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CCNA Spcor Mpls Sr Questions

75 of 193 questions · Page 1/3 · Spcor Mpls Sr topic · Answers revealed

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1

Multi-Selecteasy

An engineer is configuring MPLS LDP on a Cisco router. Which TWO statements about LDP operation are correct? (Choose two.)

Select 2 answers

A.The LFIB (Label Forwarding Information Base) in LDP is used to store only the outgoing label mapping for each FEC.

B.LDP discovers neighbors using UDP hello messages on port 646, and then establishes a TCP session on port 646.

C.LDP uses a downstream-on-demand label distribution method, where labels are sent only when requested by an upstream router.

D.In liberal label retention mode, LDP stores all received labels, even from non-next-hop neighbors, allowing faster convergence.

E.LDP label allocation happens only after the IGP converges, and labels are allocated for all IGP routes by default.

AnswersB, D

Correct. LDP uses UDP for discovery, TCP for session.

Why this answer

LDP uses downstream unsolicited mode by default in Cisco IOS; liberal label retention means it keeps all labels even if not directly reachable. Option B is correct because LDP uses UDP hello on port 646 for neighbor discovery and then TCP on port 646 for session establishment. Option D is correct because liberal label retention stores all received labels, even from non-next-hop neighbors, improving convergence.

Option C is incorrect because LDP uses downstream unsolicited, not downstream-on-demand. Options A and E are incorrect.

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2

MCQhard

A service provider is migrating from LDP-based MPLS to Segment Routing (SR-MPLS) with IS-IS. After enabling SR on all routers, the adjacency segment IDs (ADJ-SIDs) are not being advertised. Which configuration is missing?

A.The `segment-routing mpls` command is not enabled under the IS-IS process

B.The router is running OSPF instead of IS-IS

C.The interface has the `isis adjacency-sid` command incorrectly configured

D.The loopback interface does not have an ip address configured

AnswerA

IS-IS requires the segment-routing mpls command under the IS-IS process to advertise SR capabilities.

Why this answer

In IS-IS, Segment Routing (SR-MPLS) requires explicit activation under the IS-IS routing process with the `segment-routing mpls` command. Without this, the router will not generate or advertise adjacency SIDs (ADJ-SIDs) even if SR is enabled globally. This command ties the SR-MPLS data plane to the IS-IS control plane, enabling IS-IS to allocate and distribute SIDs for adjacencies.

Exam trap

Cisco often tests the distinction between global SR enablement (`segment-routing mpls` under global config) and IGP-specific activation (`segment-routing mpls` under the IS-IS or OSPF process), leading candidates to assume global configuration is sufficient.

How to eliminate wrong answers

Option B is wrong because the question explicitly states IS-IS is being used, and OSPF would require a different configuration (e.g., `segment-routing mpls` under OSPF). Option C is wrong because the `isis adjacency-sid` command is used to manually assign a specific SID value to an adjacency, but it is not required for ADJ-SIDs to be advertised; the missing global command under IS-IS is the root cause. Option D is wrong because a loopback interface IP address is necessary for prefix-SIDs (node SIDs), not for adjacency SIDs, which are interface-specific and do not depend on loopback addressing.

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3

Multi-Selecthard

Which THREE of the following are benefits of using Segment Routing over LDP in an MPLS network? (Choose three.)

Select 3 answers

A.Enables traffic engineering without RSVP-TE.

B.Reduces the number of protocols required in the network.

C.Eliminates the need for LDP and RSVP-TE in the core.

D.Provides inherent security against label spoofing.

E.Requires BGP-LU for label distribution.

AnswersA, B, C

Correct: SR-TE provides traffic engineering capabilities.

Why this answer

Segment Routing (SR) can perform traffic engineering using SR-TE policies (via a controller or PCEP) or SR Flexible Algorithm, without requiring RSVP-TE. This eliminates the complexity of RSVP-TE's soft-state signaling and per-LSP state maintenance, relying instead on source-routed segment lists encoded in the packet header.

Exam trap

Cisco often tests the misconception that Segment Routing eliminates all security concerns or that it mandates BGP-LU, when in fact SR's security model is similar to traditional MPLS and BGP-LU is optional for specific use cases like inter-domain label distribution.

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4

MCQhard

In SRv6, which endpoint behavior is used for decapsulation and forwarding to a Layer 3 VPN IPv4 destination, removing the outer IPv6 header?

A.End.X

B.End.DT6

C.End.DT2U

D.End.DT4

AnswerD

End.DT4 is for IPv4 VPN decapsulation.

Why this answer

End.DT4 is the SRv6 endpoint behavior for decapsulation and table lookup in the VPN IPv4 table.

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5

Multi-Selectmedium

An engineer is configuring EVPN IRB (Integrated Routing and Bridging) to provide IP connectivity between tenants across a service provider network. Which THREE components are essential for EVPN IRB operation? (Choose three.)

Select 3 answers

A.LDP or SR for label distribution.

B.IP-VRF for each tenant.

C.An IRB interface (BDI) that serves as the Layer 3 gateway.

D.MPLS-TE tunnels for transport.

E.Ethernet VPN Instance (EVI) with a Route Distinguisher.

AnswersB, C, E

Essential for routing.

Why this answer

EVPN IRB requires an IP-VRF to represent the tenant, a bridge domain (EVI) for Layer 2, and IRB interfaces (Layer 3 gateway) that connect the bridge domain to the VRF. BGP EVPN control plane is also needed.

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6

MCQeasy

Which tool can verify connectivity along an MPLS Label Switched Path (LSP) and detect label loops?

B.traceroute

C.MPLS LSP ping

D.ping

AnswerC

This is the dedicated OAM tool for LSP connectivity verification.

Why this answer

MPLS LSP ping is the correct tool because it specifically tests connectivity along an MPLS Label Switched Path by sending MPLS echo request packets that follow the label stack, and it can detect label loops by checking the TTL field in the MPLS header. Unlike regular ICMP-based tools, MPLS LSP ping operates at the MPLS forwarding plane, verifying that the LSP is intact and that labels are correctly swapped at each hop.

Exam trap

Cisco often tests the distinction between control-plane tools (like ping/traceroute) and data-plane verification tools (like MPLS LSP ping), trapping candidates who assume that standard IP tools can verify MPLS label switching paths.

How to eliminate wrong answers

Option A is wrong because SNMP is a management protocol used for monitoring and collecting network statistics, not for actively verifying MPLS LSP connectivity or detecting label loops. Option B is wrong because standard IP traceroute uses ICMP or UDP packets and relies on IP routing, not MPLS labels, so it cannot traverse an MPLS LSP or detect label loops within the MPLS domain. Option D is wrong because ping uses ICMP echo requests and replies at the IP layer, which are forwarded based on IP routing tables and do not follow MPLS label paths, making it unable to verify LSP connectivity or detect label loops.

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7

Multi-Selecthard

In SR-MPLS traffic engineering, which three components are essential for configuring an SR Policy on a headend router? (Choose three.)

Select 3 answers

A.Bandwidth reservation value

B.Explicit path with ERO

C.Headend router

D.Endpoint (destination) prefix

E.Color (used to steer traffic with a specific color)

AnswersC, D, E

Correct: The headend is the source of the policy.

Why this answer

In SR-MPLS traffic engineering, an SR Policy on a headend router is defined by three essential components: the headend router itself (the router that originates the policy), the endpoint (destination prefix), and a color value that distinguishes multiple policies to the same endpoint. Bandwidth reservation (A) is optional and not part of the basic policy identifier. Explicit path with ERO (B) is a way to specify the path but is not a required component of the policy definition; segment lists are used instead.

Therefore, the correct choices are C (headend), D (endpoint), and E (color).

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8

MCQmedium

A company is connecting two sites using MPLS L3VPN. The PE routers are running both LDP and Segment Routing (SR-MPLS) in the core. The CE router at Site A cannot reach the CE at Site B. On the PEs, the VRF routes are present. Which command would you use to verify the MPLS forwarding path for the VPN traffic?

A.show mpls ldp bindings

B.show ip route vrf VPN-A 10.1.1.0

C.show mpls forwarding-table vrf VPN-A 10.1.1.0

D.show bgp vpnv4 unicast labels

AnswerC

Displays the MPLS forwarding entry including the outgoing label stack and next hop.

Why this answer

The 'show mpls forwarding-table vrf VPN-A 10.1.1.0' command displays the MPLS label stack that will be imposed on packets destined for the VPN prefix, including the inner VPN label and the outer transport label (LDP or SR-MPLS). This directly verifies the end-to-end MPLS forwarding path for VPN traffic, which is necessary when VRF routes are present but connectivity fails due to label issues.

Exam trap

Cisco often tests the distinction between control-plane verification (BGP labels, VRF routes) and data-plane verification (MPLS forwarding table), so candidates mistakenly choose commands that confirm route presence but miss the actual label forwarding path.

