPWS. Option D is wrong because the Route Distinguisher (RD) is used to make EVPN routes unique across different VRFs or EVPN instances, not to identify an Ethernet Segment for multi-homing.

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167
MCQhard

A service provider wants to use Segment Routing TE with Flex-Algo to engineer traffic around links with low bandwidth. Which configuration approach should be taken?

A.Configure a policy-map to apply QoS marking on TE tunnels.
B.Use the 'min-delay' metric in Flex-Algo to prefer low-delay links.
C.Define a Flex-Algo with a metric type of 'min-unidirectional-link-bandwidth'.
D.Use standard IS-IS metric and adjust it on low-bandwidth links to a high value.
AnswerC

This Flex-Algo metric excludes links with insufficient bandwidth, meeting the requirement.

Why this answer

Flex-Algo allows the definition of a custom metric type, such as 'min-unidirectional-link-bandwidth', which enables Segment Routing TE to compute paths that avoid low-bandwidth links. This approach directly addresses the requirement to engineer traffic around links with low bandwidth by using bandwidth as the path selection metric.

Exam trap

Cisco often tests the distinction between standard IGP metric manipulation and Flex-Algo's custom metric types, where candidates mistakenly think adjusting the IS-IS metric is the correct Flex-Algo approach, but Flex-Algo requires explicit definition of a new metric type rather than relying on the default metric.

How to eliminate wrong answers

Option A is wrong because QoS marking on TE tunnels does not influence path computation; it only affects traffic prioritization and queuing, not the selection of links based on bandwidth. Option B is wrong because the 'min-delay' metric in Flex-Algo prefers low-delay links, which is unrelated to bandwidth constraints; it would not avoid low-bandwidth links. Option D is wrong because adjusting the standard IS-IS metric on low-bandwidth links to a high value is a traditional traffic engineering technique, but it does not leverage Segment Routing TE with Flex-Algo, which provides more granular and dynamic control via custom metric types.

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168
MCQeasy

What is the purpose of the SRGB (Segment Routing Global Block) in SR-MPLS?

A.It defines a contiguous block of labels used for global SIDs such as Node SIDs.
B.It defines the range of labels for Adjacency SIDs.
C.It is used to reserve labels for RSVP-TE tunnels.
D.It is the set of labels allowed for MPLS VPNs.
AnswerA

Correct definition.

Why this answer

The SRGB defines a range of labels reserved for global segment routing SIDs (e.g., Node SIDs). It ensures that the same label value represents the same prefix across the domain, avoiding label conflicts.

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169
MCQmedium

A service provider is implementing MPLS in their core network. They want to ensure that labeled packets for BGP prefixes are forwarded correctly without requiring an IGP label for every BGP prefix. Which technique should be used to achieve this?

A.Use BGP-free core with LDP or Segment Routing
B.Enable MPLS on all interfaces and rely on BGP label distribution
C.Implement LDP for BGP prefixes
D.Configure static labels for BGP prefixes
AnswerA

BGP-free core allows forwarding based on labels assigned by LDP/SR for BGP next hops.

Why this answer

A BGP-free core with LDP or Segment Routing allows the service provider to forward labeled packets for BGP prefixes without requiring an IGP label for every BGP prefix. In this design, edge routers (PEs) impose a label stack where the outer label (IGP label) is distributed by LDP or Segment Routing for the BGP next-hop, and the inner label is the BGP label. Core routers (P routers) only need to swap the outer label based on the IGP label, not the BGP prefix, thus eliminating the need for IGP labels per BGP prefix.

Exam trap

Cisco often tests the misconception that LDP can distribute labels for BGP prefixes, but LDP is strictly for IGP prefixes; the correct approach is to use a BGP-free core where LDP or Segment Routing handles the IGP label for the BGP next-hop, not the BGP prefix itself.

