Courseiva

CCNA MPLS and Segment Routing Questions

75 of 193 questions · Page 2/3 · MPLS and Segment Routing · Answers revealed

Practice these questionsDomain overviewAll questions
76
MCQhard

In an MPLS L3VPN network with route reflectors, what is the default behavior regarding the BGP next-hop attribute for reflected VPNv4 routes?

A.It sets the next-hop to 0.0.0.0.
B.It removes the next-hop attribute.
C.It sets the next-hop to the route reflector's loopback.
D.It leaves the next-hop unchanged from the originating PE.
AnswerD

Route reflectors preserve the next-hop attribute.

Why this answer

In an MPLS L3VPN network with route reflectors, the default behavior for VPNv4 routes is that the route reflector does not modify the BGP next-hop attribute. This is because the next-hop should remain the address of the originating PE router to ensure proper MPLS label switching and reachability. Changing the next-hop would break the MPLS forwarding path, as the P routers rely on the IGP to reach the PE's loopback, not the route reflector's.

Exam trap

Cisco often tests the misconception that route reflectors behave like route servers or that they must update the next-hop to themselves to ensure reachability, but in MPLS L3VPN, the next-hop must remain unchanged to preserve the MPLS forwarding path.

How to eliminate wrong answers

Option A is wrong because setting the next-hop to 0.0.0.0 would invalidate the route, as it is not a reachable address and would cause traffic to be dropped; this is not a default behavior for route reflectors. Option B is wrong because removing the next-hop attribute would make the route unusable for forwarding, as BGP requires a valid next-hop for recursive route resolution. Option C is wrong because setting the next-hop to the route reflector's loopback would break MPLS L3VPN functionality, as the P routers would attempt to reach the route reflector instead of the originating PE, disrupting label-switched path (LSP) establishment; route reflectors are not required to be in the MPLS forwarding path.

Practice this question →
77
Multi-Selecthard

Which four statements correctly describe the SRv6 micro-SID (uSID) concept? (Choose four.)

Select 4 answers
A.Micro-SID is incompatible with SR-MPLS.
B.Micro-SID encodes multiple SIDs within a single 128-bit IPv6 address.
C.Micro-SID requires a dedicated SRH extension header.
D.Micro-SID uses a common locator block shared by all nodes in the domain.
E.The micro-SID function is typically 16 bits long.
AnswersA, B, D, E

Correct. Micro-SID is an SRv6 concept and is incompatible with SR-MPLS since it uses IPv6 forwarding.

Why this answer

SRv6 micro-SID uses a compressed SID format where multiple SIDs are encoded in a single 128-bit IPv6 address. It reduces overhead by combining SIDs into a carrier SID. The uSID instruction is usually 16 bits, and the locator block is shared.

Micro-SID is incompatible with SR-MPLS because it operates in the IPv6 data plane, whereas SR-MPLS uses MPLS labels.

Practice this question →
78
MCQeasy

In MPLS, what is the term for the operation where the top label is removed and replaced with a new label?

A.Swap
B.Push
C.Pop
D.Replace
AnswerA

Correct. Swap replaces the top label with a new label.

Why this answer

Swap is the operation where the incoming label is replaced with a new outgoing label, as determined by the LFIB.

Practice this question →
79
MCQhard

An engineer configures an RSVP-TE tunnel with a bandwidth of 100 Mbps and an explicit path that excludes a specific interface. The tunnel fails to come up. Which term describes the path computation method used by RSVP-TE to find a feasible path given these constraints?

A.SPF (Shortest Path First)
B.Bellman-Ford
C.CSPF (Constrained Shortest Path First)
D.Dijkstra with ECMP
AnswerC

Correct: CSPF considers constraints.

Why this answer

CSPF (Constrained Shortest Path First) is the algorithm used by RSVP-TE to compute a path that satisfies constraints such as bandwidth, explicit path, and link affinities.

Practice this question →
80
Multi-Selectmedium

An engineer is troubleshooting an MPLS network where some routers are not exchanging labels via LDP. Which two steps are most appropriate for diagnosing the issue? (Choose two.)

Select 2 answers
A.Verify LDP neighbor adjacencies using 'show mpls ldp neighbor'.
B.Check the MTU size on all interfaces.
C.Reboot all routers to reset LDP sessions.
D.Verify that OSPF neighbor relationships are established.
E.Ensure that 'mpls ip' is enabled on the interfaces using 'show mpls ldp interface'.
AnswersA, E

Correct: This confirms session state.

Why this answer

Verifying LDP sessions with 'show mpls ldp neighbor' and checking interfaces with 'show mpls ldp interface' are standard first steps. Checking MTU or OSPF neighbors are less direct.

Practice this question →
81
MCQmedium

Refer to the exhibit. A network engineer is configuring a segment routing traffic engineering policy. The output shows two candidate paths. Why is path1 selected as the active path even though path2 has a bandwidth constraint?

A.path1 has a shorter segment list.
B.path2 is down due to insufficient bandwidth.
C.path1 is explicit and always preferred over dynamic paths.
D.path2 has a lower preference value.
AnswerD

path2 has preference 100, lower than path1's 200; the higher preference wins.

Why this answer

Path1 is selected as the active path because it has a higher preference value (200) compared to path2 (100). In segment routing traffic engineering, the preference value is the primary tie-breaker for candidate paths; the path with the highest preference is chosen regardless of bandwidth constraints. Bandwidth constraints are only evaluated after preference, and a path with a lower preference is not considered for activation even if it has a bandwidth constraint configured.

Exam trap

Cisco often tests the misconception that bandwidth constraints or path type (explicit vs. dynamic) determine path activation, when in reality the preference value is the decisive factor in SR-TE candidate path selection.

How to eliminate wrong answers

Option A is wrong because the segment list length is not a primary selection criterion; preference value takes precedence over path length. Option B is wrong because path2 is not down due to insufficient bandwidth; the output shows path2 is valid but not selected because its preference is lower. Option C is wrong because explicit paths are not inherently preferred over dynamic paths; the preference value determines which path is active, not the path type.

Practice this question →
82
Multi-Selecthard

Which THREE functions are provided by EVPN multi-homing? (Choose three.)

Select 3 answers
A.Aliasing allows other PEs to send traffic to any PE in the multi-homed group.
B.Ethernet Segment Identifier (ESI) identifies the multi-homed site.
C.Split horizon prevents loops by blocking traffic from the same ES.
D.MAC mobility is provided by multi-homing.
E.Designated Forwarder (DF) election determines which PE forwards BUM traffic.
AnswersA, B, E

Correct. Aliasing enables load balancing.

Why this answer

EVPN multi-homing uses ESI, DF election for BUM traffic, and aliasing for load balancing.

Practice this question →
83
MCQhard

An operator is deploying EVPN-VPWS over an MPLS network. Which component uniquely identifies a multihomed Ethernet segment in EVPN?

A.Ethernet Tag ID
B.Ethernet Segment Identifier (ESI)
C.Route Distinguisher (RD)
D.MPLS Label
AnswerB

Correct: ESI uniquely identifies the segment.

Why this answer

The Ethernet Segment Identifier (ESI) is a unique identifier for a multihomed segment. It is used for DF election, aliasing, and split-horizon.

Practice this question →
84
MCQeasy

An engineer is designing an MPLS network and needs to ensure that VPN traffic between two PE routers is label-switched. The PE routers are connected via a P router and have a full mesh of iBGP sessions. Which label distribution method is required for the VPN labels?

A.Segment Routing
B.MP-BGP
C.RSVP-TE
D.LDP
AnswerB

MP-BGP carries VPNv4 routes with MPLS labels.

Why this answer

In MPLS VPN architectures, VPN labels (also known as service labels) are distributed using Multiprotocol BGP (MP-BGP). MP-BGP carries VPN-IPv4 routes that include both the route distinguisher (RD) and the VPN label in the Network Layer Reachability Information (NLRI). This allows PE routers to exchange per-VRF label bindings, enabling label-switched VPN traffic across the MPLS core.

Without MP-BGP, the PE routers cannot signal the VPN-specific labels required for end-to-end label switching.

Exam trap

Cisco often tests the distinction between transport labels (distributed by LDP or RSVP-TE) and service labels (distributed by MP-BGP), leading candidates to incorrectly choose LDP or RSVP-TE for VPN label distribution.

How to eliminate wrong answers

Option A is wrong because Segment Routing (SR) is a source-routing paradigm that can be used for transport label distribution (e.g., SR-MPLS), but it does not distribute VPN service labels; VPN labels still require MP-BGP. Option C is wrong because RSVP-TE is a signaling protocol for traffic-engineered LSPs and is used for transport label distribution, not for VPN service labels. Option D is wrong because LDP distributes transport labels (IGP next-hop labels) for the MPLS core, but it cannot carry VPN-IPv4 routes or VPN labels; VPN label distribution is exclusively handled by MP-BGP.

Practice this question →
85
MCQhard

In EVPN with SRv6, which SID behavior is used for integrated routing and bridging (IRB) to forward packets between the IP fabric and the Ethernet segment?

A.End
B.End.DT2U
C.End.X
D.End.DT4
AnswerD

Correct. End.DT4 decapsulates and does IPv4 table lookup, enabling routing between IP and bridged domain.

Why this answer

End.DT2U is for L2VPN; for IRB, the router uses End.DT4/DT6 for IP forwarding and then bridges to the segment; however, the question asks for the SID behavior that enables IRB. Usually, IRB is achieved by combining a Layer 2 VPN SID and a Layer 3 VPN SID, but for the specific behavior that forwards between IP and MAC, it's often End.DT4 or End.DT6 plus a MAC lookup. But standard SRv6 IRB uses a dedicated SID like End.DT2U? Actually, IRB requires both routing and bridging; the common approach is to use a Layer 3 VPN SID for the routed part and an EVPN Ethernet segment for bridging.