How to eliminate wrong answers

Option A is wrong because 'show mpls ldp bindings' only shows LDP label bindings for the global routing table, not per-VRF VPN labels or the actual forwarding path used for VPN traffic. Option B is wrong because 'show ip route vrf VPN-A 10.1.1.0' confirms the VRF route exists but does not reveal the MPLS label stack or whether the transport label (LDP or SR-MPLS) is correctly resolved. Option D is wrong because 'show bgp vpnv4 unicast labels' displays BGP VPN label assignments from the control plane, but does not show the transport label or the actual MPLS forwarding entry used in the data plane.

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9

MCQhard

In CSPF (Constrained Shortest Path First) for MPLS-TE, which of the following is NOT a typical constraint considered during path computation?

A.Bandwidth

B.Affinity (color)

C.Explicit path inclusion/exclusion

D.Administrative distance

AnswerD

Administrative distance is not a TE constraint.

Why this answer

CSPF considers bandwidth, affinity (color), and explicit constraints, but not administrative distance, which is a routing protocol metric.

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10

MCQmedium

A network engineer is configuring an MPLS L3VPN on a PE router. The VRF is configured with route-target import/export. The PE and CE are running eBGP. The CE advertises routes to the PE, and the PE installs them in the VRF routing table. However, the remote PE does not receive these routes via BGP VPNv4. The local PE's BGP table shows the VPNv4 prefixes with the correct next hop and label. The remote PE's BGP table shows no such prefixes. The IGP between the PEs is working, and MPLS LSPs are up. What is the most likely cause and correct action?

A.Enable the 'soft-reconfiguration inbound' command

B.Reset the BGP session between the PEs

C.Check the MTU on the link between the PEs

D.Check the route-target import/export configuration on both PEs and ensure they match

AnswerD

If route-target export does not match the import, the remote PE will discard the route.

Why this answer

Despite the local PE having the routes in BGP VPNv4, the remote PE does not receive them. This often indicates that the route-target export on the local PE does not match the route-target import on the remote PE, or that the VPNv4 session is not correctly configured. Since the local PE shows the prefixes in its BGP VPNv4 table, they are being advertised to the BGP peer, but the remote PE's import policy filters them due to mismatched RT.

The correct action is to verify the route-target configuration on both PEs.

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11

MCQmedium

An SRv6 SID is composed of three parts: Locator, Function, and Arguments. Which part identifies the SRv6-capable node in the network?

A.Argument

B.Locator

C.Function

D.SRH

AnswerB

Locator identifies the node.

Why this answer

The Locator portion of an SRv6 SID identifies the node and is routable in the IGP.

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12

MCQhard

A service provider has a network with PE1, P1, P2, and PE2 connected in a chain: PE1-P1-P2-PE2. The IGP is IS-IS with wide metrics, and MPLS LDP is enabled on all interfaces. The goal is to provide L3VPN services between PE1 and PE2. The engineer has configured MP-iBGP between PE1 and PE2, and the VPNv4 routes are exchanged. However, when a CE router behind PE1 tries to reach the CE behind PE2, the traffic fails. The engineer checks the MPLS forwarding table on PE1 and sees that the label for the BGP next-hop (PE2's loopback) is 'Untagged'. The BGP next-hop is reachable via ICMP. What is the most likely cause of this issue?

A.The IS-IS metric between PE1 and P1 is too high, causing suboptimal routing.

B.The MPLS TTL propagation is disabled, causing packets to be dropped.

C.LDP is not enabled on the interface between PE1 and P1.

D.The MTU on the link between PE1 and P1 is set to 1500, causing fragmentation.

AnswerC

Without LDP, no label is received from P1 for the BGP next-hop.

Why this answer

The MPLS forwarding table on PE1 shows the label for PE2's loopback as 'Untagged', which indicates that LDP has not assigned a label for that prefix on the incoming interface. Since LDP is responsible for distributing labels for IGP routes (like loopbacks) in an MPLS LDP-enabled network, the missing label means LDP is not operational on the link between PE1 and P1. Without a label, PE1 cannot push an MPLS label for the BGP next-hop, causing the VPNv4 traffic to be dropped or forwarded as IP, which fails because the core routers (P1, P2) do not have the VPN route.

Exam trap

Cisco often tests the distinction between BGP route exchange (which works) and MPLS label assignment (which fails), leading candidates to incorrectly focus on routing protocol issues or MTU/TTL parameters instead of verifying LDP adjacency and label bindings.

How to eliminate wrong answers

Option A is wrong because a high IS-IS metric would affect path selection but would not cause the BGP next-hop label to be 'Untagged'; LDP still assigns labels regardless of metric values. Option B is wrong because disabling MPLS TTL propagation affects TTL handling in the MPLS header but does not prevent label assignment or cause an 'Untagged' entry in the forwarding table. Option D is wrong because an MTU of 1500 is standard and would not cause the label to be missing; fragmentation issues would manifest as packet drops after label imposition, not as a missing label in the forwarding table.

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13

Multi-Selecthard

Which two are benefits of using EVPN-VPWS over traditional VPWS? (Choose two.)

Select 2 answers

A.Uses MPLS-TE tunnels for path protection.

B.Eliminates the need for pseudowire configuration on each PE.

C.Support for active/active multi-homing without spanning tree.

D.Supports only point-to-point topologies.

E.Requires LDP for label distribution.

AnswersB, C

Correct: EVPN automates pseudowire signaling via BGP.

Why this answer

EVPN-VPWS provides multi-homing with fast convergence and simplifies provisioning using BGP.

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14

Multi-Selectmedium

Which TWO statements about MPLS label switching are correct? (Choose two.)

Select 2 answers

A.The transit LSR performs label swapping.

B.The CE receives a frame with an MPLS label.

C.The ingress LSR imposes a label on the packet.

D.PHP (Penultimate Hop Popping) causes the egress router to pop the label.

E.The egress LSR performs label swapping before forwarding.

AnswersA, C

Correct: Transit routers swap the incoming label with an outgoing label.

Why this answer

A transit Label Switch Router (LSR) in an MPLS network performs label swapping: it receives a labeled packet, replaces the incoming label with an outgoing label from its LFIB (Label Forwarding Information Base), and forwards the packet toward the egress LSR. This is the fundamental operation of an LSR in the core of an MPLS domain, as defined in RFC 3031.

Exam trap

Cisco often tests the distinction between which router performs label popping in PHP (penultimate hop vs. egress) and which router swaps labels (transit LSR vs. egress LSR), leading candidates to confuse the roles of the penultimate and egress routers.

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15

MCQhard

A service provider is migrating from LDP to SR-MPLS. Which mechanism allows a router to allocate labels for prefixes using both LDP and SR simultaneously during the migration?

A.LDP over SR tunnel

B.RSVP-TE interworking

C.MPLS forwarding plane adjustment

D.Segment Routing Mapping Server (SRMS)

AnswerD

Correct.

Why this answer

Cisco's SR/LDP interworking allows coexistence. Routers can run both LDP and SR, and a mapping server advertises SR labels for LDP-learned prefixes. This enables seamless migration.

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16

MCQmedium

In an MPLS network using LDP, which statement correctly describes the label distribution behavior for a given FEC?

A.Labels are advertised unsolicited to all neighbors, and all labels are retained even if not used (liberal label retention).

B.Labels are advertised only in response to a request from an upstream neighbor (downstream on demand).

C.Labels are advertised unsolicited but only the best label for each FEC is retained (conservative label retention).

D.Labels are advertised only when the LSR has an outgoing label for the FEC (ordered label distribution).

AnswerA

Correct: LDP default behavior.

Why this answer

LDP uses downstream unsolicited mode by default, where each LSR advertises labels for all FECs to all neighbors without being asked, and liberal label retention stores all received labels even if not currently used.

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17

MCQmedium

In SR-MPLS TE, a headend router is configured with an SR Policy that includes an explicit candidate path with segment list [16001, 16003]. What do these numbers represent?

A.MPLS label values representing Node SIDs or Adjacency SIDs

B.Interface indices

C.Metric values for path calculation

D.IPv4 addresses encoded as numbers

AnswerA

Correct: Segment lists contain MPLS labels.

Why this answer

In SR-MPLS, segment lists are composed of MPLS labels. Node SIDs are typically from the SRGB (e.g., 16001 could be a Node SID for a router). Adjacency SIDs would be link-specific labels.

The numbers are labels, not interface indices or metric values.

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18

MCQeasy

An engineer is deploying MPLS in the core and wants to ensure that all core routers use the same label for a specific prefix, regardless of which router originated it. Which MPLS label allocation mode should be used?

A.Per-interface label mode

B.Per-next-hop label mode

C.Per-prefix label mode

D.Per-VRF label mode

AnswerC

Per-prefix allocates one label per prefix, ensuring same label across all routers.

Why this answer

Per-prefix label mode (option C) is correct because it assigns a single label for a specific prefix across all core routers, regardless of which router originated the route. This ensures label consistency, which is critical for proper MPLS forwarding and troubleshooting. In contrast, per-next-hop or per-interface modes would create different labels for the same prefix based on the next hop or interface, breaking the requirement for uniform label allocation.

Exam trap

Cisco often tests the distinction between per-prefix and per-next-hop label modes, and the trap here is that candidates confuse per-next-hop (which creates multiple labels for the same prefix) with per-prefix, thinking that per-next-hop ensures consistency when it actually does the opposite.