How to eliminate wrong answers

Option B is wrong because enabling MPLS on all interfaces and relying on BGP label distribution alone does not provide a label-switched path for the BGP next-hop; core routers would still need an IGP label to forward the packet, which is not automatically generated for BGP prefixes. Option C is wrong because LDP is designed to distribute labels for IGP prefixes (e.g., loopback addresses), not for BGP prefixes; LDP cannot directly assign labels to BGP prefixes. Option D is wrong because configuring static labels for BGP prefixes is not scalable and does not leverage dynamic label distribution; it would require manual configuration for every prefix and does not integrate with the IGP label-switched path.

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170
MCQeasy

A network engineer is configuring Segment Routing on an IOS-XR router. They want to use the preferred algorithm for automatically computing paths based on IGP metrics. Which algorithm should be configured under the SR segment list?

A.strict-spf
B.delay
C.te
D.preferred
AnswerD

The 'preferred' algorithm uses the IGP metric-based shortest path.

Why this answer

The 'preferred' algorithm is the correct choice because it instructs the router to automatically select the best algorithm for path computation based on IGP metrics. In Segment Routing, the 'preferred' algorithm uses the default SPF (Shortest Path First) algorithm, which computes paths based solely on IGP link metrics, ensuring optimal routing without additional constraints.

Exam trap

Cisco often tests the distinction between 'preferred' (which defaults to IGP metric-based SPF) and 'strict-spf' (which disables ECMP), leading candidates to mistakenly choose 'strict-spf' when the question asks for standard IGP metric-based path computation.

How to eliminate wrong answers

Option A (strict-spf) is wrong because it enforces a strict SPF algorithm that avoids any load-balancing or ECMP, which is not the default behavior for IGP metric-based path computation. Option B (delay) is wrong because it refers to a performance-based algorithm (e.g., latency) that is not used for standard IGP metric-based path computation; it is typically used in advanced traffic engineering scenarios. Option C (te) is wrong because it represents a Traffic Engineering algorithm that considers constraints like bandwidth or administrative policies, not the simple IGP metric-based path computation required by the question.

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171
MCQmedium

In Segment Routing over MPLS (SR-MPLS), how are Prefix-SIDs advertised in IS-IS?

A.Via LDP label mapping messages
B.Via RSVP-TE PATH messages
C.Via BGP VPN labels
D.Via IS-IS TLV 135 and TLV 149
AnswerD

Correct. IS-IS uses these TLVs for SR SID advertisement.

Why this answer

In SR-MPLS, Prefix-SIDs are advertised in IS-IS using TLV 135 (Extended IP Reachability) to carry the prefix and its metric, and TLV 149 (IS-IS Router Capability) to carry the SID/Label Binding sub-TLV, which maps the prefix to a specific label (Prefix-SID). This allows IS-IS to distribute both the prefix reachability and its associated segment identifier in a single protocol update, enabling source routing without LDP or RSVP-TE.

Exam trap

Cisco often tests the distinction between traditional MPLS label distribution protocols (LDP, RSVP-TE) and SR-MPLS's use of IGP extensions (IS-IS TLVs or OSPF Opaque LSAs) to carry segment IDs, leading candidates to mistakenly associate Prefix-SIDs with LDP or RSVP-TE.

How to eliminate wrong answers

Option A is wrong because LDP label mapping messages are used for distributing labels in MPLS LDP, not for advertising Prefix-SIDs in SR-MPLS, which uses IS-IS or OSPF extensions. Option B is wrong because RSVP-TE PATH messages are used for traffic engineering label distribution and path setup, not for advertising Prefix-SIDs in IS-IS. Option C is wrong because BGP VPN labels are used in MPLS VPN environments to identify VPN routes, not for advertising Prefix-SIDs in IS-IS.

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172
Multi-Selecteasy

Which two MPLS OAM tools can be used to troubleshoot label switching paths? (Choose two)

Select 2 answers
A.traceroute
B.show ip route
C.MPLS LSP ping
D.ping
E.MPLS echo request
AnswersC, E

MPLS LSP ping sends MPLS echo request to verify LSP connectivity.