In SRv6, the endpoint behavior for IRB is typically End.DT4 or End.DT6 for the routing side, and for bridging, it's End.DT2U. But the question expects a single behavior? Perhaps End.DT2U is for L2VPN only. Let's think: Cisco documentation often describes 'End.DT2U' for L2VPN, and 'End.DT4' for L3VPN.

For IRB, the service is provided by a combination. However, a specific SID behavior called 'End.DT2U' can be used for EVPN VPWS. To align with common SIDs, the answer is End.DT2U for L2VPN IRB? Actually, IRB typically uses a Layer 3 gateway SID.

I'll choose End.DT4 as it routes IP, and then the router bridges locally. But the question says 'forward packets between the IP fabric and the Ethernet segment' - that is routing. So End.DT4 is correct.

Practice this question →
86
MCQmedium

In SR-MPLS, what is the purpose of the SRGB (Segment Routing Global Block)?

A.To define the range of labels used for LDP
B.To provide a global label space for prefix SIDs
C.To reserve labels for MPLS-TE tunnels
D.To allocate labels for adjacency SIDs
AnswerB

Correct: SRGB is for prefix SIDs.

Why this answer

SRGB is a reserved label range used for globally significant prefix SIDs. All routers in the domain use the same SRGB to ensure consistent label mapping.

Practice this question →
87
Multi-Selectmedium

Which two statements about LDP liberal label retention are true? (Choose two.)

Select 2 answers
A.It requires explicit configuration of the label retention mode in Cisco IOS.
B.It allows faster convergence after a topology change.
C.It conserves memory by only storing labels for next-hop neighbors.
D.It retains labels from all neighbors regardless of whether they are used for forwarding.
E.It requires manual configuration of label filtering.
AnswersB, D

Correct: Labels are already available.

Why this answer

Liberal label retention is the default mode in Cisco IOS and does not require explicit configuration; it retains all labels even if not used (D) and speeds convergence because labels are already available after a topology change (B). Option C describes conservative retention, and option E is false because liberal retention is default and does not require manual filtering.

Practice this question →
88
MCQmedium

An engineer is configuring MPLS LDP on a Cisco router. The router uses downstream unsolicited mode with liberal label retention. Which behavior describes the label distribution and retention on this router?

A.The router requests labels from neighbors and retains only labels for the best path.
B.The router sends label bindings only for directly connected routes and discards labels from non-reachable next hops.
C.The router sends label bindings only in response to requests from upstream neighbors and retains only labels for directly connected next hops.
D.The router sends label bindings to all neighbors without receiving requests and retains all received label bindings.
AnswerD

Correct description.

Why this answer

Downstream unsolicited means the downstream router sends label bindings to all neighbors without being asked. Liberal label retention means the router keeps all received label bindings, even if the next hop is not reachable.

Practice this question →
89
MCQhard

What does the output indicate about the TI-LFA protection on R1?

A.All prefixes have backup paths.
B.Only 1 prefix has a backup path.
C.0 prefixes have backup paths.
D.2 prefixes have backup paths.
AnswerB

The line 'Number of prefixes with backup paths: 1' confirms this.

Why this answer

The output shows that for the protected prefix 10.1.1.1/32, the backup path is listed as 'N/A', meaning no backup path was computed. However, for the other prefix (likely a different /32), a backup path is present. This indicates that only one prefix has a backup path, making option B correct.

TI-LFA computes a backup path only when a viable post-convergence path exists without microloops; if the topology doesn't allow it, the backup is marked as not available.

Exam trap

Cisco often tests the misconception that TI-LFA always provides a backup path for every prefix, but in reality, the backup is only installed when a loop-free post-convergence path exists; candidates may incorrectly assume all prefixes are protected.

How to eliminate wrong answers

Option A is wrong because the output explicitly shows that not all prefixes have backup paths (one shows 'N/A'). Option C is wrong because at least one prefix does have a backup path (the other prefix shows a valid backup next hop). Option D is wrong because only one prefix has a backup path, not two.

Practice this question →
90
MCQeasy

Which MPLS label operation is performed by the penultimate hop router in PHP (Penultimate Hop Popping)?

A.No operation
B.Swap
C.Push
D.Pop
AnswerD

PHP pops the label.

Why this answer

In PHP, the penultimate hop router pops the label before forwarding the packet to the egress router, reducing the egress's workload.

Practice this question →
91
Multi-Selectmedium

Which three are required components for deploying Segment Routing in an MPLS network? (Choose three.)

Select 3 answers
A.RSVP-TE signaling protocol
B.MPLS forwarding capability on routers
C.Node-SID and Adj-SID assignments
D.IGP with Segment Routing extensions (OSPF or IS-IS)
E.LDP for label distribution
AnswersB, C, D

Routers must support MPLS forwarding to process labels.

Why this answer

B is correct because Segment Routing relies on the existing MPLS forwarding plane; routers must be capable of pushing, swapping, and popping MPLS labels based on the Segment Routing label stack. Without MPLS forwarding hardware or software support, the SR-MPLS data plane cannot function, even if the control plane is configured.

Exam trap

Cisco often tests the misconception that Segment Routing requires RSVP-TE or LDP, but the correct answer is that SR uses IGP extensions and the existing MPLS forwarding plane, eliminating the need for these legacy signaling protocols.

Practice this question →
92
MCQmedium

Refer to the exhibit. An engineer notices that R1 has an LDP neighbor but 'show mpls forwarding-table' on R1 shows no label bindings for prefixes learned from R2. What is the most likely cause?

A.The LDP discovery source is incorrect
B.The peer LDP identifier is not reachable
C.LDP session is not established
D.R1 is configured for 'label distribution on-demand'
AnswerD

Downstream on-demand means labels are not sent until requested; if no request, no labels.

Why this answer

R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.

Exam trap

Cisco often tests the distinction between LDP session state (neighbor adjacency) and label binding exchange, tricking candidates into thinking a working session guarantees label bindings, when in fact 'label distribution on-demand' can suppress label advertisements.

How to eliminate wrong answers

Option A is wrong because the LDP discovery source being incorrect would prevent neighbor discovery entirely, but the exhibit shows an LDP neighbor is present, so discovery is working. Option B is wrong because if the peer LDP identifier were not reachable, the LDP session would not establish; the exhibit confirms a session exists, so reachability is fine. Option C is wrong because the LDP session is established (neighbor is shown), so the session is not the issue; the problem is specifically with label binding exchange, not session state.

Practice this question →
93
MCQhard

A service provider is deploying a new MPLS core with Segment Routing and requires fast convergence upon link failure. They plan to use TI-LFA (Topology Independent Loop-Free Alternate). What is a prerequisite for TI-LFA to provide protection against any single link failure?

A.IGP must be a link-state protocol with complete topology information (OSPF or IS-IS).
B.BGP-LU must be enabled for label distribution.
C.LDP must be enabled on all interfaces.
D.RSVP-TE must be configured with FRR.
AnswerA

TI-LFA uses the link-state database to compute backup paths.

Why this answer

TI-LFA relies on the IGP having a complete view of the network topology to compute a post-convergence path that avoids the failed link. OSPF and IS-IS are link-state protocols that flood link-state advertisements (LSAs) or link-state packets (LSPs) to provide this full topology database, which is essential for TI-LFA to calculate a loop-free backup path for any single link failure.

Exam trap

Cisco often tests the misconception that TI-LFA requires LDP or RSVP-TE, but the key prerequisite is a link-state IGP (OSPF or IS-IS) with complete topology information, as TI-LFA is a Segment Routing feature that uses IGP-based SIDs for path computation.

How to eliminate wrong answers

Option B is wrong because BGP-LU (BGP Labeled Unicast) is used for inter-domain label distribution and does not provide the link-state topology information required by TI-LFA; TI-LFA operates within a single IGP domain. Option C is wrong because LDP is not a prerequisite for TI-LFA; in fact, Segment Routing can operate without LDP by using IGP-distributed labels (Prefix-SIDs), and TI-LFA is designed for SR-based networks. Option D is wrong because RSVP-TE with FRR is a separate MPLS fast-reroute mechanism that requires explicit tunnel signaling and resource reservation, whereas TI-LFA is a topology-based, signaling-free protection mechanism that works with Segment Routing and a link-state IGP.

Practice this question →
94
MCQmedium

The CE router behind the VRF interface is unable to reach the remote CE. Which configuration error is most likely the cause?

A.The BGP neighbor is configured with a directly connected interface address but uses update-source Loopback0.
B.The VRF is missing the 'rd' command.
C.The 'mpls ip' command is missing on the core interface.
D.The VRF interface does not have an IP address.
AnswerA

This mismatch causes BGP to try to source from the loopback while expecting a session to the interface address, likely leading to session failure.

Why this answer

When BGP is configured with a neighbor using a directly connected interface address (e.g., 10.0.0.2) but the update-source is set to Loopback0, the BGP session will attempt to establish using the loopback IP as the source. If the loopback address is not reachable via the directly connected link (e.g., no IGP route for the loopback across the MPLS/VPN core), the BGP session will fail, preventing the CE router behind the VRF from reaching the remote CE. This mismatch between the neighbor address and the update-source is a common misconfiguration in MPLS L3VPN environments.

Exam trap

Cisco often tests the subtle interaction between BGP update-source and neighbor address configuration in MPLS VPN scenarios, where candidates mistakenly think the directly connected interface address is always sufficient for BGP peering, ignoring that the update-source overrides the source IP and must be routable.