How to eliminate wrong answers

Option A is wrong because per-interface label mode assigns a unique label per interface for each FEC, which would cause the same prefix to have different labels on different interfaces, not a single label across all routers. Option B is wrong because per-next-hop label mode allocates a label per next hop for a given prefix, leading to multiple labels for the same prefix if multiple next hops exist, violating the requirement for a single label. Option D is wrong because per-VRF label mode is used in MPLS VPNs to assign a label per VRF, not per prefix, and would not ensure a single label for a specific prefix across the core.

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19

MCQmedium

A service provider is deploying MPLS L3VPN and notices that BGP next-hop resolution for VPNv4 routes fails on the PE routers. The PE routers are running OSPF as the IGP and have loopback interfaces used for LDP and BGP peering. Which configuration change should the engineer implement to ensure that the BGP next-hop is reachable?

A.Configure 'neighbor x.x.x.x update-source loopback0' under the BGP router configuration.

B.Configure 'neighbor x.x.x.x allowas-in 1' under the BGP VRF configuration.

C.Apply the 'neighbor x.x.x.x next-hop-self' command under the BGP VRF configuration.

D.Increase the 'maximum-paths' value under the BGP address-family VPNv4.

AnswerA

This ensures BGP uses the loopback as the source IP, making the next-hop reachable via IGP.

Why this answer

The BGP next-hop for VPNv4 routes is typically the loopback interface of the remote PE router. For BGP to consider the next-hop reachable, the local PE must have an IGP route to that loopback address. The 'neighbor x.x.x.x update-source loopback0' command ensures that BGP uses the loopback interface as the source IP for the TCP session, which aligns the BGP peering address with the IGP-advertised loopback, making the next-hop reachable via OSPF.

Exam trap

Cisco often tests the distinction between BGP session establishment (which requires reachability to the neighbor's IP) and BGP next-hop resolution (which requires reachability to the next-hop address carried in the route); candidates confuse these two separate requirements and incorrectly apply 'next-hop-self' or 'allowas-in'.

How to eliminate wrong answers

Option B is wrong because 'allowas-in' permits the local AS to appear in the AS_PATH, which is used for route acceptance in VRF contexts, not for next-hop resolution. Option C is wrong because 'next-hop-self' changes the next-hop to the local PE's address on routes sent to a BGP neighbor, but the issue is that the original next-hop (remote PE loopback) is unreachable due to IGP routing, not that the next-hop needs to be changed. Option D is wrong because 'maximum-paths' controls the number of equal-cost paths for load balancing, not next-hop reachability.

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20

MCQhard

Refer to the exhibit. The output shows an MPLS forwarding entry with FRR protection. What is the purpose of the backup path shown?

A.To handle penultimate hop popping for the primary path.

B.To forward traffic if the primary outgoing interface fails.

C.To provide a load-balancing alternative for the primary path.

D.To carry the VPN label separately.

AnswerB

The backup path activates when primary fails, ensuring fast convergence.

Why this answer

The backup path in an MPLS Fast Reroute (FRR) configuration is precomputed and installed in the forwarding table to provide sub-50ms protection against link or node failures. When the primary outgoing interface fails, traffic is immediately switched to the backup path without waiting for IGP convergence, ensuring minimal packet loss.

Exam trap

Cisco often tests the distinction between FRR backup paths (for protection) and ECMP load-balancing paths (for traffic distribution), leading candidates to confuse redundancy with load sharing.

How to eliminate wrong answers

Option A is wrong because penultimate hop popping (PHP) is a mechanism where the egress LSR removes the label before forwarding the packet to the CE, and it is unrelated to FRR backup paths; the backup path handles label imposition and forwarding, not PHP. Option C is wrong because load-balancing alternatives distribute traffic across multiple equal-cost paths for efficiency, not for protection against failures; FRR backup paths are solely for redundancy and are not used for load sharing. Option D is wrong because VPN labels are carried in the primary and backup paths as part of the label stack, but the backup path's purpose is not to separate VPN labels; it is to provide a precomputed alternate route for fast failover.

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21

MCQmedium

In an MPLS-TE tunnel configuration, which RSVP message carries the Explicit Route Object (ERO) that specifies the explicit path the tunnel must follow?

A.RSVP_TEAR message

B.RSVP_ERR message

C.PATH message

D.RESV message

AnswerC

PATH carries the ERO.

Why this answer

In RSVP-TE, the PATH message includes the ERO to define the explicit route the traffic should take. The RESV message is sent back to confirm the reservation.

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22

MCQmedium

Which of the following best describes the function of CSPF in MPLS-TE?

A.It distributes labels to LDP neighbors.

B.It calculates the shortest path based solely on IGP metric.

C.It signals RSVP-TE PATH messages.

D.It prunes links that do not satisfy constraints and then runs SPF on the remaining topology.

AnswerD

Correct. CSPF applies constraints first.

Why this answer

CSPF computes a path that meets constraints (bandwidth, affinity, etc.) and avoids links/ nodes not meeting them.

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23

MCQeasy

A service provider is designing a greenfield MPLS core and wants to minimize control-plane complexity while still supporting traffic engineering. They plan to use Segment Routing with MPLS. Which statement about Segment Routing in this context is accurate?

A.Segment Routing only works with IPv6 data plane.

B.Segment Routing uses RSVP-TE for label distribution.

C.Segment Routing reduces the number of protocols required in the core.

D.Segment Routing requires TI-LFA to be enabled for traffic engineering.

AnswerC

SR eliminates LDP and RSVP-TE, relying only on an IGP with SR extensions, thus reducing protocol complexity.

Why this answer

Segment Routing (SR-MPLS) eliminates the need for a separate label distribution protocol like LDP or RSVP-TE. The MPLS labels are derived directly from the IGP (IS-IS or OSPF) extensions, reducing control-plane complexity while still enabling traffic engineering via SR-TE policies and flexible path computation.

Exam trap

Cisco often tests the misconception that Segment Routing requires a separate label distribution protocol like LDP or RSVP-TE, when in fact it uses IGP extensions (IS-IS or OSPF) to distribute labels, reducing protocol complexity.

How to eliminate wrong answers

Option A is wrong because Segment Routing supports both MPLS (SR-MPLS) and IPv6 (SRv6) data planes; it is not limited to IPv6. Option B is wrong because SR-MPLS does not use RSVP-TE for label distribution; labels are signaled via IGP extensions (IS-IS or OSPF) with the Segment Routing extensions, not via RSVP-TE. Option D is wrong because TI-LFA (Topology Independent Loop-Free Alternate) is a fast-reroute mechanism that can be used with SR but is not required for traffic engineering; SR-TE can be achieved using explicit paths or SR policies without TI-LFA.

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24

Matchingmedium

Match each IS-IS term to its definition.

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

Concepts

Matches

Link State PDU containing routing information

Designated Intermediate System on a broadcast network

Network Service Access Point, the IS-IS address

Subnetwork Point of Attachment, e.g., MAC address

IS-IS Hello PDU used for neighbor discovery

Why these pairings

In IS-IS, an area is identified by a common area address, LSPs carry link state data, and Level-1 routers operate intra-area. Common confusions involve swapping area definitions (system ID vs. area address) and mixing Level-1/Level-2 roles.

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25

Multi-Selecthard

An SP network uses RSVP-TE for MPLS traffic engineering with FRR link protection. Which THREE statements are correct about RSVP-TE operation and FRR? (Choose three.)

Select 3 answers

A.CSPF (Constrained Shortest Path First) is used to compute the best path based on IGP metrics only, ignoring TE constraints.

B.In FRR link protection, the 1:1 detour method creates a separate backup LSP for each protected LSP, while the bypass tunnel method protects multiple LSPs over a single backup tunnel.

C.The RSVP 'fixed-filter' (FF) reservation style is the only style used in MPLS-TE to allow multiple senders.

D.The PATH message travels from headend to tailend, while the RESV message travels from tailend back to headend, establishing reservation state.

E.The Explicit Route Object (ERO) in the PATH message lists the strict or loose hops that the traffic must traverse.

AnswersB, D, E

Correct. 1:1 detour per LSP; bypass tunnel protects many.

Why this answer

RSVP-TE uses PATH messages traveling downstream from headend to tailend and RESV messages traveling upstream from tailend to headend, establishing reservation state (D correct). The Explicit Route Object (ERO) in the PATH message specifies the strict or loose hops the traffic must traverse (E correct). For FRR link protection, the 1:1 detour method creates a separate backup LSP for each protected LSP, while the bypass tunnel method protects multiple LSPs over a single backup tunnel (B correct).

Option A is incorrect because CSPF computes paths based on both IGP metrics and TE constraints (e.g., bandwidth, affinity), not only IGP metrics. Option C is incorrect because the 'fixed-filter' (FF) style is for per-sender reservation in RSVP, but MPLS-TE typically uses the 'shared-explicit' (SE) style to allow multiple senders (LSPs) to share the same reservation.

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26

MCQhard

Which EVPN component ensures that only one of the multi-homed PEs forwards traffic to the CE for a given Ethernet segment?