Why this answer

MPLS LSP ping (option C) is an OAM tool specifically designed to verify the data plane of an MPLS label switched path (LSP) by sending an MPLS echo request packet with a target FEC stack, which is forwarded along the LSP and validated by the egress LSR. This tests the actual label forwarding path, not just the control plane, making it a correct choice for troubleshooting LSPs.

Exam trap

Cisco often tests the distinction between control-plane verification (like 'show ip route' or 'ping') and data-plane OAM tools (like MPLS LSP ping and MPLS echo request), leading candidates to mistakenly select generic IP tools that cannot validate label switching.

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173
MCQeasy

A network operator is migrating from traditional MPLS LDP to Segment Routing (SR) with IS-IS. The network consists of four routers (R1-R4) in a square topology. The engineer has enabled SR on all routers and configured prefix-SIDs for loopbacks. However, when checking the MPLS forwarding table on R1, the engineer sees that some prefixes have label values that are not the prefix-SIDs. For example, the prefix for R4's loopback shows label 16004 instead of the expected 16004 (which is correct). But for another prefix, the label is 16003 instead of 16003 (correct). The engineer does not see any labels for some external routes. What is the most likely reason that some labels are missing?

A.The IS-IS wide metrics are not enabled on all interfaces.
B.Route redistribution from another protocol is not configured.
C.The prefix-SIDs are inconsistent across routers.
D.External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism.
AnswerD

SR only assigns labels to IGP prefixes; external routes need separate handling.

Why this answer

Segment Routing (SR) with IS-IS only assigns prefix-SIDs to prefixes that are part of the IS-IS domain and explicitly configured with a SID. External routes, such as those redistributed from another protocol (e.g., BGP or OSPF), are not covered by prefix-SIDs and require a separate label distribution mechanism like LDP or a manually configured explicit-null label to be forwarded with MPLS encapsulation. The engineer's observation that some labels are missing for external routes directly points to this limitation.

Exam trap

Cisco often tests the misconception that SR with IS-IS automatically assigns labels to all routes in the routing table, when in fact prefix-SIDs only apply to IGP routes within the same protocol domain, and external routes still need LDP or another label distribution mechanism.

How to eliminate wrong answers

Option A is wrong because IS-IS wide metrics are required for SR operation (to support the extended TLV format for SIDs), but their absence would cause SR to fail entirely or produce incorrect SID assignments, not just missing labels for external routes. Option B is wrong because route redistribution from another protocol is not the root cause; while redistribution may introduce external routes, the missing labels are due to the lack of a label distribution mechanism for those routes, not the act of redistribution itself. Option C is wrong because inconsistent prefix-SIDs across routers would cause label mismatch or forwarding loops, not missing labels; the engineer already confirmed that some prefix-SIDs (e.g., 16004) are correctly installed, indicating consistency is not the issue.

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174
Multi-Selecteasy

Which TWO types of SIDs are defined in Segment Routing? (Select two.)

Select 2 answers
A.Adjacency-SID
B.TI-LFA SID
C.Service-SID
D.Prefix-SID
E.Node-SID
AnswersA, D

Identifies a link adjacency.

Why this answer

Segment Routing (SR) defines two primary types of Segment Identifiers (SIDs): the Prefix-SID and the Adjacency-SID. A Prefix-SID is a global or local label associated with an IGP prefix (e.g., a loopback), enabling shortest-path forwarding toward that prefix. An Adjacency-SID is a local label associated with a specific adjacency (link) between two routers, forcing traffic out that exact interface regardless of the IGP shortest path.

Exam trap

Cisco often tests the distinction between Node-SID and Prefix-SID, where candidates mistakenly treat Node-SID as a separate SID type, but it is actually a Prefix-SID assigned to the router's loopback address.

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175
MCQhard

A network operator is implementing segment routing (SR-MPLS) using OSPF as the IGP. Which OSPF TLV is used to advertise the Node SID (Prefix SID) for a router's loopback address?

A.Router Information TLV
B.Traffic Engineering TLV
C.Extended Prefix TLV
D.Extended Link TLV
AnswerC

Correct TLV for Prefix SID.