How to eliminate wrong answers

Option B is wrong because the 'rd' (route distinguisher) command is required for VRF configuration to make customer routes unique in the MPLS VPN backbone, but its absence would not directly prevent a CE router from reaching a remote CE via BGP; it would affect route import/export into the VRF, not BGP session establishment. Option C is wrong because the 'mpls ip' command on the core interface enables MPLS label switching on that interface, but if the core is already running MPLS (e.g., via other interfaces or a full mesh), its absence on one interface would not necessarily break BGP peering between CEs; the issue is at the CE-PE edge, not the core. Option D is wrong because the VRF interface must have an IP address for Layer 3 connectivity, but the question states the CE router is behind the VRF interface, implying the interface is configured; the problem is specifically with BGP update-source, not missing IP addressing.

Practice this question →
95
Multi-Selecthard

Which two are characteristics of the Anycast SID in Segment Routing? (Choose two.)

Select 2 answers
A.It identifies an adjacency between two routers.
B.It is typically used as an Adj-SID for load balancing.
C.It is unique per node and identifies a specific router.
D.It provides redundancy and fast convergence by allowing traffic to be rerouted to another router.
E.It is shared among multiple routers in the same anycast group.
AnswersD, E

If one anycast router fails, traffic goes to another.

Why this answer

Anycast SIDs are shared among multiple routers in the same anycast group, allowing any of those routers to process traffic destined for that SID. This provides redundancy and fast convergence because if one router fails, traffic is automatically rerouted to another router in the group without requiring control-plane signaling changes.

Exam trap

Cisco often tests the distinction between Node SID (unique per router) and Anycast SID (shared among routers), so the trap here is confusing the 'shared' nature of Anycast SID with the 'unique' nature of Node SID, leading candidates to incorrectly select Option C.

Practice this question →
96
MCQeasy

In SRv6, which Segment Identifier (SID) format is used to encode a function that performs IPv6 decapsulation and forwarding?

A.End.X
B.End.B6
C.End.DT4
D.End
AnswerC

Correct.

Why this answer

SRv6 SIDs are 128-bit and structured as Locator:Function:Arguments. End.DT4 is a function that decapsulates the outer IPv6 header and forwards the inner IPv4 packet based on destination.

Practice this question →
97
MCQhard

An engineer is troubleshooting an MPLS VPN issue where CE1 cannot ping CE2 across the provider network. The PE routers are configured with MP-BGP and LDP. On PE1, 'show ip bgp vpnv4 vrf CUST' shows the route to CE2's loopback as valid, but 'show mpls forwarding-table' does not list a label for that prefix. What is the most likely cause?

A.MTU mismatch on the MPLS core interfaces
B.LDP is not enabled on the core link between PE1 and P
C.Route target mismatch between PE1 and PE2
D.BGP session between PE1 and PE2 is not established
AnswerC

Incorrect RT prevents the VPN label from being installed in the forwarding table despite BGP advertisement.

Why this answer

The route is present in the VRF BGP table on PE1 (valid), but no MPLS label is assigned for that prefix in the forwarding table. This indicates that PE1 has learned the route via MP-BGP but cannot install it into the MPLS forwarding table because the route target (RT) import policy on PE1 does not match the RT export policy on PE2. Without a matching RT, PE1 does not consider the VPNv4 route as belonging to the CUST VRF, so it cannot resolve the next hop or assign a label for forwarding.

Exam trap

Cisco often tests the distinction between BGP route validity (learned via MP-BGP) and VRF route installation (controlled by RT import), leading candidates to mistakenly suspect BGP session or LDP issues when the real problem is a route target mismatch.

How to eliminate wrong answers

Option A is wrong because an MTU mismatch on core interfaces would cause fragmentation or packet drops, not a missing label in the MPLS forwarding table for a specific VPN prefix. Option B is wrong because LDP not being enabled on the core link would prevent label assignment for all transit prefixes, not just a single VPN route, and the issue here is specific to a VRF route. Option D is wrong because if the BGP session between PE1 and PE2 were not established, the route to CE2's loopback would not appear as valid in 'show ip bgp vpnv4 vrf CUST'; the route is present, so the session is up.

Practice this question →
98
MCQmedium

A network operator is configuring RSVP-TE tunnels for traffic engineering in an MPLS core. They want to enforce that the tunnel path strictly follows a predefined set of hops. Which explicit path option should be used?

A.Exact
B.Loose
C.Strict
D.Dynamic
AnswerC

Strict explicit path requires each hop to be adjacent.

Why this answer

(Strict) is correct because RSVP-TE explicit paths with the 'strict' keyword require each hop in the path to be directly adjacent (connected) to the previous hop. This enforces that the tunnel path follows exactly the predefined set of hops without any intermediate routers being inserted, which is the requirement in the question.

Exam trap

Cisco often tests the distinction between 'strict' and 'loose' explicit paths, where candidates may confuse 'exact' as a valid option or assume 'loose' provides the same strict enforcement.

How to eliminate wrong answers

Option A (Exact) is wrong because 'exact' is not a valid keyword for RSVP-TE explicit path configuration; the valid keywords are 'strict' and 'loose'. Option B (Loose) is wrong because a 'loose' explicit path allows intermediate routers not listed in the path to be inserted between specified hops, which does not enforce strict adherence to the predefined set of hops. Option D (Dynamic) is wrong because a 'dynamic' path is computed by the router using CSPF (Constrained Shortest Path First) based on traffic engineering constraints, not a predefined set of hops.

Practice this question →
99
Drag & Dropmedium

Drag and drop the steps for the BGP route selection process (best path selection) in order.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4

Why this order

BGP best path selection follows a specific order: weight, local preference, locally originated, AS_PATH length, MED.

Practice this question →
100
MCQmedium

An engineer is implementing MPLS-TE Fast Reroute for link protection. Which protection method provides a backup tunnel that bypasses a specific link, protecting all LSPs traversing that link?

A.Node protection bypass tunnel
B.Bypass tunnel protecting the link
C.1:1 detour tunnel per LSP
D.FRR using RSVP-TE path pre-emption
AnswerB

Correct: Bypass tunnel protects the entire link.

Why this answer

A bypass tunnel is a single tunnel protecting a link, carrying traffic for all LSPs that traverse that link. Detour tunnels are per-LSP.

Practice this question →
101
MCQhard

In EVPN with Integrated Routing and Bridging (IRB), which type of interface is used to connect the bridge domain to the IP-VRF for inter-subnet forwarding?

A.IRB interface
B.BD interface
C.L3VPN interface
D.SVI
AnswerA

IRB interface is the correct term for EVPN IRB.

Why this answer

IRB uses an Integrated Routing and Bridging interface (IRB interface) or BVI (Bridge Virtual Interface) to route between VLANs within the EVPN.

Practice this question →
102
MCQeasy

Which label distribution mode does LDP use by default in Cisco IOS?

A.Downstream on-demand
B.Upstream unsolicited
C.Downstream unsolicited
D.Upstream on-demand
AnswerC

Correct: LDP default is downstream unsolicited.

Why this answer

LDP uses downstream unsolicited mode by default, where a router advertises labels to all neighbors without being asked.

Practice this question →
103
MCQeasy

An SP network engineer is designing a new segment routing traffic engineering deployment within a single IGP area. The network consists of 50 core routers running IS-IS and MPLS. The engineer needs to steer traffic from Router A to Router D over a path that avoids high latency links. Which technology should be used to define and instantiate the explicit path?

A.Use LDP to distribute labels and rely on IGP shortest path.
B.Set up a Path Computation Element (PCE) and delegate path computation.
C.Implement BGP LU to create an explicit path via local policies.
D.Configure an SR-TE policy with an explicit path using segment lists.
AnswerD

SR-TE policies provide explicit path steering via segment lists specifying nodes or adjacency SIDs.

Why this answer

SR-TE policies with explicit paths using segment lists allow steering traffic over a specific sequence of nodes or links, avoiding high latency links. Option A is wrong because LDP distributes labels for IGP shortest path forwarding, not explicit path control. Option B is wrong because while PCE can assist with path computation, the question asks for the technology to define and instantiate the explicit path, which is the SR-TE policy itself.

Option C is wrong because BGP-LU distributes labels for BGP prefixes, not for traffic engineering paths.

Practice this question →
104
Multi-Selecthard

Which THREE are valid SRv6 endpoint behaviors for VPN services? (Choose three.)

Select 3 answers
A.End.DT2U
B.End.B6
C.End.DT4
D.End.X
E.End.DT6
AnswersA, C, E

Decapsulation and L2VPN forwarding.

Why this answer

End.DT4 for IPv4 L3VPN, End.DT6 for IPv6 L3VPN, and End.DT2U for L2VPN are defined SRv6 behaviors.

Practice this question →
105
MCQeasy

What is the default range of the Segment Routing Global Block (SRGB) on Cisco IOS XR?

A.16000-23999
B.15000-25000
C.16000-24000
D.16-23999
AnswerA

Correct. This is the default SRGB.

Why this answer

Default SRGB is 16000-23999, but can be configured. The label range is global across SR domain.

Practice this question →
106
MCQeasy

In MPLS, which forwarding table is used to determine the outgoing label and interface for a labeled packet?

A.FIB
B.LIB
C.LFIB
D.RIB
AnswerC

Correct. LFIB is used for label switching.

Why this answer

The LFIB (Label Forwarding Information Base) is the MPLS forwarding table that maps incoming labels to outgoing labels and interfaces.

Practice this question →
107
MCQmedium

In MPLS, which label operation is performed by a Penultimate Hop Popping (PHP) router?

A.Swap the top label with a new label
B.Push a new label onto the stack
C.Pop the top label from the stack
D.Swap and push an additional label
AnswerC

Correct: PHP removes the label.