A.DF Election

B.Split Horizon

C.Aliasing

D.ESI (Ethernet Segment Identifier)

AnswerA

Correct: DF election selects one PE to forward.

Why this answer

DF election determines which PE forwards traffic for a given VLAN to avoid duplication.

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27

MCQhard

In SR-TE, a headend router is configured with an SR Policy that includes an explicit candidate path with segment list [16001, 16003, 16005]. What does this segment list represent?

A.A list of labels for an RSVP-TE LSP

B.A list of adjacency SIDs for each link

C.A list of node SIDs representing the path through routers with those SIDs

D.A list of anycast SIDs for redundancy

AnswerC

Correct. Node SIDs identify routers.

Why this answer

Each SID in the list is a node SID (Prefix-SID) of intermediate routers, forming a strict hop-by-hop path.

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28

MCQmedium

What is the primary purpose of the SRGB (Segment Routing Global Block) when configuring SR-MPLS on a network?

A.To provide a common label space for global segments, ensuring consistent label to prefix mapping.

B.To set aside labels for MPLS-TE tunnels.

C.To allocate labels for adjacency SIDs only.

D.To define the range for LDP labels used in SR interworking.

AnswerA

Correct.

Why this answer

SRGB defines a range of labels reserved for global segments (e.g., Node SIDs). It ensures that a given Node SID maps to the same label across all routers in the SR domain, simplifying operations.

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29

Multi-Selecthard

Which THREE are mechanisms used in EVPN to support multi-homing and load balancing? (Choose three.)

Select 3 answers

A.Aliasing

B.Ethernet Segment Identifier (ESI)

C.Spanning Tree Protocol (STP)

D.Designated Forwarder (DF) election

E.Virtual Router Redundancy Protocol (VRRP)

AnswersA, B, D

Allows load balancing to any PE in the ES.

Why this answer

ESI identifies the segment, DF election avoids duplicates, and aliasing allows traffic to be forwarded to any PE in the multi-homing group.

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30

MCQmedium

An engineer is troubleshooting MPLS label switching in a service provider core. They notice that packets are being forwarded correctly between provider edge routers, but when they check the MPLS forwarding table on a P router, they see only implicit-null labels for some destinations. What is the most likely reason for this?

A.The router is using explicit-null label due to security policies.

B.The router has a directly connected neighbor that is the egress LER.

C.The router is misconfigured to use implicit-null for all labels.

D.The router is performing penultimate hop popping (PHP) incorrectly.

AnswerB

Implicit-null is used in PHP; the egress LER advertises it to the penultimate hop.

Why this answer

When a P router sees only implicit-null labels (label 3) for some destinations in its MPLS forwarding table, it indicates that the router is performing penultimate hop popping (PHP). This occurs when the P router is the penultimate hop before the egress Label Edge Router (LER). The egress LER advertises an implicit-null label to its directly connected neighbor, causing the P router to pop the label and forward the packet using only the IP header.

Option B is correct because the P router has a directly connected neighbor that is the egress LER, which triggers PHP.

Exam trap

The trap here is that candidates often confuse implicit-null with a misconfiguration or error, when in fact it is a normal and expected result of PHP being triggered by a directly connected egress LER.

How to eliminate wrong answers

Option A is wrong because the explicit-null label (label 0) is used for traffic engineering or QoS preservation, not for security policies, and it would appear as label 0 in the forwarding table, not implicit-null. Option C is wrong because implicit-null is not a misconfiguration; it is a standard label advertised by the egress LER to request PHP, and a router does not globally configure implicit-null for all labels—it is per-prefix based on LDP or RSVP-TE signaling. Option D is wrong because the router is performing PHP correctly, not incorrectly; the presence of implicit-null labels in the forwarding table is the expected result of proper PHP operation.

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31

MCQmedium

Refer to the exhibit. An engineer configured Segment Routing on a router. However, the router does not advertise a Prefix-SID for the loopback0 interface. What is the most likely reason?

A.The loopback is not included in any OSPF network statement.

B.The 'prefix-sid index' command is not associated with the loopback prefix; it should be configured under the OSPF process for that specific prefix.

C.The 'segment-routing mpls' command is missing under the loopback interface.

D.The 'mpls ip' command is missing on the loopback interface.

AnswerB

The prefix-sid index under OSPF is generic; it needs to be tied to the specific prefix, e.g., 'prefix-sid index 100 192.168.1.1/32'.

Why this answer

In Cisco IOS XR, the 'prefix-sid index' command must be configured under the OSPF process for the specific prefix (e.g., 'prefix-sid index 100' under 'router ospf 1' for the loopback0 prefix). Configuring it under the interface does not associate the SID with the prefix in OSPF's link-state database, so the router will not advertise the Prefix-SID. The exhibit shows the command under the loopback interface, which is incorrect for OSPF Segment Routing.

Exam trap

Cisco often tests the distinction between interface-level and protocol-level configuration for Segment Routing, trapping candidates who assume the 'prefix-sid index' command belongs on the interface (like MPLS or IPv6 commands) rather than under the OSPF or IS-IS process.

How to eliminate wrong answers

Option A is wrong because OSPF network statements are not required for loopback interfaces; loopbacks are automatically advertised as stub networks if the interface is enabled under OSPF. Option C is wrong because 'segment-routing mpls' is a global command that enables Segment Routing MPLS on the router, not an interface-level command; it is not missing on the loopback. Option D is wrong because 'mpls ip' is used for LDP, not for Segment Routing; Segment Routing does not require MPLS IP on the interface for Prefix-SID advertisement.

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32

Multi-Selectmedium

An engineer is troubleshooting an MPLS-TE tunnel that is not coming up. The configuration includes an explicit path with strict hops. Which TWO conditions could prevent the tunnel from being established? (Choose two.)

Select 2 answers

A.The tunnel interface has an IP address.

B.RSVP is not enabled on an interface along the path.

C.The destination loopback is not advertised in IGP.

D.The path includes a router that does not support MPLS.

E.One of the strict hops is not directly connected to the previous router.

AnswersB, E

RSVP must be enabled.

Why this answer

An explicit path with strict hops requires each hop to be directly connected. If a hop is not directly connected, the path fails. Also, if there is no RSVP capability along the path, the tunnel cannot be signaled.

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33

Multi-Selectmedium

Which TWO are characteristics of SR-MPLS when compared to LDP-based MPLS? (Choose two.)

Select 2 answers

A.SR-MPLS uses IPv6 SIDs only.

B.Labels are distributed using LDP.

C.Labels are distributed via IGP extensions.

D.SR-MPLS requires RSVP-TE for traffic engineering.

E.SR-MPLS reduces protocol state by eliminating LDP and RSVP.

AnswersC, E

IGP (IS-IS/OSPF) carries Segment Routing labels.

Why this answer

SR-MPLS eliminates LDP and RSVP state, using IGP for label distribution (e.g., via IS-IS TLVs).

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34

MCQeasy

Which component in EVPN is used to uniquely identify an Ethernet segment across multiple PE devices in a multi-homing scenario?

A.RT-2

B.MAC-VRF

C.ESI

D.EVI

AnswerC

ESI identifies the Ethernet segment.

Why this answer

The Ethernet Segment Identifier (ESI) is a 10-byte value that identifies a specific multihomed site.

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35

MCQmedium

Based on the exhibit, which prefix is missing a label binding and likely causing traffic to be dropped?

A.10.2.2.0/24

B.10.1.1.0/24

C.10.4.4.0/24

D.10.3.3.0/24

E.10.5.5.0/24

AnswerC

'No Label' means no label binding exists; traffic to this prefix will be dropped.

Why this answer

The exhibit shows an MPLS LDP label binding table. Prefix 10.4.4.0/24 has no local label assigned (shown as 'no label' or missing), meaning LDP has not established a label binding for this FEC. Without a label, traffic destined for 10.4.4.0/24 cannot be label-switched and will be dropped or forwarded via IP lookup (if enabled), but in an MPLS core, this typically results in packet loss.

Exam trap

Cisco often tests the concept that a missing local label in the LDP binding table indicates the prefix is not reachable via LDP, and candidates may mistakenly focus on remote labels or assume a label of '0' or 'implicit null' means a problem, when the real issue is the absence of a local label.

How to eliminate wrong answers

Option A is wrong because 10.2.2.0/24 has a valid local label binding (e.g., label 22), so LDP has successfully assigned a label and traffic can be label-switched. Option B is wrong because 10.1.1.0/24 shows a local label (e.g., label 21), indicating proper LDP label allocation. Option D is wrong because 10.3.3.0/24 has a local label (e.g., label 23), so no missing binding exists.

Option E is wrong because 10.5.5.0/24 also has a local label (e.g., label 25), confirming LDP has completed label binding for that prefix.

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36

Multi-Selectmedium

Which three are characteristics of SR-TE policies? (Choose three.)

Select 3 answers

A.They are defined at the headend router with a destination and color.

B.They use explicit segment lists to encode the path.

C.They require RSVP-TE signaling to maintain state.

D.They can be used to steer traffic onto a specific path for service-level agreement (SLA) compliance.