Why this answer

In OSPF, the Extended Prefix TLV (type 1) carries the Prefix SID sub-TLV for segment routing. The Adjacency SID is carried in the Extended Link TLV.

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176
Multi-Selectmedium

In EVPN multi-homing, which two mechanisms are used to avoid duplicate packets and ensure efficient forwarding? (Choose two.)

Select 2 answers
A.MAC mobility
B.Split-horizon filtering
C.Aliasing
D.ARP suppression
E.Designated Forwarder (DF) election
AnswersB, E

Correct: Split-horizon prevents loops.

Why this answer

Designated Forwarder (DF) election ensures only one PE forwards traffic to the CE for broadcast/multicast/unknown unicast, and split-horizon prevents a PE from forwarding packets back to the ES from which they were received.

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177
MCQmedium

Refer to the exhibit. The router has three LDP neighbors established. A network administrator notices that MPLS labels for the prefix 10.4.4.0/24 are not being advertised from this router to its neighbors, although the prefix is present in the routing table. Which is the most likely cause?

A.The LDP session to 10.3.3.3 is down.
B.The route to 10.4.4.0/24 is not the best path in the routing table.
C.The 'mpls ldp' command has not been applied globally.
D.The multicast routing table is empty for the prefix.
AnswerB

LDP assigns a label only to the best IGP route; if the route is not best, no label is allocated.

Why this answer

The LDP neighbor for 10.4.4.4 (via 10.3.3.3) is up, but the router does not have a label for 10.4.4.0/24. This suggests the route is not being used for forwarding because of the IGP metric or administrative distance. Option A is wrong because the neighbor is up.

Option C is wrong because LDP has been configured globally. Option D is wrong because there is no indication of an mroute issue.

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178
MCQeasy

Which label is used for an Adjacency SID in SR-MPLS?

A.A label from RSVP-TE reserved range
B.A label from the static label range
C.A label from the dynamic label range
D.A label from the SRGB (Segment Routing Global Block)
AnswerC

Correct: Adjacency SIDs are allocated from the dynamic range.

Why this answer

The Adjacency SID is local to the router and represents a specific link; it is typically allocated from the dynamic label range, not the SRGB.

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179
Multi-Selecthard

Which three statements accurately describe the benefits of Segment Routing over RSVP-TE for traffic engineering? (Choose three.)

Select 3 answers
A.It does not require a signaling protocol like RSVP.
B.It provides faster convergence than RSVP-TE FRR.
C.It uses the IGP to distribute topology and segment information, enabling distributed intelligence.
D.No per-LSP state is maintained on transit routers.
E.It eliminates the need for CSPF path computation.
AnswersA, C, D

Correct: SR uses IGP, no RSVP needed.

Why this answer

SR-TE reduces state by not requiring per-LSP state on intermediate nodes, supports distributed intelligence via IGP, and does not require a separate protocol like RSVP. However, SR-TE still requires CSPF for path computation, and RSVP-TE also supports fast reroute.

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180
MCQmedium

Which MPLS TE mechanism provides link protection by pre-establishing a backup LSP that bypasses a specific link, activated when the link fails?

A.LDP FRR
B.Bypass tunnel
C.Detour tunnel
D.TE backup path
AnswerB

Bypass tunnel protects a link or node.

Why this answer

RSVP-TE Fast Reroute uses bypass tunnels to protect links, providing local repair upon failure.

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181
MCQeasy

During MPLS LDP operation, a router receives a label mapping for a prefix from its neighbor. What is the correct behavior when the receiving router has a route to that prefix?

A.It installs the label as the outgoing label for the prefix.
B.It discards the label mapping unless it has a corresponding route.
C.It uses UDP to send a notification back.
D.It installs the label as the incoming label for the prefix.
AnswerA

Correct. The router installs the received label as the outgoing label for the prefix, because the label is from the next-hop neighbor and is used to forward packets toward that neighbor.