Why this answer

In PHP, the penultimate router pops the label before forwarding to the egress, reducing the egress router's work.

Practice this question →
108
Multi-Selectmedium

Which two statements about MPLS label distribution are correct? (Choose two)

Select 2 answers
A.RSVP-TE allocates labels based on LDP.
B.LDP can establish sessions with non-directly connected neighbors using targeted LDP.
C.LDP uses the BGP next-hop as the label FEC.
D.LDP distributes labels for all IGP routes in the routing table.
E.TDP is the Cisco proprietary version of LDP.
AnswersB, D

Targeted LDP allows LDP sessions between non-adjacent routers.

Why this answer

LDP can establish sessions with non-directly connected neighbors using targeted LDP, which uses UDP discovery (port 646) and TCP (port 646) to form a session even when the peers are not directly connected at Layer 3. This is defined in RFC 5036 and is commonly used in MPLS VPN inter-AS options and AToM scenarios.

Exam trap

Cisco often tests the distinction between LDP and TDP, and candidates mistakenly think TDP is a version of LDP, but TDP is a separate, obsolete protocol that was replaced by the standards-based LDP.

Practice this question →
109
MCQeasy

Which label distribution protocol is replaced by segment routing in SR-MPLS deployments?

A.RSVP-TE
B.LDP
C.IGP itself
D.BGP
AnswerB

Correct: SR replaces LDP.

Why this answer

In SR-MPLS deployments, segment routing replaces LDP as the label distribution protocol. LDP traditionally distributes labels for each prefix in the IGP, while segment routing uses the IGP itself to advertise prefix-SIDs and adjacency-SIDs, eliminating the need for a separate label distribution protocol. This simplifies the control plane and removes the dependency on LDP.

Exam trap

Cisco often tests the misconception that segment routing replaces RSVP-TE for traffic engineering, but the question specifically asks about the label distribution protocol replaced by SR-MPLS, which is LDP, not RSVP-TE.

How to eliminate wrong answers

Option A is wrong because RSVP-TE is a traffic engineering protocol that provides explicit path setup and bandwidth reservation, not a general label distribution protocol replaced by segment routing; segment routing can actually replace RSVP-TE for traffic engineering using segment lists, but the question specifically asks about the label distribution protocol replaced by SR-MPLS. Option C is wrong because the IGP itself is not a label distribution protocol; segment routing leverages the IGP to distribute SIDs, but the IGP is not replaced—it is enhanced. Option D is wrong because BGP is used for inter-domain label distribution (e.g., BGP-LU) and is not replaced by segment routing; segment routing can work with BGP for inter-domain scenarios, but BGP remains in use.

Practice this question →
110
Multi-Selectmedium

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

Select 2 answers
A.Label operations are performed based on the IP destination address.
B.At the ingress LSR, a label is pushed onto the packet.
C.At the ingress LSR, the label is popped.
D.At the egress LSR, a label is swapped.
E.At an intermediate LSR, a label is swapped.
AnswersB, E

Correct. The ingress adds the label.

Why this answer

At the ingress, a label is pushed; at the egress, the label is popped. Swap occurs at intermediate routers.

Practice this question →
111
Multi-Selectmedium

Which THREE steps are required when configuring MPLS L3VPN on a PE router? (Choose three.)

Select 3 answers
A.Configure BGP on the CE router to advertise routes to PE
B.Create a VRF definition
C.Configure BGP address-family ipv4 vrf to exchange VPN routes
D.Configure route-target import/export using route-map
E.Assign the customer-facing interface to the VRF
AnswersB, C, E

The VRF must be created on the PE.

Why this answer

Creating a VRF definition on the PE router is required to separate customer routing tables and forwarding instances in an MPLS L3VPN. The VRF provides the logical isolation needed to keep each customer's routes and traffic distinct, and it is the foundation for associating interfaces, route targets, and VRF-specific routing processes.

Exam trap

Cisco often tests the distinction between configuring route-target values directly under the VRF versus using a route-map; candidates mistakenly think a route-map is required for RT import/export, but route-maps are only used for advanced filtering or attribute modification, not for setting the RT values themselves.

Practice this question →
112
MCQmedium

An engineer is configuring SR-MPLS in an IS-IS network. Which IS-IS TLV is used to advertise the Segment Routing Global Block (SRGB) and Node-SID assignments?

A.TLV 135
B.TLV 229
C.TLV 242
D.TLV 22
AnswerC

TLV 242 contains SR-Capabilities and SR-Algorithm.

Why this answer

The IS-IS SR-Capabilities sub-TLV (TLV 242) advertises the SRGB range, while the SR-Algorithm sub-TLV (TLV 242) and Prefix-SID sub-TLV (TLV 135) advertise Node-SIDs.

Practice this question →
113
MCQeasy

In SR-MPLS, which mechanism replaces LDP for label distribution?

A.RSVP-TE
B.IGP (IS-IS or OSPF) with Segment Routing extensions
C.MPLS-TE
D.BGP-LU
AnswerB

Correct: IGP distributes SIDs.

Why this answer

Segment Routing replaces LDP by using IGP extensions (IS-IS or OSPF) to advertise Prefix-SIDs (Node-SIDs) and Adjacency-SIDs. In IS-IS, TLV 242 (SR Capabilities) advertises the SRGB, while Node-SIDs are advertised via Prefix-SID sub-TLVs (e.g., TLV 135 for IPv4), eliminating the need for a separate label distribution protocol.

Practice this question →
114
MCQeasy

In MPLS-TE, what is the purpose of the Explicit Route Object (ERO) in the RSVP PATH message?

A.To specify the desired path as a list of nodes or links.
B.To label the path with a tunnel ID.
C.To reserve bandwidth along the path.
D.To record the actual path taken by the LSP.
AnswerA

Correct.

Why this answer

ERO specifies the strict or loose hops that the TE LSP must traverse. It allows the headend to enforce a specific path through the network.

Practice this question →
115
MCQeasy

A network engineer is designing an MPLS core using Segment Routing. They want to minimize label stack depth while still providing fast convergence using TI-LFA. Which prefix-SID assignment strategy should be used?

A.Assign a node-SID to each loopback interface
B.Use per-interface label mode from SRGB
C.Allocate labels per-VRF on PE routers
D.Advertise prefix-SIDs for all IGP prefixes
AnswerD

Prefix-SIDs enable TI-LFA with minimal label depth.

Why this answer

Advertising prefix-SIDs for all IGP prefixes allows TI-LFA to compute backup paths using any prefix in the network, not just loopbacks. This minimizes label stack depth by enabling TI-LFA to use a single prefix-SID for the repair path, avoiding the need for additional labels or explicit path constructs. Fast convergence is achieved because TI-LFA precomputes a backup next-hop that can be activated immediately upon failure, and having all prefixes as SIDs ensures the backup path can be expressed with minimal label imposition.

Exam trap

Cisco often tests the misconception that node-SIDs (loopbacks) are sufficient for TI-LFA, but the trap is that TI-LFA requires prefix-SIDs for all prefixes to achieve optimal label stack depth and fast convergence, as node-SIDs alone may force deeper stacks or suboptimal backup paths.

How to eliminate wrong answers

Option A is wrong because assigning a node-SID only to loopback interfaces restricts TI-LFA to using only those loopback addresses as repair targets, which may force deeper label stacks or suboptimal backup paths when the failure involves a non-loopback prefix. Option B is wrong because per-interface label mode from SRGB is used for adjacency-SIDs, not for prefix-SIDs, and TI-LFA relies on prefix-SIDs for fast reroute; adjacency-SIDs would increase label stack depth and are not designed for TI-LFA's loop-free alternate computation. Option C is wrong because per-VRF label allocation on PE routers is a BGP/MPLS VPN concept unrelated to Segment Routing prefix-SID assignment and does not affect TI-LFA convergence or label stack depth in the MPLS core.

Practice this question →
116
MCQeasy

Which label operation is performed on a router when an MPLS packet arrives with a top label that matches a local label assigned to a directly connected prefix?

A.No operation, label is ignored
B.Push
C.Pop
D.Swap
AnswerC

Correct operation for directly connected prefix.

Why this answer

If the top label corresponds to a directly connected prefix, the router will pop the label (penultimate hop popping or explicit null) and forward the packet based on the inner header.

Practice this question →
117
MCQhard

In SRv6 micro-SID (uSID) compression, which part of the SID is used to carry a compressed instruction set, enabling efficient forwarding with a shorter SID list?

A.Block
B.Function
C.Locator
D.Argument
AnswerB

The function part encodes the behavior in compressed form.

Why this answer

The micro-SID block consists of a locator followed by compressed function and argument, reducing the SID list size.

Practice this question →
118
MCQhard

In an MPLS L3VPN network, a customer has overlapping IP addresses between two VPNs. The provider edge routers are configured with VRF instances. Which method ensures that traffic from one VPN does not leak into the other when using MPLS labels?

A.Assign different MPLS label ranges to each VRF.
B.Apply BGP community strings to filter routes.
C.Use distinct route targets for import and export per VRF.
D.Configure different route distinguishers for each VPN.
AnswerC

RTs control which routes are imported into a VRF, preventing leakage.

Why this answer

Route targets (RTs) control the import and export of VPNv4 routes between VRFs on a PE router. By assigning distinct import and export RTs per VRF, the PE ensures that routes from one VPN are not installed into the routing table of another VPN, even if the customer uses overlapping IP addresses. MPLS labels are allocated per VRF forwarding instance, but the isolation is enforced by the RT-based filtering of BGP VPNv4 routes, not by label ranges or route distinguishers alone.

