CCNA MPLS and Segment Routing Questions — Page 2 of 2

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

This forces OSPF to re-flood LSAs and run SPF, ensuring the restored link is considered in the shortest path tree.

Why this answer

Option D is correct: The issue is likely that OSPF link-state advertisement (LSA) flooding is delayed or blocked, preventing the repair of the LSDB. 'clear ip ospf process' forces a fresh LSA flood and SPF computation. Option A is wrong because SR-TE policies are not used. Option B is wrong because the problem is not with label allocation but with routing.

Option C is wrong because adjacency SIDs are automatically allocated and not the root cause.

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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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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 definition of custom metric types; 'min-unidirectional-link-bandwidth' excludes low-bandwidth links from the topology.

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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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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 in SR-MPLS uses IGP metric-based SPF computation. Option A is incorrect because 'strict-spf' enforces strict shortest path. Option C is incorrect because 'delay' uses latency, not IGP metric.

Option D is incorrect because 'te' uses traffic engineering constraints.

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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 (echo request) and MPLS traceroute are the primary OAM tools for LSP verification.

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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

Option D is correct because 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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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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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

Option B is correct because 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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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.

AnswerD

The router updates its LIB with the received label as the outgoing label for the prefix.

Why this answer

The router installs the received label as the incoming label for that prefix in its LIB and switches to that label. Option A is incorrect because it does not install an outgoing label; it stores the mapping. Option C is incorrect because it does not discard; it uses it.

Option D is incorrect because LDP uses UDP for discovery, but TCP for session; the label mapping is over TCP.

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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 IPv4 labeled unicast routes using MP-eBGP

AnswerD

MP-eBGP with label is used for inter-AS option B.

Why this answer

In Inter-AS Option B, ASBRs exchange IPv4 labeled unicast routes using MP-eBGP (Multiprotocol BGP) with the IPv4 labeled unicast address family. This allows the ASBRs to pass VPNv4 routes between autonomous systems without requiring per-VRF sessions or VRFs on the ASBRs, enabling scalable inter-AS connectivity.

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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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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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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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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