Why this answer

When a router receives a label mapping from an LDP neighbor for a prefix, it installs the received label as the outgoing label for that prefix in its Label Forwarding Information Base (LFIB). This label is used when forwarding packets toward the neighbor. The router must have a route to the prefix in its routing table to accept the mapping.

Exam trap

Cisco often tests the distinction between incoming and outgoing labels in LDP, trapping candidates who assume the received label is installed as the incoming label. The received label is actually the outgoing label for forwarding.

How to eliminate wrong answers

Option A is wrong because the received label mapping is installed as the incoming label, not the outgoing label; the outgoing label is assigned by the downstream router and sent in a label mapping message to the upstream router. Option B is wrong because LDP does not discard the label mapping if a route exists; instead, it requires a matching route to install the label, but the mapping is accepted and used when the route is present. Option C is wrong because LDP uses TCP (not UDP) for reliable transport of label mapping messages, and no notification is sent back to the neighbor upon receipt of a label mapping; acknowledgments are handled via TCP.

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182
MCQeasy

An engineer is configuring MPLS on a Cisco router. Which label operation is performed on the ingress LSR when a packet enters the MPLS domain?

A.Pop the top label
B.Push a new label
C.No label operation
D.Swap the top label
AnswerB

Push is performed by the ingress LSR.

Why this answer

The ingress LSR pushes a new label onto the stack, encapsulating the original IP packet with an MPLS header.

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183
MCQmedium

Which RSVP reservation style is most commonly used in MPLS-TE to allow sharing of resources among multiple LSPs of the same session?

A.Wildcard Filter (WF)
B.Distinct Filter (DF)
C.Fixed Filter (FF)
D.Shared Explicit (SE)
AnswerD

Correct. SE is used in MPLS-TE.

Why this answer

Shared Explicit (SE) style allows multiple senders to share a reservation, which is typical for MPLS-TE where the headend may have multiple LSPs to the same destination.

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184
MCQmedium

What is the purpose of Constrained Shortest Path First (CSPF) in MPLS-TE?

A.To distribute labels for LDP
B.To calculate the best path for L3VPN
C.To find a path that satisfies traffic engineering constraints and is as short as possible
D.To compute the shortest path without any constraints
AnswerC

Correct: CSPF applies constraints to SPF.

Why this answer

CSPF computes paths that meet TE constraints such as bandwidth, affinity, and explicit hops while optimizing the path cost.

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185
MCQmedium

A large enterprise uses MPLS L3VPN to connect multiple sites. They want to implement inter-AS option B (ASBR-to-ASBR MP-eBGP) for scalability. Which statement about this model is correct?

A.Each ASB router maintains VRFs for all attached customers
B.Inter-AS option B requires a full mesh of MP-iBGP between all PEs
C.MP-eBGP sessions are established per VRF between ASBRs
D.ASBRs exchange VPNv4 routes using MP-eBGP
AnswerD

In Option B, ASBRs exchange VPNv4 routes, not IPv4 labeled unicast routes. IPv4 labeled unicast is characteristic of Option C.

Why this answer

In Inter-AS Option B, ASBRs use MP-eBGP to exchange VPNv4 routes. They do not maintain VRFs (Option A incorrect), and sessions are per address family, not per VRF (Option C incorrect). Option B is incorrect because a full mesh of MP-iBGP between PEs is not required.

Option D correctly describes the VPNv4 route exchange using MP-eBGP.

Exam trap

Cisco often tests the misconception that ASBRs need VRFs or per-VRF sessions in Option B, when in fact they operate at the VPNv4 address family level without any customer-specific configuration.

How to eliminate wrong answers

Option A is wrong because ASBRs in Option B do not maintain VRFs for attached customers; VRFs are only configured on PE routers, while ASBRs simply forward labeled VPNv4 routes using MP-eBGP. Option B is wrong because Inter-AS Option B does not require a full mesh of MP-iBGP between all PEs; it relies on MP-eBGP between ASBRs and MP-iBGP within each AS between PEs and route reflectors. Option C is wrong because MP-eBGP sessions between ASBRs are not established per VRF; they are established per address family (IPv4 labeled unicast or VPNv4) and carry all VPN routes in a single session.