Exam trap

Cisco often tests the misconception that route distinguishers (RDs) alone provide VPN isolation, but RDs only ensure IP uniqueness in the BGP table, while route targets (RTs) are the actual mechanism for controlling route import/export between VRFs.

How to eliminate wrong answers

Option A is wrong because MPLS label ranges are allocated per forwarding equivalence class (FEC) or per platform, not per VRF; different label ranges do not prevent route leakage since labels are only locally significant and do not filter route distribution. Option B is wrong because BGP community strings are used for route policy and attribute manipulation, but they do not provide the mandatory import/export filtering mechanism that RTs enforce; communities are optional and can be overridden. Option D is wrong because route distinguishers (RDs) make IPv4 prefixes unique across VPNs, but they do not control route import/export; two VRFs with different RDs can still leak routes if they share the same RTs.

Practice this question →
119
MCQmedium

Which BGP extended community is used to signal the OSPF domain ID between PE routers in an MPLS L3VPN when OSPF is the PE-CE protocol?

A.Site of Origin
B.OSPF Domain ID
C.OSPF Route Type
D.Route Target
AnswerB

This community specifically carries the OSPF domain ID.

Why this answer

In an MPLS L3VPN where OSPF is the PE-CE protocol, the OSPF Domain ID extended community is used to signal the OSPF domain identifier between PE routers. This allows the receiving PE to determine whether the OSPF route originated from the same OSPF domain (and thus should be redistributed as an intra-area or inter-area route) or from a different domain (requiring a Type 5 LSA). The OSPF Domain ID is carried as a BGP extended community (type 0x0005 or 0x8005) and is critical for maintaining OSPF route type semantics across the MPLS backbone.

Exam trap

Cisco often tests the confusion between the OSPF Domain ID and the OSPF Route Type extended communities, where candidates mistakenly think the Route Type carries the domain information, but in reality the Route Type only encodes the OSPF path type and metric, while the Domain ID identifies the originating OSPF domain.

How to eliminate wrong answers

Option A is wrong because the Site of Origin (SoO) extended community is used to prevent routing loops in MPLS L3VPN environments, not to signal the OSPF domain ID. Option C is wrong because the OSPF Route Type extended community carries the OSPF route type (e.g., intra-area, inter-area, external) and the metric type, but it does not convey the domain identifier. Option D is wrong because the Route Target extended community controls VPN route import/export between VRF instances and has no role in signaling OSPF domain identity.

Practice this question →
120
MCQmedium

Refer to the exhibit. An engineer is troubleshooting packet loss in the MPLS core. For prefix 10.3.3.0/24, the outgoing label is 'Untagged'. What does this indicate?

A.The packet will have an implicit null label (label 3) imposed
B.The prefix is not reachable and packets will be dropped
C.The outgoing label is the same as the local label (label 18)
D.No MPLS label is being imposed on outgoing packets for this prefix
AnswerD

Untagged means no label.

Why this answer

When the outgoing label for a prefix in the MPLS forwarding table shows 'Untagged', it means that for packets destined to that prefix, no MPLS label is imposed. The router will forward the packet as a standard IP packet (without an MPLS header) out the egress interface. This typically occurs when the next-hop router has signaled an implicit null label (label 3) via LDP, instructing the upstream router to pop the label stack and send the packet as IP.

Exam trap

Cisco often tests the distinction between 'Untagged' (meaning no label is imposed, typically due to PHP with implicit null) and 'Pop' (which explicitly indicates the label is removed), leading candidates to confuse the two or incorrectly associate 'Untagged' with unreachability.

How to eliminate wrong answers

Option A is wrong because an implicit null label (label 3) causes the upstream router to pop the top label and forward the packet without any label, not to impose label 3. Option B is wrong because 'Untagged' does not indicate unreachability; the prefix is reachable and packets are forwarded as IP. Option C is wrong because the outgoing label being the same as the local label (label 18) would be described as 'Pop' or 'Implicit Null', not 'Untagged'.

Practice this question →
121
MCQhard

A network operator is deploying Segment Routing with TI-LFA across an OSPF network. After configuring OSPF with 'segment-routing mpls' and 'fast-reroute per-prefix', they notice that backup paths are not being installed for some prefixes. 'Show ip ospf segment-routing protected' shows 'No FRR backup' for those prefixes. What is a possible reason?

A.LDP label distribution is still active
B.The prefix-SID index is not globally unique
C.OSPF does not support TI-LFA for per-prefix prefixes
D.Adjacency-SIDs are not configured on neighboring routers
AnswerD

TI-LFA uses adjacency-SIDs to create the repair path.

Why this answer

TI-LFA (Topology Independent Loop-Free Alternate) for per-prefix fast-reroute in OSPF requires that adjacency-SIDs be configured on neighboring routers. Without adjacency-SIDs, OSPF cannot compute the necessary repair paths to provide backup protection, resulting in 'No FRR backup' for those prefixes.

Exam trap

Cisco often tests the misconception that TI-LFA works automatically with just 'fast-reroute per-prefix', when in fact adjacency-SIDs are a prerequisite for the repair path computation in OSPF.

How to eliminate wrong answers

Option A is wrong because LDP label distribution being active does not prevent OSPF from installing TI-LFA backup paths; OSPF Segment Routing and LDP can coexist, and the issue is specifically about SR-TI-LFA operation. Option B is wrong because a non-globally unique prefix-SID index would cause label conflicts or forwarding issues, but it would not prevent the installation of backup paths via TI-LFA; the 'No FRR backup' output indicates a failure in repair path computation, not a label allocation problem. Option C is wrong because OSPF does support TI-LFA for per-prefix prefixes when properly configured with adjacency-SIDs; the statement that OSPF does not support it is incorrect.

Practice this question →
122
MCQeasy

A service provider is migrating from a legacy MPLS network using LDP to Segment Routing (SR-MPLS). They have enabled SR on all routers and are using IS-IS. The migration plan is to keep LDP running alongside SR during the transition. After enabling SR, some traffic is being forwarded using the SR path, but the network operator notices that some CEs in an L2VPN are experiencing packet loss during failover scenarios. Troubleshooting shows that the primary pseudowire is using SR labels, but the backup pseudowire is still using LDP labels. The backup path is not working correctly. What is the most likely cause?

A.The backup path uses a different IGP metric
B.The backup pseudowire is still using LDP labels and the LDP session for the backup path is down
C.The MTU on the backup path is smaller than on the primary
D.The prefix-SID for the remote PE is missing in the SR-TE database
AnswerB

If LDP sessions are down for the backup path, the backup pseudowire cannot use SR labels, leading to packet loss.

Why this answer

During coexistence of LDP and SR, there might be a mismatch in label switching. The backup pseudowire might be using LDP labels that are not consistent with the SR forwarding plane. The most likely cause is that the backup path is not properly programmed to use SR labels, or the LDP session for the backup path is down.

The correct action is to ensure that the pseudowire's backup signaling is using the same transport mechanisms as the primary, or to disable LDP gradually.

Practice this question →
123
MCQhard

An MPLS network is experiencing micro-loops during convergence after a link failure. The network uses LDP and IS-IS as IGP. Which of the following solutions can prevent micro-loops during IGP convergence without requiring additional protocols?

A.Implement MPLS-TE FRR using bypass tunnels.
B.Enable BGP PIC (Prefix Independent Convergence).
C.Configure LFA (Loop-Free Alternate) on the IGP.
D.Deploy RSVP-TE with fast-reroute.
AnswerC

LFA computes backup paths that avoid loops during convergence.

Why this answer

LFA (Loop-Free Alternate) is a feature built directly into IGPs like IS-IS and OSPF that precomputes a backup next-hop that does not loop back to the failed link. This prevents micro-loops during IGP convergence without requiring any additional protocols such as RSVP-TE or BGP, making it a lightweight, protocol-integrated solution.

Exam trap

Cisco often tests the distinction between IGP-integrated solutions (LFA) and overlay protocols (RSVP-TE, BGP), so the trap here is assuming that any fast-reroute mechanism requires an additional protocol like RSVP-TE, when LFA is a native IGP feature.

How to eliminate wrong answers

Option A is wrong because MPLS-TE FRR using bypass tunnels requires RSVP-TE, an additional protocol beyond the IGP, and is designed for link/node protection in MPLS-TE, not specifically for preventing micro-loops during IGP convergence. Option B is wrong because BGP PIC (Prefix Independent Convergence) is a BGP feature that improves convergence for BGP routes, not for IGP routes, and does not address micro-loops caused by IGP convergence. Option D is wrong because RSVP-TE with fast-reroute also requires RSVP-TE as an additional protocol and is focused on MPLS-TE tunnel protection, not on preventing micro-loops during IGP convergence.

Practice this question →
124
MCQmedium

A service provider is deploying MPLS L2VPN using Virtual Private Wire Service (VPWS). After configuration, the two CEs cannot ping each other. The PE routers show the xconnect interface is up, but no MAC addresses are learned. What is the most likely cause?

A.The MTU on the core links is less than 1500
B.The control word is enabled on one PE but not the other
C.The VC ID mismatch between the two PEs
D.The encapsulation type on the AC is different on the two PEs
AnswerC

VC ID must match for the pseudowire to be established.

Why this answer

In MPLS L2VPN VPWS, the VC ID (Virtual Circuit ID) must match on both PE routers to establish the pseudowire between them. A mismatch prevents the pseudowire from coming up, so even though the local xconnect interface shows as up (because the attachment circuit is operational), no MAC learning can occur because the pseudowire control plane (e.g., LDP or BGP signaled) fails to bind the two endpoints. This directly explains why the CEs cannot ping and no MAC addresses are learned.

Exam trap

Cisco often tests the distinction between local interface status (attachment circuit up) and pseudowire status (VC ID match required), leading candidates to mistakenly focus on MTU or encapsulation issues when the real problem is a control plane mismatch.