E.They require per-flow state at intermediate routers.

AnswersA, B, D

Correct: Headend defines policy.

Why this answer

SR-TE policies are stateful at the headend, use segment lists for path steering, and can be associated with a color for traffic engineering.

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37

MCQmedium

An engineer configures an MPLS-TE tunnel with a bandwidth of 100 Mbps and an explicit path that avoids a specific link. Which RSVP message carries the Explicit Route Object (ERO) to signal the path?

A.PATH message

B.RESV Confirm message

C.PATH Tear message

D.RESV message

AnswerA

PATH message contains the ERO for explicit routing.

Why this answer

In RSVP-TE, the PATH message includes the ERO to specify the explicit route for the tunnel.

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38

MCQhard

A network engineer notices that BGP-LU prefixes learned from an eBGP neighbor are not being installed in the MPLS forwarding table (LFIB). The BGP table shows the prefixes with the correct label. The IGP route to the neighbor is reachable. What additional configuration is needed?

A.Enable `no bgp default ipv4-unicast`

B.Configure `mpls ldp autoconfig` on the interface

C.Add `allowas-in` to accept routes with AS-path containing own AS

D.Set `label mode per-prefix` under the BGP neighbor or address-family

AnswerD

BGP-LU requires per-prefix label allocation for LFIB installation.

Why this answer

BGP-LU (BGP Labeled Unicast) installs labels into the BGP table, but for those labels to be used for MPLS forwarding, the router must be configured to allocate labels on a per-prefix basis. By default, BGP may use per-VRF or per-next-hop label allocation, which does not populate the LFIB with per-prefix entries. Setting `label mode per-prefix` under the BGP neighbor or address-family forces BGP to allocate a unique label for each prefix, enabling the LFIB installation required for MPLS forwarding.

Exam trap

Cisco often tests the distinction between BGP label allocation modes (per-prefix vs. per-next-hop) and the requirement for per-prefix mode to install BGP-LU prefixes into the LFIB, confusing candidates who assume any BGP label automatically populates the MPLS forwarding table.

How to eliminate wrong answers

Option A is wrong because disabling the default IPv4 unicast exchange (`no bgp default ipv4-unicast`) affects BGP session establishment for IPv4 unicast but does not influence MPLS label allocation or LFIB population. Option B is wrong because `mpls ldp autoconfig` enables LDP on an interface, which is unrelated to BGP-LU; BGP-LU uses BGP to distribute labels, not LDP. Option C is wrong because `allowas-in` permits routes with the local AS in the AS_PATH, which is a loop-prevention override for specific topologies (e.g., MPLS VPN inter-AS), but it does not affect label allocation or LFIB installation.

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39

MCQeasy

In MPLS, which operation occurs when a router receives a labeled packet and replaces the top label with a new label before forwarding?

A.Lookup

B.Pop

C.Swap

D.Push

AnswerC

Swap exchanges the top label for a new one.

Why this answer

In MPLS, the swap operation replaces the incoming top label with a new outgoing label as per the LFIB entry.

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40

MCQhard

In SRv6, which behavior is used for L3VPN decapsulation (removing the outer IPv6 header and SRH, and forwarding the inner IPv4 packet based on the VPN routing table)?

A.End.DT2U

B.End.DT6

C.End.DT4

D.End

AnswerC

Correct for IPv4 L3VPN.

Why this answer

End.DT4 is the SRv6 endpoint behavior for decapsulation and table lookup for IPv4 L3VPN. End.DT6 is for IPv6 L3VPN, End.DT2U is for L2VPN.

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41

MCQeasy

A network architect is designing an MPLS network that must provide fast convergence in case of a link failure. The network uses Segment Routing. Which mechanism provides sub-50ms protection by using a precomputed backup path?

A.BFD (Bidirectional Forwarding Detection)

B.TI-LFA (Topology-Independent Loop-Free Alternate)

C.LFA (Loop-Free Alternate)

D.LDP FRR

AnswerB

TI-LFA uses segment lists to guarantee backup path regardless of topology.

Why this answer

B is correct because TI-LFA (Topology-Independent Loop-Free Alternate) is a Segment Routing mechanism that precomputes a backup path using a segment list, enabling sub-50ms convergence upon link failure. It guarantees loop-free forwarding even in complex topologies where traditional LFA may fail, by leveraging SR's source routing capabilities.

Exam trap

Cisco often tests the distinction between failure detection (BFD) and failure repair (FRR mechanisms), and the trap here is that candidates confuse BFD's fast detection with the actual precomputed backup path provided by TI-LFA.

How to eliminate wrong answers

Option A is wrong because BFD is a fast failure detection protocol (e.g., sub-second hello intervals), not a precomputed backup path mechanism; it detects failures but does not provide an alternate forwarding path. Option C is wrong because LFA (Loop-Free Alternate) computes a backup next-hop but may not provide coverage in all topologies (e.g., ring or square topologies), and it does not guarantee sub-50ms protection in all cases. Option D is wrong because LDP FRR (Label Distribution Protocol Fast Reroute) uses LDP labels for backup paths but is not specific to Segment Routing; it also lacks the topology-independent guarantee and segment list optimization of TI-LFA.

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42

MCQhard

In SR-TE, a headend router is configured with an SR Policy using an explicit candidate path that includes a segment list [100, 200, 300]. If all routers have the same SRGB starting at 10000, what is the actual outgoing label stack for the first segment?

A.100, 200, 300

B.10100, 10200, 10300

C.10000, 20000, 30000

D.Labels depend on LDP allocation

AnswerB

Correct: SRGB base + index.

Why this answer

In SR-MPLS, the SID values in the segment list are relative to the SRGB. If the SRGB starts at 10000, a SID value of 100 corresponds to label 10100 (10000 + 100). So the first segment label is 10100.

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43

MCQmedium

In Segment Routing (SR-MPLS), which IGP TLV is used by IS-IS to advertise the Node SID for a prefix?

A.TLV 135 (Extended IP Reachability) with sub-TLV 3 for Prefix-SID.

B.TLV 22 (IS Reachability) with sub-TLV 22 for Adjacency SID.

C.TLV 130 (IP Address) with sub-TLV 1 for Node SID.

D.TLV 137 (Dynamic hostname) with sub-TLV 5 for SID.

AnswerA

Correct.

Why this answer

IS-IS uses the IS-IS Router CAPABILITY TLV (135) with sub-TLV 19 to carry the SR-capable information and prefix-SID. Specifically, the Prefix-SID is advertised in the IS-IS Extended IP Reachability TLV (135) with sub-TLV 3.

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44

Multi-Selectmedium

Which TWO are benefits of using Segment Routing (SR) over traditional LDP-based MPLS?

Select 2 answers

A.Eliminates the need for LDP and RSVP-TE protocols.

B.Requires only OSPF as the IGP.

C.Reduces label imposition at the ingress PE.

D.Faster convergence due to BGP PIC.

E.Supports traffic engineering without RSVP-TE.

AnswersA, E

SR uses IGP to distribute labels, removing LDP and RSVP-TE.

Why this answer

Segment Routing (SR) eliminates the need for the Label Distribution Protocol (LDP) and Resource Reservation Protocol with Traffic Engineering (RSVP-TE) by encoding MPLS labels directly into the Interior Gateway Protocol (IGP), such as OSPF or IS-IS. This simplifies the control plane by removing these protocols entirely, reducing operational complexity and resource overhead.

Exam trap

Cisco often tests the misconception that Segment Routing requires a specific IGP (like OSPF only) or that it reduces label imposition, when in fact SR is IGP-agnostic and label depth depends on the path encoding, not the protocol itself.

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45

MCQhard

An engineer is deploying MPLS-TE with FRR using the 1:1 detour method. Which statement about link protection in this scenario is true?

A.The detour LSP merges back with the main LSP after the protected link.

B.The detour LSP is created dynamically after a failure is detected.

C.Link protection requires node protection to be configured.

D.A single bypass tunnel protects multiple LSPs over the same link.

AnswerA

Correct.

Why this answer

1:1 detour creates a one-to-one backup LSP for each protected LSP. The detour LSP is pre-signaled and protects against link failure by re-routing traffic around the failed link.

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46

MCQmedium

A network engineer is troubleshooting MPLS L3VPN issues on a PE router. The CE router is receiving routes from the PE, but the CE cannot ping the remote CE. The PE shows the remote prefix in its routing table and in the VRF. What is the most likely cause?

A.The VRF route-target import/export is misconfigured

B.The MTU on the core links is too small

C.The RD is different on the two PEs

D.The LSP between the PEs is not operational

AnswerD

The LSP must be up to forward MPLS-encapsulated packets between PEs.

Why this answer

The CE can receive routes from the PE (indicating that the VRF and route-target import/export are functioning correctly), but the CE cannot ping the remote CE. This points to a failure in the data plane between the PEs. If the LSP (Label Switched Path) between the PEs is not operational, MPLS labels cannot be swapped or forwarded correctly, causing packets to be dropped even though the control plane (routing table and VRF) shows the remote prefix.