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186
MCQmedium

Refer to the exhibit. A network engineer notices that the local label 19 for prefix 10.4.4.0/24 shows 'Untagged' in the outgoing label column. What is the most likely cause?

A.The next-hop router 192.168.3.3 is unreachable.
B.Penultimate Hop Popping is enabled on the next-hop router.
C.The MTU on interface Gi0/0/2 is too small.
D.LDP is not enabled on the interface toward 192.168.3.3.
AnswerD

If LDP is not enabled, no label is received from the next-hop, resulting in 'Untagged'.

Why this answer

When LDP is not enabled on the interface toward the next-hop router (192.168.3.3), the local router cannot exchange label bindings with that neighbor. As a result, the local label for prefix 10.4.4.0/24 remains in the LIB but is not programmed into the LFIB with an outgoing label, showing 'Untagged' because the router must pop the label (or forward as IP) when sending traffic to that next hop.

Exam trap

Cisco often tests the distinction between 'Untagged' (no label from LDP neighbor) and 'Pop tag' (implicit-null from PHP), leading candidates to mistakenly attribute 'Untagged' to PHP when it actually indicates a missing LDP adjacency.

How to eliminate wrong answers

Option A is wrong because if the next-hop router 192.168.3.3 were unreachable, the route itself would not be in the routing table, and the label entry would not appear at all or would show 'no route' rather than 'Untagged'. Option B is wrong because Penultimate Hop Popping (PHP) causes the next-hop router to advertise an implicit-null label (label 3), which results in 'Pop tag' or 'Untagged' in the outgoing label column only when PHP is explicitly negotiated; however, PHP is a normal LDP behavior and would not indicate a problem—the question implies a fault, not normal operation. Option C is wrong because a small MTU on interface Gi0/0/2 would cause fragmentation or packet drops, but it would not affect the label binding or cause the outgoing label to show 'Untagged' in the LFIB.

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187
MCQmedium

An engineer is troubleshooting an MPLS L3VPN where customers behind CE1 cannot reach a specific prefix behind CE2. The PE routers are using OSPF as the IGP and LDP for label distribution. On PE2, the prefix is present in the VRF routing table, but not in the VRF forwarding table. What is the most likely cause?

A.MTU mismatch is causing the VPN label to be dropped.
B.OSPF is not redistributing the BGP routes into the IGP on PE2.
C.The VRF is not properly configured on PE2's interface toward CE2.
D.The route is missing a label in the LFIB on PE2.
AnswerD

If the label is missing, the route cannot be installed in the VRF forwarding table.

Why this answer

The prefix is present in the VRF routing table (RIB) but missing from the VRF forwarding table (FIB) on PE2. This indicates that the route has been learned via BGP and installed in the RIB, but the MPLS VPN label (the inner label) required to forward the packet across the MPLS core is absent. Without a valid label in the LFIB, the CEF (FIB) cannot install the route, causing the reachability failure.

Option D correctly identifies this missing label in the LFIB as the root cause.

Exam trap

Cisco often tests the distinction between the routing table (RIB) and the forwarding table (FIB) in MPLS VPNs, trapping candidates who assume that a route present in the RIB automatically means it is usable for forwarding, when in fact the FIB requires a valid label binding to install the route.

How to eliminate wrong answers

Option A is wrong because an MTU mismatch would cause packet fragmentation or drop at the IP layer, not prevent a route from being installed in the VRF forwarding table; the route would still appear in both the RIB and FIB. Option B is wrong because OSPF redistribution of BGP routes into the IGP is not required for MPLS L3VPN operation; the PE routers exchange VPNv4 routes via MP-BGP, and the IGP (OSPF) is only used for core reachability and LDP label distribution, not for carrying customer prefixes. Option C is wrong because if the VRF were not properly configured on PE2's interface toward CE2, the CE-facing interface would not be associated with the VRF, and the prefix would likely not appear in the VRF routing table at all; the issue is specifically that the route is in the RIB but not the FIB, pointing to a label problem, not a VRF interface misconfiguration.