How to eliminate wrong answers

Option A is wrong because an MTU less than 1500 on core links would cause fragmentation or drop of large packets, but it would not prevent the pseudowire from being established or MAC learning; the xconnect interface would still show up and MACs could be learned if the control plane is functional. Option B is wrong because the control word is an optional feature for operations like sequencing or fragmentation; a mismatch in its configuration does not prevent pseudowire establishment or MAC learning—it may cause packet corruption or drop, but the pseudowire would still come up and MACs could be learned. Option D is wrong because the encapsulation type on the attachment circuit (e.g., Ethernet, VLAN, PPP) must match the CE-facing configuration, but a mismatch between the two PEs would affect how traffic is encapsulated on the local AC, not the pseudowire signaling or MAC learning across the MPLS core; the pseudowire itself would still be established if VC IDs match.

Practice this question →
125
Multi-Selecteasy

Which TWO are characteristics of Segment Routing (SR-MPLS)? (Choose two.)

Select 2 answers
A.It relies on source routing
B.It requires a centralized controller for traffic engineering
C.It forwards packets based on destination IP address at each hop
D.It requires MPLS LDP for label distribution
E.It supports both MPLS and IPv6 data planes
AnswersA, E

SR performs source routing by encoding the path as a segment list.

Why this answer

Segment Routing (SR-MPLS) is a source routing paradigm where the ingress node encodes a list of segment identifiers (SIDs) into the packet header, specifying the path the packet must take through the network. This eliminates the need for per-hop signaling protocols like LDP or RSVP-TE, as each intermediate node simply forwards based on the top SID.

Exam trap

Cisco often tests the misconception that SR-MPLS requires a centralized controller for traffic engineering, but the distributed IGP-based control plane is fully capable of steering traffic using segment lists without any controller.

Practice this question →
126
MCQmedium

A service provider is migrating from LDP to Segment Routing in its MPLS core. The team has enabled IS-IS as the IGP and configured segment routing under the IS-IS process on all core routers. However, after the migration, some LSPs are not being signaled correctly and traffic is blackholing. Which action should be taken to ensure seamless interworking between LDP and SR during the migration?

A.Configure 'segment-routing mpls sr-prefer' under the IS-IS process.
B.Change the IGP to OSPF with segment routing enabled.
C.Remove the 'mpls ldp' configuration from all routers that have SR enabled.
D.Enable 'mpls ldp igp sync' on the IGP interfaces.
AnswerA

Correct. The 'segment-routing mpls sr-prefer' command ensures SR labels are preferred but LDP labels are maintained for fallback, enabling seamless coexistence during migration.

Why this answer

The 'segment-routing mpls sr-prefer' command under IS-IS configures the router to prefer SR labels for forwarding while maintaining LDP forwarding entries for fallback. This ensures seamless interworking between LDP and SR during migration, preventing blackholing when some routers are not fully SR-capable. Option B is incorrect because changing the IGP to OSPF is unnecessary and disruptive; IS-IS can support both LDP and SR simultaneously.

Option C is incorrect because removing LDP from all routers would break LDP signaling for any router not yet migrated to SR, causing traffic loss. Option D is incorrect because 'mpls ldp igp sync' synchronizes LDP with IGP convergence, not SR-LDP interworking, and does not address label preference during coexistence.

Practice this question →
127
MCQmedium

Which BGP address family must be used to exchange VPNv4 routes between PE routers in an MPLS L3VPN?

A.RT constraint
B.IPv4 unicast
C.L2VPN VPLS
D.VPNv4 unicast
AnswerD

VPNv4 unicast is the correct address family for exchanging VPN-IPv4 routes.

Why this answer

In an MPLS L3VPN, VPNv4 routes carry both the IPv4 prefix and the Route Distinguisher (RD) to ensure uniqueness across overlapping customer address spaces. The VPNv4 unicast address family (address-family ipv4 vpn) is the mandatory BGP address family used between PE routers to exchange these VPNv4 routes, enabling MPLS-based VPN reachability.

Exam trap

Cisco often tests the distinction between the address family used for route exchange (VPNv4 unicast) versus the filtering mechanism (RT constraint), leading candidates to confuse the RT constraint address family as the primary exchange method.

How to eliminate wrong answers

Option A is wrong because the RT constraint (Route Target constraint) address family is used to filter route advertisements based on RT membership, not to exchange VPNv4 routes themselves. Option B is wrong because IPv4 unicast address family carries only standard IPv4 routes without RD or VPN attributes, making it unsuitable for MPLS L3VPN route exchange. Option C is wrong because L2VPN VPLS address family is used for Layer 2 VPN services like Virtual Private LAN Service, not for Layer 3 VPNv4 route exchange.

Practice this question →
128
MCQmedium

When configuring an MPLS-TE tunnel with affinity/constraint-based routing, what is the effect of assigning a link attribute of 'red' and setting the tunnel's affinity to include 'red' with the 'include-all' keyword?

A.The tunnel will prefer links with 'red' but may use others if necessary.
B.The tunnel will use a strict explicit path.
C.The tunnel will avoid links with 'red'.
D.The tunnel must use only links that have the 'red' attribute.
AnswerD

Correct: 'include-all' requires all links to have the attribute.

Why this answer

The 'include-all' affinity means the tunnel path must include all links that have the specified color attribute. If a link does not have 'red' attribute, it is excluded from the CSPF computation.

Practice this question →
129
Multi-Selecthard

Which THREE are EVPN multi-homing functions? (Choose three.)

Select 3 answers
A.Ethernet Segment Identifier (ESI) uniquely identifies the multi-homed segment.
B.Split horizon prevents loops among PEs.
C.MAC mobility provides MAC address move detection.
D.Designated Forwarder (DF) election ensures only one PE forwards broadcast traffic.
E.Aliasing allows a PE to advertise all MAC addresses from a segment even if not attached.
AnswersA, D, E

ESI is key for multi-homing.

Why this answer

EVPN multi-homing uses ESI for identification, DF election to avoid duplicate frames, and aliasing for load balancing.

Practice this question →
130
MCQmedium

An engineer is configuring MPLS LDP on a Cisco router. The router is using downstream unsolicited mode and liberal label retention. Which statement correctly describes the label advertisement behavior?

A.Labels are advertised to all LDP peers for all known prefixes without a request.
B.Labels are retained only for directly connected neighbors.
C.Label advertisements are sent only for prefixes learned via BGP.
D.Labels are advertised only when explicitly requested by a neighbor.
AnswerA

Correct for downstream unsolicited mode.

Why this answer

In downstream unsolicited mode, the router advertises labels for all known prefixes to all neighbors, even if the neighbor did not request them. Liberal label retention keeps all received labels, even if not currently used.

Practice this question →
131
Drag & Dropmedium

Drag and drop the steps to configure a PPPoE client on a Cisco router into the correct order.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4

Why this order

PPPoE client configuration involves creating a dialer interface, setting encapsulation, dialer pool, authentication, and IP address.

Practice this question →
132
MCQhard

In SRv6, which endpoint behavior is used for decapsulating the outer IPv6 header and performing an IPv4 lookup (e.g., for L3VPN)?

A.End.DT6
B.End.DT2U
C.End.DX4
D.End.DT4
AnswerD

Correct: End.DT4 decapsulates and does IPv4 lookup.

Why this answer

End.DT4 is the SRv6 endpoint behavior that decapsulates the outer IPv6 header and then performs a lookup in the IPv4 VPN table. End.DT6 is for IPv6, and End.DT2U is for L2VPN.

Practice this question →
133
Multi-Selecthard

In SR-MPLS, which THREE statements correctly describe the properties of Adjacency SIDs? (Choose three.)

Select 3 answers
A.Adjacency SIDs can be used in segment lists for SR-TE policies.
B.Adjacency SIDs are allocated from the SRGB.
C.Adjacency SIDs force a packet to traverse a specific link.
D.Adjacency SIDs are global and must be unique across the SR domain.
E.Adjacency SIDs are advertised in IGP as sub-TLVs under IS reachability.
AnswersA, C, E

Correct.

Why this answer

Adjacency SIDs are local to a router and represent a specific link. They are not global and are used for strict path control. They are allocated from a local label range (not SRGB).

Practice this question →
134
Multi-Selecthard

In an MPLS L3VPN network, a route reflector (RR) is used to distribute VPNv4 routes between PE routers. After a new PE router (PE4) is added, some VPN routes are not being received by other PEs. Which TWO actions should be investigated to resolve the issue? (Choose two.)

Select 2 answers
A.Ensure the VRF route-target import/export values are correctly configured on the new PE.
B.Configure OSPF as an additional IGP to redistribute VPN routes.
C.Verify that BGP VPNv4 neighbor relationship is established between the new PE and the RR.
D.Check physical connectivity and IGP adjacency between the new PE and the RR.
E.Issue a 'clear ip bgp * refresh' on the RR.
AnswersA, C

Mismatched RTs prevent routes from being imported into the VRF.

Why this answer

The new PE must have matching VRF route-target import/export values to receive and advertise VPN routes. Option C is correct because a BGP VPNv4 neighbor relationship between the new PE and the RR is necessary for VPNv4 route exchange. Option B is incorrect: OSPF as an additional IGP is not required for VPN route distribution.

Option D is incorrect: while physical connectivity and IGP adjacency are prerequisites, the specific issue with missing VPN routes points to BGP session or RT misconfiguration. Option E is incorrect: a route refresh does not fix missing route-target or BGP session problems.

Practice this question →
135
Multi-Selectmedium

A service provider is transitioning from LDP-based MPLS to Segment Routing (SR-MPLS). Which TWO statements correctly describe differences or interoperability considerations? (Choose two.)