Exam trap

Cisco often tests the distinction between control plane and data plane failures; the trap here is that candidates see routes in the VRF and assume the data path is working, but they overlook the requirement for a functional LSP to forward MPLS-encapsulated traffic between PEs.

How to eliminate wrong answers

Option A is wrong because if the VRF route-target import/export were misconfigured, the PE would not import the remote prefix into the VRF, and the CE would not receive the route at all. Option B is wrong because an MTU issue on core links would typically cause fragmentation problems or packet drops for large packets, but it would not prevent the CE from receiving routes or the PE from having the prefix in its routing table and VRF. Option C is wrong because the Route Distinguisher (RD) is used to make prefixes unique across VRFs; different RDs on the two PEs do not affect reachability or ping success, as long as the route targets match for import/export.

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47

MCQeasy

In MPLS, which operation is performed when a packet arrives at a transit router and the top label matches the router's own label assigned for the FEC?

A.Pop the top label.

B.Push a new label onto the stack.

C.Drop the packet.

D.Swap the top label with a new label.

AnswerA

Correct.

Why this answer

When the top label is the router's own label (often an implicit null or explicit null), the router pops the label and forwards the packet based on the next label or IP header.

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48

MCQmedium

An engineer wants to configure SR-TE without using RSVP. Which mechanism does SR-TE use to specify the explicit path?

A.ERO in RSVP PATH message

B.Label stack with PHP

C.IGP shortest path only

D.Segment list in SR Policy

AnswerD

SR-TE uses segment lists in SR Policies.

Why this answer

SR-TE uses a segment list consisting of Node SIDs and/or Adjacency SIDs to define the explicit path, avoiding RSVP state.

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49

MCQmedium

Which RSVP-TE message is used to reserve resources along the path and includes the Explicit Route Object (ERO)?

A.RESV_CONFIRM

B.PATH

C.PATH_TEAR

D.RESV

AnswerB

Correct. The PATH message carries the ERO to specify the explicit route for the LSP.

Why this answer

The PATH message is sent from the source to the destination to request resource reservation and includes the Explicit Route Object (ERO), which specifies the path to be taken. The RESV message, sent upstream in response, includes objects like LABEL, STYLE, and RECORD_ROUTE, but not the ERO.

Exam trap

Many candidates mistakenly think RESV carries the ERO because both relate to path setup, but the ERO is only in the PATH message.

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50

MCQhard

A service provider is implementing Segment Routing over IPv6 (SRv6) in their core. They want to provide path isolation for different services using SRv6 SIDs. Which SID structure is used to encode both the locator and the function?

A.The SID is divided into locator and function (and optionally arguments).

B.The SID uses a separate label stack for function encoding.

C.The SID is an IPv6 address without any encoding.

D.The SID consists of a prefix only.

AnswerA

SRv6 SID is structured as Locator:Function:Args.

Why this answer

In SRv6, the SID is encoded as a 128-bit IPv6 address that is explicitly divided into three parts: the Locator (identifies the node or prefix), the Function (identifies the specific behavior or instruction), and optionally Arguments (additional parameters). This structure allows path isolation by assigning unique locator/function combinations per service, enabling per-service steering without additional label stacks.

Exam trap

The trap here is that candidates confuse SRv6 with MPLS-based Segment Routing, assuming a separate label stack is needed for function encoding, when in fact SRv6 embeds everything into the IPv6 address itself.

How to eliminate wrong answers

Option B is wrong because SRv6 does not use a separate label stack; the function is encoded directly within the IPv6 address, not in a separate MPLS label stack. Option C is wrong because an SRv6 SID is not a plain IPv6 address without encoding; it has a structured format with locator, function, and optional arguments as defined in RFC 8986. Option D is wrong because the SID is not just a prefix; it includes the function and optional arguments to specify the forwarding behavior, not merely a network prefix.

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51

MCQeasy

Which LDP mode uses downstream unsolicited label distribution and liberal label retention?

A.Downstream on demand with liberal retention

B.Downstream unsolicited with liberal retention

C.Upstream unsolicited with liberal retention

D.Downstream on demand with conservative retention

AnswerB

This is the default LDP mode.

Why this answer

Downstream unsolicited (DU) label distribution is the default mode in LDP, where each LSR advertises labels for all its directly connected prefixes to its neighbors without waiting for a request. Liberal label retention means the LSR stores all received label bindings, even if they are not currently used for forwarding, which enables rapid convergence upon topology changes.

Exam trap

Cisco often tests the distinction between label distribution modes (DU vs. DoD) and label retention modes (liberal vs. conservative), and candidates may confuse 'downstream unsolicited' with 'upstream unsolicited' or incorrectly associate liberal retention with DoD mode.

How to eliminate wrong answers

Option A is wrong because downstream on demand (DoD) mode requires an explicit label request from an upstream neighbor before a label is advertised, which is the opposite of unsolicited distribution. Option C is wrong because LDP does not use an 'upstream unsolicited' mode; label distribution is always from the downstream LSR to the upstream LSR. Option D is wrong because conservative label retention discards label bindings that are not currently used for forwarding, which contradicts the liberal retention specified in the question.

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52

MCQmedium

Which SRv6 endpoint behavior is used for L3VPN decapsulation and IP forwarding to the destination in a VRF?

A.End.DT2U

B.End

C.End.X

D.End.DT4

AnswerD

Correct. End.DT4 decapsulates and does IPv4 VRF lookup.

Why this answer

End.DT4 is the correct SRv6 endpoint behavior for L3VPN decapsulation and IP forwarding to the destination in a VRF. It removes the outer IPv6 header and SRH, then performs a lookup in the specified VRF IPv4 routing table to forward the inner IPv4 packet. This behavior is defined in RFC 8986 and is the direct SRv6 equivalent of the MPLS-based per-VRF label allocation for L3VPN.

Exam trap

Cisco often tests the distinction between endpoint behaviors by using similar acronyms (e.g., End.DT2U vs End.DT4) to confuse candidates who may not recall that '2U' refers to Layer 2 unicast and '4' refers to IPv4 VRF forwarding.

How to eliminate wrong answers

Option A is wrong because End.DT2U is used for L2VPN decapsulation and unicast MAC forwarding to a bridge domain, not for L3VPN IP forwarding in a VRF. Option B is wrong because End (without any flavor) performs simple decapsulation and IPv6 forwarding based on the destination address in the inner IPv6 header, not VRF-based IPv4 forwarding. Option C is wrong because End.X is used for decapsulation and forwarding to a specific neighbor via a Layer 3 interface, often with a next-hop address, but it does not involve VRF lookup for L3VPN.

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53

MCQmedium

In an MPLS network using LDP, which LDP mode allocates a label for every prefix in the routing table and advertises it to all LDP neighbors, regardless of whether the neighbor requests it?

A.Downstream on demand with liberal label retention

B.Downstream unsolicited with conservative label retention

C.Downstream unsolicited with liberal label retention

D.Downstream on demand with conservative label retention

AnswerC

This is the default mode in Cisco IOS.

Why this answer

Downstream unsolicited (DU) mode means a router advertises labels for all its prefixes to all neighbors without waiting for a request.

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54

MCQeasy

An engineer is configuring an MPLS Traffic Engineering tunnel using RSVP-TE. The tunnel is intended to carry voice traffic with strict delay requirements. Which RSVP-TE object should be used to enforce a maximum delay path?

A.Enable 'record-route' on the tunnel

B.Configure 'fast-reroute' protection

C.Specify an explicit-path (ERO) manually

D.Use the 'path-option' with 'metric delay' under the tunnel

AnswerD

This allows CSPF to use delay as the metric for path computation.

Why this answer

The 'path-option with metric delay' command instructs RSVP-TE to compute a path that minimizes cumulative delay rather than IGP cost or hop count. This directly enforces a maximum delay constraint for voice traffic by selecting the path with the lowest measured or configured delay metric across the MPLS network.

Exam trap

Cisco often tests the distinction between path selection tools (like metric types) and path recording or protection mechanisms, leading candidates to confuse 'record-route' or 'fast-reroute' with delay enforcement.

How to eliminate wrong answers

Option A is wrong because 'record-route' merely logs the actual path taken by the tunnel for troubleshooting or loop detection; it does not influence path selection or enforce delay constraints. Option B is wrong because 'fast-reroute' provides local protection against link or node failures by pre-signaling backup tunnels, but it does not impose a delay requirement on the primary path. Option C is wrong because manually specifying an explicit-path (ERO) forces a fixed sequence of hops, which may not be the lowest-delay path and requires static engineering; it does not dynamically enforce a delay metric.

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55

MCQeasy

In Segment Routing over IPv6 (SRv6), what is the function of the 'Locator' part of an SRv6 SID?

A.It indicates the length of the SID.

B.It encodes an argument for the SRv6 function.

C.It identifies the node and is used for routing the packet to that node.

D.It specifies the function to be performed on the packet.

AnswerC

Correct: The locator is routable and identifies the node.

Why this answer

The Locator is the portion of the SRv6 SID that identifies the node (or the segment endpoint) and is used for routing to that node.

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56

Multi-Selectmedium

Which TWO statements correctly describe RSVP-TE reservation styles? (Choose two.)