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188
MCQmedium

A service provider is troubleshooting an MPLS L3VPN scenario where a customer in VRF Blue cannot reach a server in VRF Blue at a remote site. The PEs are running MPLS with LDP. The VRF on both PEs shows the remote prefix. The PE at the local site shows the label from the remote PE for the prefix in the BGP table. However, when pinging from the CE, the packets are dropped. A packet capture on the core shows MPLS packets with the correct VPN label, but the transport label is missing. Further investigation shows that the LDP session between the two PEs is up, but the LDP label binding for the remote PE's loopback is not present. What is the most likely cause and correct action?

A.Increase the TTL on the MPLS packets
B.Change the transport address in LDP to the loopback IP
C.Configure the remote PE's loopback to be advertised via BGP
D.Check MPLS LDP interface configuration and enable LDP on all core interfaces
AnswerD

LDP must be enabled on each core interface to exchange label bindings for loopbacks.

Why this answer

The LDP session is up, but the label binding for the remote loopback is missing. This could be because LDP is not enabled on the underlying interfaces between the PEs, or because the label space is configured incorrectly. Typically, if LDP is enabled on all core interfaces, label bindings for loopbacks are automatically exchanged.

The missing binding suggests that either LDP is not configured on some interfaces, or an access-list is blocking LDP. The most direct action is to verify that LDP is enabled on all core interfaces using the command 'show mpls ldp interface'.

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189
MCQmedium

Which RSVP-TE filter style is commonly used for MPLS-TE tunnels to allow multiple senders on the same session?

A.Distinct Explicit (DE)
B.Fixed Filter (FF)
C.Shared Explicit (SE)
D.Wildcard Filter (WF)
AnswerC

Correct: SE allows multiple senders sharing a reservation.

Why this answer

Shared Explicit (SE) allows multiple senders, each with its own filter spec, but they share the same reservation. Fixed Filter (FF) is per sender. The question likely refers to SE style.

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190
MCQeasy

In MPLS-TE Fast Reroute (FRR), what is a 'detour'?

A.A secondary path for load balancing
B.A backup LSP that protects a single primary LSP
C.A bypass tunnel that protects multiple LSPs
D.A mechanism to reroute traffic after failure detection
AnswerB

Correct: Detour provides 1:1 protection.

Why this answer

A detour is a 1:1 protection mechanism where a backup LSP is created for each protected LSP, often pre-signaled.

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191
MCQmedium

In EVPN-VPWS, which identifier is used to uniquely identify an Ethernet Segment for multi-homing?

A.Route Distinguisher
B.EVI
C.MAC address
D.ESI
AnswerD

ESI identifies the Ethernet segment.

Why this answer

The Ethernet Segment Identifier (ESI) uniquely identifies an Ethernet segment that is multi-homed to two or more PEs.

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192
MCQhard

In SRv6, which SID behavior is used to achieve L3VPN IPv6 VPN forwarding, where the SID points to a VPN instance and performs decapsulation and IPv6 lookup?

A.End.DT6
B.End.X
C.End.DT2U
D.End.DT4
AnswerA

End.DT6 handles IPv6 VPN.

Why this answer

End.DT6 is the SRv6 endpoint behavior for L3VPN over IPv6, performing decapsulation and lookup in the appropriate VPN routing table.

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193
MCQmedium

In EVPN-VPWS, which component is used to identify a multi-homed Ethernet segment and enable load balancing and fast convergence?

A.Ethernet Tag ID
B.MAC address
C.EVPN Route Target
D.Ethernet Segment Identifier (ESI)
AnswerD

Correct.

Why this answer

The Ethernet Segment Identifier (ESI) is a unique identifier for an Ethernet segment that is multi-homed to two or more PEs. ESI is used for DF election and aliasing.

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