Select 2 answers
A.In SR-MPLS, the Node-SID is allocated from the dynamic label range and is unique only within the IGP area.
B.LDP and SR-MPLS cannot coexist on the same router; the router must be configured exclusively for one or the other.
C.The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.
D.Adjacency-SID in SR-MPLS is a local label representing a specific link and is used for traffic steering over that link.
E.In SR-MPLS, the label stack is built by the ingress router and nodes swap the top label without pushing additional labels.
AnswersC, D

Correct. SRGB is a global label range for Prefix-SIDs.

Why this answer

In SR-MPLS, the Segment Routing Global Block (SRGB) is a reserved label range used to allocate Prefix-SIDs, ensuring end-to-end label consistency across the SR domain. An Adjacency-SID is a local label that represents a specific link and is used for traffic steering over that link. LDP and SR-MPLS can coexist with proper mapping, making option B incorrect.

Node-SIDs are allocated from the SRGB and are unique domain-wide, not just within an area. In SR-MPLS, the ingress router pushes a label stack, and intermediate nodes perform penultimate hop popping (PHP) or continue to forward, but they do not swap labels as in LDP. Therefore, options C and D are correct.

Practice this question →
136
MCQhard

A service provider is designing a new MPLS core network using Segment Routing with MPLS data plane. They require traffic engineering capabilities to optimize bandwidth utilization. Which technology should be used to compute optimal paths based on IGP link attributes and bandwidth constraints?

A.RSVP-TE with FRR
B.LDP over SR
C.SR-TE (Segment Routing Traffic Engineering)
D.OSPF with MPLS-TE extensions
AnswerC

SR-TE computes paths using segment lists and can enforce bandwidth constraints.

Why this answer

SR-TE (Segment Routing Traffic Engineering) is the correct choice because it uses a centralized or distributed controller to compute optimal paths based on IGP link attributes (such as metric, TE metric, affinity) and bandwidth constraints, encoding the path as a segment list in the packet header. Unlike RSVP-TE, SR-TE does not require per-flow state in the core routers, making it more scalable for bandwidth optimization in an MPLS Segment Routing network.

Exam trap

Cisco often tests the misconception that OSPF with MPLS-TE extensions alone provides traffic engineering, but in reality, it only advertises link attributes and requires a separate path computation mechanism like SR-TE or RSVP-TE to enforce TE paths.

How to eliminate wrong answers

Option A is wrong because RSVP-TE with FRR is a traditional MPLS TE solution that requires per-tunnel state maintenance and signaling, which contradicts the stateless nature of Segment Routing and does not leverage IGP link attributes for path computation in the same way as SR-TE. Option B is wrong because LDP over SR is a label distribution mechanism that provides basic MPLS forwarding but lacks traffic engineering capabilities to compute optimal paths based on bandwidth constraints or link attributes. Option D is wrong because OSPF with MPLS-TE extensions only floods TE link attributes (via opaque LSAs) but does not compute or enforce traffic-engineered paths; it requires an external component like RSVP-TE or SR-TE to perform the actual path computation.

Practice this question →
137
Multi-Selectmedium

Which TWO statements correctly describe Segment Routing characteristics? (Select two.)

Select 2 answers
A.All routers must be configured with the same SRGB value
B.The path is encoded as a label stack at the source router
C.Label distribution does not rely on LDP or RSVP
D.SR eliminates all per-prefix state from core routers
E.Traffic engineering policies are distributed via BGP without any IGP extension
AnswersB, C

SR uses a label stack to specify the path.

Why this answer

Segment Routing (SR) encodes the forwarding path as an ordered list of segment identifiers (SIDs) pushed onto a label stack at the source router. This source-routing paradigm allows the ingress node to specify the exact path through the network without requiring intermediate routers to maintain per-flow state.

Exam trap

Cisco often tests the misconception that SR eliminates all per-prefix state from core routers, but in reality, core routers still maintain IGP per-prefix state and may hold SR-MPLS labels for those prefixes.

Practice this question →
138
MCQmedium

An engineer is configuring RSVP-TE fast reroute for link protection. Which protection method uses a pre-configured bypass tunnel that protects multiple LSPs on the same link?

A.Local Path Protection
B.Global Repair
C.1:1 detour
D.Facility backup
AnswerD

Bypass tunnel protects multiple LSPs.

Why this answer

In RSVP-TE FRR, facility backup uses a bypass tunnel that can protect multiple LSPs on the same link. In contrast, 1:1 detour creates a separate detour LSP for each protected LSP.

Practice this question →
139
MCQhard

A service provider is troubleshooting an MPLS L3VPN issue where a CE router is receiving the VPN route from the PE but cannot ping the remote CE's loopback. The PE shows that the VPN label is assigned but cannot route the packets. Which command would help determine if the remote PE is correctly resolving the BGP next-hop via IGP?

A.show ip route bgp
B.show mpls ldp neighbor
C.show ip route vrf CUST
D.show mpls forwarding-table vrf CUST
AnswerA

Shows the BGP route and its next-hop reachability (via IGP).

Why this answer

'show ip route bgp', is correct because the issue is that the remote PE cannot route packets to the BGP next-hop of the VPN route. The CE receives the route, but the PE cannot forward packets, indicating a missing or incorrect IGP route to the next-hop address. This command displays BGP routes in the global routing table, allowing you to verify if the next-hop is reachable via IGP (e.g., OSPF or IS-IS) and if the recursive routing is successful.

Exam trap

Cisco often tests the distinction between VRF-specific commands and global routing table commands; the trap here is that candidates assume 'show ip route vrf CUST' will show the next-hop reachability, but it only shows the VPN route itself, not the underlying IGP route required for recursive forwarding.

How to eliminate wrong answers

Option B is wrong because 'show mpls ldp neighbor' checks LDP session status and label exchange between directly connected LSRs, but it does not verify IGP reachability to the BGP next-hop, which is the root cause here. Option C is wrong because 'show ip route vrf CUST' shows routes within the VRF, including the VPN route received from the remote PE, but it does not show the global IGP route to the BGP next-hop; the issue is in the global routing table, not the VRF. Option D is wrong because 'show mpls forwarding-table vrf CUST' displays the MPLS forwarding entries for the VRF, including the VPN label and outgoing interface, but it does not reveal whether the BGP next-hop is reachable via IGP; the forwarding table assumes the next-hop is reachable, which is the problem here.

Practice this question →
140
MCQmedium

A service provider is deploying MPLS-TE with RSVP-TE in their core network. They notice that some LSPs are not being established due to resource contention. Which action would best address this issue without redesigning the entire traffic engineering deployment?

A.Enable preemption on RSVP-TE LSPs with appropriate priority levels.
B.Increase the bandwidth of all core links.
C.Configure LSP path-option explicit paths with strict hops.
D.Disable RSVP-TE and use LDP for label distribution.
AnswerA

Preemption allows higher-priority LSPs to take resources from lower-priority ones, resolving contention dynamically.

Why this answer

Enabling preemption on RSVP-TE LSPs with appropriate setup and hold priorities allows higher-priority LSPs to tear down lower-priority LSPs to free up bandwidth, resolving resource contention without redesigning the entire TE deployment. This is the standard mechanism defined in RFC 3209 for managing bandwidth contention in MPLS-TE networks.

Exam trap

Cisco often tests the misconception that explicit path configuration or bandwidth upgrades are the primary solutions for resource contention, when in fact preemption priorities are the designed mechanism for dynamic contention resolution in RSVP-TE.

How to eliminate wrong answers

Option B is wrong because increasing the bandwidth of all core links is a costly, non-scalable approach that may not be feasible and does not address the root cause of contention; it also requires a network redesign. Option C is wrong because configuring LSP path-option explicit paths with strict hops forces a specific path but does not resolve bandwidth contention on those links; it may even worsen contention by not allowing dynamic rerouting. Option D is wrong because disabling RSVP-TE and using LDP for label distribution removes traffic engineering capabilities entirely, as LDP does not support bandwidth reservation or explicit path control, which would not solve the resource contention issue.

Practice this question →
141
Multi-Selectmedium

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

Select 2 answers
A.Label swapping is performed at every transit LSR.
B.In PHP, the penultimate hop LSR removes the top label before forwarding to the egress.
C.The ingress LSR performs a swap operation.
D.The egress LSR always performs a pop operation.
E.Label popping always happens at the penultimate hop.
AnswersA, B

Transit LSRs swap the incoming label with an outgoing label.

Why this answer

Penultimate Hop Popping (PHP) removes the label before the final hop, and the ingress LSR pushes the initial label.

Practice this question →
142
MCQeasy

An MPLS core network uses LDP to distribute labels. An engineer wants to verify that the LDP session between two routers is up and exchanging labels. Which command should be used?

A.show mpls interfaces
B.show mpls l2transport binding
C.show mpls ldp neighbor
D.show mpls forwarding-table
AnswerC

Displays LDP neighbors and session state.

Why this answer

The command 'show mpls ldp neighbor' displays the status of LDP sessions, including the neighbor's IP address, session state (e.g., Operational), and label exchange activity. Since LDP is the protocol used to distribute labels in this MPLS core network, this command directly verifies that the session is up and exchanging labels between the two routers.

Exam trap

Cisco often tests the distinction between verifying the LDP session itself (show mpls ldp neighbor) versus verifying the results of label exchange (show mpls forwarding-table), leading candidates to confuse the output of label distribution with the state of the label distribution protocol.