Select 2 answers

A.Shared Explicit (SE) does not allow any sharing.

B.Shared Explicit (SE) allows multiple senders to share a reservation but with explicit filters.

C.Fixed Filter (FF) is the only style used in MPLS-TE.

D.Wildcard Filter (WF) is commonly used in MPLS-TE.

E.Fixed Filter (FF) creates a separate reservation for each sender.

AnswersB, E

SE style uses explicit filters but allows sharing.

Why this answer

Shared Explicit (SE) allows sharing among senders with different selectors, and Fixed Filter (FF) creates distinct reservations per sender.

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57

MCQeasy

A service provider is designing a new MPLS core network and wants to simplify traffic engineering without deploying a centralized controller. Which technology should be used to enable source-routed paths with explicit path control?

A.BGP-LU

B.LDP

C.RSVP-TE

D.Segment Routing (SR-MPLS)

AnswerD

SR-MPLS enables source routing with segment lists, supporting traffic engineering without a controller.

Why this answer

Segment Routing (SR-MPLS) enables source-routed paths with explicit path control by encoding the path as a list of segment identifiers (SIDs) in the packet header, allowing the ingress router to steer traffic along a specific path without a centralized controller. This simplifies traffic engineering in an MPLS core network by eliminating the need for per-flow signaling and state maintenance on intermediate nodes, which is exactly what the question requires.

Exam trap

Cisco often tests the misconception that RSVP-TE is the only way to achieve explicit path control, but the trap here is that RSVP-TE requires per-flow state and signaling, while Segment Routing provides source-routed paths without a centralized controller or per-node state.

How to eliminate wrong answers

Option A (BGP-LU) is wrong because BGP-LU is used for distributing MPLS labels for IPv4/IPv6 prefixes across BGP peers, not for source-routed path control or explicit traffic engineering; it lacks the ability to encode a path as a segment list. Option B (LDP) is wrong because LDP is a label distribution protocol that creates hop-by-hop label-switched paths based on IGP routing, without any explicit path control or source-routing capability; it cannot enforce a specific path beyond what the IGP provides. Option C (RSVP-TE) is wrong because while RSVP-TE does provide explicit path control through signaling, it requires a centralized controller or per-path configuration on each router, and it maintains per-flow state on every intermediate node, which contradicts the goal of simplifying traffic engineering without a centralized controller.

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58

Matchingmedium

Match each Ethernet OAM protocol to its function.

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

Concepts

Matches

Connectivity Fault Management for end-to-end Ethernet OAM

Performance monitoring including delay and loss measurement

Ethernet in the First Mile OAM for link-level monitoring

Link Aggregation Control Protocol for bundling links

Link Layer Discovery Protocol for neighbor discovery

Why these pairings

These are Ethernet OAM and management protocols used in service provider access.

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59

MCQmedium

Which component of SRv6 SID format is used to identify the specific function (e.g., End, End.X) on the node?

A.Argument

B.Block

C.Function

D.Locator

AnswerC

Correct: Function identifies the behavior.

Why this answer

The SRv6 SID is structured as Locator:Function:Argument. The Function part identifies the specific behavior to be executed on the node, such as End or End.X.

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60

MCQmedium

An SR-TE policy is configured with a candidate path containing multiple segment lists. Which selection criteria is used to choose the active segment list among the candidate paths?

A.Color

B.Preference value

C.Metric type (IGP or TE)

D.Hop count

AnswerB

The candidate path with highest preference is selected.

Why this answer

The preference value (or priority) is used to select the best candidate path; higher preference wins.

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61

MCQeasy

Which protocol is typically used to distribute MPLS labels between directly connected LSRs?

A.LDP

B.RSVP

AnswerA

LDP is specifically designed to distribute labels for MPLS forwarding.

Why this answer

LDP (Label Distribution Protocol) is the standard protocol used to distribute MPLS labels between directly connected LSRs (Label Switch Routers) in a non-traffic-engineered MPLS network. It operates by establishing LDP sessions over TCP (port 646) and exchanging label mappings for each FEC (Forwarding Equivalence Class) based on the underlying IGP routing table, enabling hop-by-hop label switching without requiring explicit path setup.

Exam trap

Cisco often tests the distinction between label distribution protocols (LDP) and routing protocols (OSPF, BGP) or signaling protocols (RSVP), trapping candidates who confuse the role of RSVP-TE for traffic engineering with the basic label distribution function of LDP.

How to eliminate wrong answers

Option B (RSVP) is wrong because RSVP is used for traffic engineering (RSVP-TE) to reserve bandwidth and establish explicit paths, not for basic label distribution between directly connected LSRs; it requires signaling of path and reservation messages and is more complex than LDP. Option C (BGP) is wrong because BGP distributes labels for VPN or inter-AS MPLS services (e.g., labeled unicast or VPNv4 routes) but does not handle label distribution for directly connected LSRs in the core; it relies on LDP or RSVP-TE for the underlying LSP. Option D (OSPF) is wrong because OSPF is an IGP that distributes routing information, not labels; while OSPF can carry opaque LSAs for traffic engineering (OSPF-TE), it does not perform label distribution itself.

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62

MCQhard

An engineer is configuring MPLS-TE FRR for link protection. Which of the following options describes a 1:1 protection scheme where a dedicated detour LSP is created for each protected LSP?

A.Shared mesh protection

B.Bypass tunnel protecting multiple LSPs

C.Detour LSP for each protected LSP

D.Node protection bypass tunnel

AnswerC

Correct. 1:1 detour is per-LSP.

Why this answer

In MPLS-TE Fast Reroute (FRR), a 1:1 protection scheme means a dedicated detour LSP is created for each protected LSP. This provides per-LSP backup, ensuring that if a link fails, traffic is immediately switched to the detour tunnel. Option C correctly describes this exact mechanism, where each protected LSP has its own pre-established detour path.

Exam trap

Cisco often tests the distinction between 1:1 (detour LSP) and 1:N (bypass tunnel) protection schemes, and the trap here is that candidates confuse 'bypass tunnel protecting multiple LSPs' (facility backup) with a dedicated detour, or assume 'node protection' is the same as link protection.

How to eliminate wrong answers

Option A is wrong because shared mesh protection is a recovery mechanism where multiple LSPs share a pool of backup resources, not a 1:1 dedicated detour per LSP. Option B is wrong because a bypass tunnel protecting multiple LSPs is a facility backup (1:N) scheme, where a single bypass tunnel protects many LSPs over a common link or node, not a dedicated detour per LSP. Option D is wrong because node protection bypass tunnel protects against a node failure, not a link failure, and it is a facility backup (1:N) scheme, not a 1:1 detour per LSP.

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63

MCQeasy

Which component of an MPLS-TE FRR (Fast Reroute) configuration provides protection for the next-hop node by bypassing it?

A.Node protection

B.Path protection

C.Link protection

D.Bandwidth protection

AnswerA

Node protection protects the next-hop node.

Why this answer

Node protection uses a bypass tunnel that bypasses the protected node, while link protection only bypasses the link.

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64

MCQeasy

Which LDP label retention mode stores all received label bindings even if they are not currently used for forwarding?

A.Conservative label retention

B.Liberal label retention

C.Ordered label distribution

D.Downstream unsolicited

AnswerB

Correct: Liberal retains all bindings.

Why this answer

Liberal label retention stores all label bindings, allowing quick convergence when a path changes, while conservative retention stores only bindings from the next-hop.

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65

MCQmedium

An MPLS TE tunnel is configured with an explicit path. Which RSVP message is used by the headend to signal the path and carry the Explicit Route Object (ERO) listing the hops?

A.RESV

B.PATH_TEAR

C.RESV_CONFIRM

D.PATH

AnswerD

PATH carries the ERO and label request.

Why this answer

The PATH message is sent downstream from the headend and contains the ERO that specifies the explicit route.

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66

MCQmedium

Which SR-MPLS SID is advertised by an IS-IS router to identify a specific link and is distributed only to adjacent neighbors?

A.Adjacency SID

B.Prefix SID

C.Node SID

D.Anycast SID

AnswerA

Adjacency SID is link-specific and local.

Why this answer

An Adjacency SID is a local label for a specific adjacency, distributed only to neighbors.

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67

MCQhard

Refer to the exhibit. A PE router in an MPLS L3VPN network shows the above output. The VRF CUSTOMER contains two routes. Which statement about forwarding for these routes is true?

A.Both routes are reachable via MPLS.

B.Traffic to 10.1.1.0/24 will be label-switched with an implicit null label.

C.Traffic to 10.2.2.0/24 will be forwarded using the BGP next hop label.

D.Traffic to 10.2.2.0/24 will be forwarded using IP routing.

AnswerB

'Pop' means implicit-null label (label 3), which is popped by the penultimate hop.

Why this answer

The output shows that for the VRF CUSTOMER, the route to 10.1.1.0/24 has a next hop of 192.168.1.2 and is associated with label 3 (implicit null). In MPLS L3VPN, label 3 instructs the upstream router to pop the MPLS label and forward the packet using IP routing to the directly connected next hop. Therefore, traffic to 10.1.1.0/24 will be label-switched with an implicit null label, meaning the label is removed before forwarding to the CE router.