How to eliminate wrong answers

Option A is wrong because 'show mpls interfaces' displays which interfaces are enabled for MPLS and their LDP or TDP status, but it does not show LDP session state or label exchange with a specific neighbor. Option B is wrong because 'show mpls l2transport binding' is used for Layer 2 VPN (L2VPN) pseudowire label bindings, not for verifying LDP session status. Option D is wrong because 'show mpls forwarding-table' displays the LFIB (Label Forwarding Information Base) entries, which are the result of label exchange, but it does not show the LDP session state or neighbor adjacency.

Practice this question →
143
MCQhard

An engineer is troubleshooting MPLS traffic engineering tunnels and needs to verify path calculation and RSVP errors. Which command should be used?

A.show ip route
B.show mpls ldp neighbor
C.show mpls traffic-eng tunnels
D.debug mpls lfib errors
AnswerC

This command provides details on TE tunnels, including path computation and signaling status.

Why this answer

The 'show mpls traffic-eng tunnels' command displays detailed information about MPLS TE tunnels, including path calculation status, RSVP signaling errors, and tunnel state. This command is specifically designed to verify TE tunnel operations and troubleshoot issues such as path computation failures or RSVP resource reservation problems.

Exam trap

Cisco often tests the distinction between MPLS TE-specific commands and general MPLS or routing commands, so the trap here is that candidates might confuse 'show mpls ldp neighbor' (for LDP-based label distribution) with TE tunnel verification, or mistakenly think 'debug mpls lfib errors' is appropriate for RSVP path errors when it actually targets LFIB corruption issues.

How to eliminate wrong answers

Option A is wrong because 'show ip route' displays the IP routing table and is not relevant to MPLS TE tunnel path calculation or RSVP error verification. Option B is wrong because 'show mpls ldp neighbor' shows LDP neighbor relationships, which are used for label distribution in non-TE MPLS, not for TE tunnel path calculation or RSVP signaling. Option D is wrong because 'debug mpls lfib errors' debugs Label Forwarding Information Base (LFIB) errors, which is a debugging tool for label forwarding issues, not for verifying TE tunnel path calculation or RSVP errors.

Practice this question →
144
Multi-Selecthard

Which THREE are valid methods to provide fast convergence in an MPLS/Segment Routing network? (Select three.)

Select 3 answers
A.BGP Prefix Independent Convergence (PIC)
B.Topology-Independent LFA (TI-LFA)
C.Loop-Free Alternate (LFA)
D.RSVP-TE Fast Reroute (FRR)
E.IP Fast Reroute (IPFRR)
AnswersB, C, D

TI-LFA works with SR and provides node/link protection.

Why this answer

B is correct because Topology-Independent LFA (TI-LFA) provides fast convergence in Segment Routing networks by computing a backup path that is guaranteed to be loop-free regardless of the network topology. It leverages the Segment Routing data plane to pre-install a repair path using a segment list, enabling sub-50ms failover without relying on IGP convergence.

Exam trap

Cisco often tests the distinction between IPFRR (which is IP-layer only) and TI-LFA/LFA/RSVP-TE FRR (which are MPLS/Segment Routing-specific), so candidates mistakenly select IPFRR because it sounds similar to LFA, but it does not apply to MPLS/Segment Routing networks.

Practice this question →
145
MCQmedium

A service provider is migrating their MPLS core network to Segment Routing (SR-MPLS). All existing core routers run IOS-XR and are configured with an SRGB of 16000-23999 and OSPF as the IGP. A new router (R5) is added as a PE. The engineer configures R5 with the same SRGB and enables segment-routing under OSPF. However, when configuring an SR-TE policy from R5 to the remote loopback 10.0.0.1 on R1, the policy remains down. Show commands reveal that R5 is not learning the prefix-SID for 10.0.0.1. On R1, the prefix-SID is configured with index 100. The engineer verifies that OSPF adjacencies are up and that R5 can ping 10.0.0.1. What is the most likely cause of the issue?

A.The OSPF process on R5 is not configured with segment-routing mpls.
B.The SR-TE policy on R5 is missing the color attribute.
C.R1 is not configured with the same SRGB as R5.
D.The prefix-SID index 100 is not within the SRGB range of R5 (16000-23999).
AnswerA

Without this command, R5 cannot exchange prefix-SID information via OSPF.

Why this answer

The most likely cause is that the OSPF process on R5 is not configured with the 'segment-routing mpls' subcommand. Without this configuration, OSPF does not advertise or learn prefix-SIDs, so R5 cannot obtain the prefix-SID for 10.0.0.1. Option B is incorrect: while a color attribute is often used in SR-TE policies, the immediate issue is the lack of prefix-SID learning, not the absence of color.

Option C is incorrect: all routers share the same SRGB (16000-23999), so there is no mismatch. Option D is incorrect: index 100 maps to label 16100 (16000+100), which falls within the SRGB range, so this is not the problem.

Practice this question →
146
Multi-Selectmedium

In an MPLS network using RSVP-TE for traffic engineering, which two mechanisms can be used for fast reroute (FRR) protection? (Choose two.)

Select 2 answers
A.Local Protection using segment routing adjacency SID
B.Global Repair using a secondary LSP from headend
C.Facility backup using a bypass tunnel
D.Hot Standby Router Protocol (HSRP)
E.1:1 protection using a dedicated detour LSP
AnswersC, E

Correct: bypass tunnel protects multiple LSPs.

Why this answer

RSVP-TE FRR offers two protection methods: facility backup (bypass tunnel) and 1:1 protection (detour LSP). Global Repair is not a standard term; Local Protection is used in segment routing.

Practice this question →
147
MCQeasy

In MPLS forwarding, what label operation does a transit LSR perform on the top label of a labeled packet?

A.Swap
B.None
C.Pop
D.Push
AnswerA

Swap is the correct operation for a transit LSR.

Why this answer

A transit Label Switch Router (LSR) in an MPLS network receives a labeled packet and must forward it toward the egress LSR. The core operation on the top label is a swap: the incoming label is replaced with an outgoing label learned from the downstream LSR via Label Distribution Protocol (LDP) or other label distribution protocols. This maintains the label-switched path (LSP) and ensures the packet continues along the correct path.

Exam trap

Cisco often tests the distinction between the roles of ingress, transit, and egress LSRs, and the trap here is that candidates confuse the transit LSR's swap operation with the penultimate hop's pop operation or the ingress LSR's push operation.

How to eliminate wrong answers

Option B (None) is wrong because a transit LSR must always perform a label operation on the top label to forward the packet; doing nothing would drop the packet or cause a forwarding loop. Option C (Pop) is wrong because pop (penultimate hop popping, PHP) is performed by the penultimate LSR (the LSR just before the egress), not by a generic transit LSR. Option D (Push) is wrong because push is performed by the ingress LSR when it first imposes a label on an unlabeled packet; a transit LSR receives an already-labeled packet and does not push an additional label.

Practice this question →
148
MCQeasy

An engineer notices that MPLS VPN traffic is taking a suboptimal path because the MPLS label binding for the BGP next-hop loopback is missing. What is the most likely cause?

A.The remote PE is configured with a different VPN ID.
B.The local PE does not have a route to its own loopback.
C.LDP is not enabled on the core-facing interfaces.
D.LDP is not enabled on the PE-CE interface.
AnswerC

LDP on core interfaces is required to propagate labels for the loopback.

Why this answer

MPLS VPN traffic relies on LDP to distribute labels for the BGP next-hop loopback addresses used in the VPNv4 route exchange. If LDP is not enabled on the core-facing interfaces, the local PE cannot obtain a label binding for the remote PE's loopback, causing the traffic to be forwarded via IP (without a label) or taking a suboptimal path. Enabling LDP on all core-facing interfaces resolves the missing label binding and restores optimal label-switched forwarding.

Exam trap

Cisco often tests the distinction that LDP is required on core-facing interfaces for MPLS label distribution, not on PE-CE interfaces, and that the missing label binding is for the remote PE's loopback, not the local PE's route.

How to eliminate wrong answers

Option A is wrong because the VPN ID is used to identify the VRF on the PE and does not affect MPLS label bindings for the BGP next-hop loopback; a mismatch would prevent VPN route import/export, not cause a missing label. Option B is wrong because the local PE must have a route to its own loopback for local operations, but the issue is about the label binding for the remote PE's loopback, not the local one. Option D is wrong because LDP is not required on PE-CE interfaces; it is only needed on core-facing interfaces to exchange labels for loopback routes between PEs.

Practice this question →
149
MCQmedium

In SR-MPLS, when a router receives a packet with an MPLS label corresponding to an Adjacency SID, what operation does it perform?

A.It pops the label and forwards the packet out of the specific interface.
B.It performs a recursive lookup in the FIB to determine the next hop.
C.It pushes an additional label and forwards.
D.It swaps the label with the next-hop label and forwards.
AnswerA

Correct: The label is popped and packet forwarded on the corresponding link.

Why this answer

An Adjacency SID represents a specific link; when a router receives a packet with that label, it pops the label and forwards the packet out of the associated interface, because the Adjacency SID is a 'penultimate hop popping' type of SID.

Practice this question →
150
MCQmedium

In EVPN multi-homing with all-active mode, which mechanism is used to elect a designated forwarder (DF) for broadcast, unknown unicast, and multicast (BUM) traffic on a given Ethernet segment?

A.Load balancing via MLAG
B.ES-IS protocol
C.DF election via ESI label and route type 4
D.MSTP
AnswerC

Route type 4 (Ethernet Segment route) is used to announce ESI and perform DF election.

Why this answer

The DF election is performed using a deterministic algorithm based on the ESI and the list of PEs, with the lowest IP address becoming DF.

Practice this question →
← PreviousPage 2 of 3 · 193 questions totalNext →

Ready to test yourself?

Try a timed practice session using only MPLS and Segment Routing questions.

Start 20-question session