Exam trap

Cisco often tests the distinction between implicit null (label 3) and explicit null (label 0), and the trap here is that candidates assume any route with a label in the VRF table is fully MPLS-switched end-to-end, not realizing that label 3 means the label is popped before the final hop.

How to eliminate wrong answers

Option A is wrong because not both routes are reachable via MPLS; the route to 10.1.1.0/24 uses implicit null (label 3), which means the MPLS label is popped before forwarding, so the packet is not MPLS-switched on the final hop. Option C is wrong because traffic to 10.2.2.0/24 will be forwarded using the BGP next hop label (label 16000), not an implicit null label; the output shows label 16000 for that route. Option D is wrong because traffic to 10.2.2.0/24 will be forwarded using MPLS label switching (label 16000), not IP routing, as indicated by the presence of a non-null label in the VRF table.

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68

MCQhard

Refer to the exhibit. An operator configures an SR-TE policy on a headend router. Which statement is true about the traffic steered into this policy?

A.The segment-list consists of adjacency-SIDs

B.The path is automatically computed using CSPF based on IGP metrics

C.Traffic is forwarded using MPLS LDP labels

D.The headend will impose a label stack containing labels 16000, 16001, and 16002

AnswerD

Explicit segment-lists define the ordered list of labels.

Why this answer

The SR-TE policy uses a segment-list with three labels (16000, 16001, 16002), which are MPLS labels assigned to prefix-SIDs for specific nodes. The headend imposes this label stack to steer traffic along the explicit path defined by the segment-list, ensuring strict source routing without relying on dynamic CSPF computation or LDP.

Exam trap

Cisco often tests the distinction between explicit segment-lists (which require manual label configuration) and dynamic path computation (CSPF), leading candidates to mistakenly assume CSPF is always used in SR-TE policies.

How to eliminate wrong answers

Option A is wrong because the segment-list consists of prefix-SIDs (labels 16000, 16001, 16002), not adjacency-SIDs; adjacency-SIDs are typically local and used for link-level forwarding, not for node-to-node path segments. Option B is wrong because the path is explicitly defined by the segment-list, not automatically computed using CSPF; SR-TE policies can use explicit paths or dynamic paths, but the exhibit shows an explicit segment-list. Option C is wrong because traffic is forwarded using MPLS labels from the segment-list (prefix-SIDs), not LDP labels; SR-TE uses segment routing labels, and LDP is not involved unless interworking is configured.

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69

Multi-Selecteasy

Which two statements about MPLS label operations are true? (Choose two.)

Select 2 answers

A.Push operation can only add one label at a time.

B.Pop operation can remove multiple labels at once.

C.Push operation adds a label to the packet.

D.Swap operation removes and replaces two labels.

E.Pop operation removes the top label and may be triggered by implicit-null.

AnswersC, E

Push adds an MPLS header.

Why this answer

The MPLS push operation adds a new label onto the top of the label stack, encapsulating the packet for forwarding through an LSP. Option E is correct because the pop operation removes the top label, and implicit-null (label value 3) triggers the penultimate hop to pop the label before forwarding the packet to the egress LSR, reducing the egress router's processing load.

Exam trap

Cisco often tests the misconception that push and pop operations are limited to one label at a time, but push can impose multiple labels (e.g., for label stacking), and pop with implicit-null is a specific mechanism to optimize egress processing.

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70

MCQhard

In RSVP-TE, which filter spec style allows multiple senders to share the same reserved resources on a link, as long as their flows do not exceed the reservation?

A.Shared Explicit (SE)

B.Distinct Explicit (DE)

C.Fixed Filter (FF)

D.Wildcard Filter (WF)

AnswerA

SE provides sharing with explicit sender list.

Why this answer

Shared Explicit (SE) style allows multiple senders to share a single reservation, with explicit sender list.

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71

Multi-Selecthard

When deploying EVPN with SR-MPLS or SRv6, which THREE statements about EVPN multi-homing and related concepts are correct? (Choose three.)

Select 3 answers

A.EVPN IRB (Integrated Routing and Bridging) enables both Layer 2 and Layer 3 forwarding over the same EVPN instance using an anycast gateway.

B.Aliasing allows a PE to forward traffic to a multi-homed site even if it has not learned the MAC address, based on the ESI knowledge.

C.The Ethernet Segment Identifier (ESI) is a unique identifier for a multi-homed site, used to detect multi-homing and for split-horizon filtering.

D.EVPN VPWS (Virtual Private Wire Service) uses a single EVPN instance for both point-to-point and multipoint services.

E.Designated Forwarder (DF) election is performed per (ESI, VLAN) to avoid duplicate traffic from multiple PEs to the CE.

AnswersA, B, E

Correct. IRB provides L2 and L3 with anycast gateway.

Why this answer

The three correct statements about EVPN multi-homing are A, B, and E. EVPN IRB does use an anycast gateway for Layer 3 forwarding. Aliasing allows forwarding based on ESI knowledge even without learning the specific MAC address.

Designated Forwarder (DF) election is performed per (ESI, VLAN) to avoid duplicate traffic. Option C is incorrect because the Ethernet Segment Identifier (ESI) is used for identifying multi-homed sites and DF election, but split-horizon filtering is achieved through the ESI label, not the ESI itself. Option D is incorrect because EVPN VPWS is a point-to-point service only.

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72

Multi-Selectmedium

An engineer is designing an EVPN IRB (Integrated Routing and Bridging) solution. Which two statements about EVPN IRB are correct? (Choose two.)

Select 2 answers

A.EVPN IRB does not support multi-homing.

B.EVPN IRB uses an anycast default gateway IP and MAC that is shared across all PEs.

C.EVPN IRB only supports IPv6 traffic.

D.EVPN IRB requires a separate VRF for each MAC-VRF.

E.In asymmetric IRB, routing occurs only at the ingress PE.

AnswersB, E

Correct: common anycast gateway.

Why this answer

EVPN IRB provides bridging at Layer 2 and routing at Layer 3 with an anycast default gateway. It uses asymmetric IRB (bridging locally, routing via VRF) and symmetric IRB (both directions route). The anycast gateway MAC is shared across all PEs.

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73

MCQeasy

Refer to the exhibit. The show command output displays the LDP neighbor state. What does the 'Downstream' label mode indicate about label distribution?

A.Labels are sent only to the peer that advertises the prefix.

B.Labels are only distributed when explicitly requested.

C.The neighbor must request labels via a Label Request message.

D.Labels are advertised to all neighbors without a request.

AnswerD

Downstream mode means unsolicited label advertisement.

Why this answer

In the context of LDP, 'Downstream' label mode means that a router advertises label bindings for all prefixes in its routing table to its LDP neighbors without waiting for an explicit request. This is the default label distribution mode for LDP, where labels are proactively sent to all peers. Option D correctly identifies this behavior.

Exam trap

Cisco often tests the distinction between 'Downstream Unsolicited' and 'Downstream on Demand' modes, where candidates confuse the default DU behavior with the request-driven DoD behavior described in options B and C.

How to eliminate wrong answers

Option A is wrong because 'Downstream Unsolicited' mode does not require the peer to advertise the prefix first; labels are sent regardless. Option B is wrong because it describes 'Downstream on Demand' mode, where labels are only distributed when explicitly requested via a Label Request message. Option C is wrong because it also describes 'Downstream on Demand' mode, where a Label Request message is required; in 'Downstream Unsolicited' mode, no such request is needed.

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74

MCQhard

An engineer configures an SR-TE policy with a segment list containing adjacency SIDs. The required path includes a strict loose hop through a specific router. Which segment type should be used for the strict hop?

A.Anycast SID for the group containing the router.

B.Adjacency SID for the desired link.

C.Binding SID (BSID) of another policy.

D.Node SID (Prefix SID) of the router.

AnswerB

Correct.

Why this answer

Adjacency SIDs force the packet to traverse a specific link. For a strict hop, the adjacency SID ensures the packet goes through that exact link. A Node SID would allow any path to the node.

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75

Multi-Selectmedium

Which TWO statements about SR-MPLS compared to LDP are correct? (Choose two.)

Select 2 answers

A.SR-MPLS uses a global label block (SRGB) that is consistent across the domain.

B.SR-MPLS requires LDP for label distribution.

C.SR-MPLS supports only dynamic label allocation.

D.SR-MPLS allocates a per-prefix label (Node SID) for each router's loopback.

E.SR-MPLS requires RSVP-TE for TE tunnels.

AnswersA, D

Correct. SRGB is configured consistently.

Why this answer

SR-MPLS uses a Segment Routing Global Block (SRGB), which is a reserved label range (e.g., 16000–23999 per RFC 8660) that is configured consistently across all routers in the SR domain. This global consistency allows any router to interpret a label from the SRGB as the same topological or service instruction, eliminating the need for per-hop label negotiation.

Exam trap

Cisco often tests the misconception that SR-MPLS is dependent on LDP or RSVP-TE, when in fact SR-MPLS is designed to replace these protocols by leveraging IGP for label distribution and SR-TE policies for traffic engineering.

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