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CCNA Networking Questions

75 of 218 questions · Page 2/3 · Networking · Answers revealed

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76
Multi-Selecteasy

Which THREE of the following are required for successful operation of OSPFv3 in an IPv6 service provider network?

Select 3 answers
A.Interface activation with 'ipv6 ospf <process> area <area>'
B.OSPFv3 process configuration with router-id
C.IPv6 unicast routing enabled globally
D.OSPFv3 authentication configured
E.OSPFv3 LSA type 9 for link-local addresses
AnswersA, B, C

Each interface must be explicitly activated for OSPFv3.

Why this answer

OSPFv3 requires interface-level activation using the 'ipv6 ospf <process> area <area>' command to enable the OSPFv3 process on a specific interface and associate it with an area. Without this, the interface will not participate in OSPFv3 neighbor discovery or routing updates, even if the process is configured globally.

Exam trap

Cisco often tests the misconception that OSPFv3 authentication is mandatory, but it is optional and uses IPsec AH/ESP headers, not the simple authentication mechanisms of OSPFv2.

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77
Multi-Selecthard

Which TWO of the following are valid BGP extended community attributes? (Choose two.)

Select 2 answers
A.Site of Origin (SoO)
B.Route Target (RT)
C.Route Origin (RO)
D.Standard community
E.Large community
AnswersA, B

Correct. Site of Origin (SoO) is a BGP extended community used to prevent routing loops.

Why this answer

Site of Origin (SoO) and Route Target (RT) are well-known BGP extended communities. Route Origin (RO) is a standard community, not an extended community. Standard and Large communities are also not extended communities.

Thus, only SoO and RT are correct among the options, but the question asks for three; note that Color or Bandwidth are other valid extended communities not listed here.

Exam trap

Candidates often confuse Route Origin (standard community) with Route Target (extended community).

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78
Multi-Selecthard

A service provider is deploying IPv6 transition mechanisms. Which three technologies use encapsulation over an IPv4 network? (Choose three.)

Select 3 answers
A.6VPE
B.NAT64
C.MAP-T
D.6PE
E.6rd
AnswersA, D, E

6VPE encapsulates IPv6 VPN traffic over MPLS.

Why this answer

6PE, 6VPE, and 6rd encapsulate IPv6 packets over IPv4; MAP-T uses translation, not encapsulation.

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79
MCQeasy

Which IS-IS level is used for routing within an area?

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

L1 is used for routing within a single area.

Why this answer

IS-IS uses Level 1 (L1) for intra-area routing and Level 2 (L2) for inter-area routing.

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80
Multi-Selectmedium

An SP network is running OSPFv3 for IPv6 and needs to support traffic engineering extensions for RSVP-TE. Which three statements about OSPFv3 TE extensions are true?

Select 3 answers
A.OSPFv3 TE uses TLV 1 (Router Address) in Opaque LSAs.
B.OSPFv3 uses Opaque LSA Type 10 for TE information.
C.OSPFv3 TE requires the use of wide metrics.
D.OSPFv3 TE extensions are defined in RFC 5329.
E.OSPFv3 TE only supports point-to-point links.
AnswersA, B, D

TLV 1 is the Router Address TLV used in OSPF TE.

Why this answer

OSPFv3 uses Opaque LSA types 9, 10, and 11 for TE, with Type 10 carrying TE information link-state. OSPFv3 TE requires the OSPFv3 process to be configured for TE.

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81
MCQmedium

In OSPF Traffic Engineering (TE) extensions for RSVP-TE, which LSA type is used to carry TE information?

A.Type 5 LSA
B.Type 11 LSA
C.Type 9 LSA
D.Type 10 LSA
AnswerD

Type 10 is area-scoped opaque, used for TE.

Why this answer

Opaque LSA Type 10 (area-scoped) is used for OSPF TE, containing sub-TLVs for link attributes.

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82
MCQeasy

In OSPF, which type of area does not allow external routes and only accepts a default route?

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

Stub areas block Type 5 LSAs and use a default route.

Why this answer

A stub area blocks Type 5 LSAs and injects a default route; it does not accept external routes from other areas.

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83
MCQmedium

An SP network is implementing BGP PIC (Prefix Independent Convergence) for fast failover. What is the primary mechanism used by BGP PIC to achieve sub-second convergence?

A.It pre-computes and installs a backup path in the FIB for each prefix
B.It uses route summarization to reduce the number of prefixes
C.It implements BGP multipath to load balance traffic
D.It relies on BFD to detect failures quickly
AnswerA

Correct. BGP PIC installs a backup path so that upon failure, traffic is immediately switched to the backup.

Why this answer

BGP PIC uses a backup path (often a pre-computed alternate path) that is installed in the FIB along with the primary path. Upon failure of the primary path, traffic is switched to the backup path without waiting for BGP convergence.

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84
MCQhard

An SP engineer is designing a BGP-based MPLS L3VPN service. The PE routers are fully meshed via iBGP for VPNv4 routes, and an RR is deployed to reduce sessions. The engineer notices that all PE routers are receiving duplicate routes from different PEs, causing suboptimal path selection. Which BGP feature should be enabled to ensure proper load balancing across multiple equal-cost paths?

A.Enable BGP additional paths on the RR
B.Deploy a second RR to reduce the number of iBGP sessions
C.Configure the BGP best path selection algorithm to ignore interior cost
D.Enable BGP multipath on the PE routers
AnswerD

Enabling BGP multipath on the PE routers allows them to install up to a configured number of equal-cost paths in the routing table. This enables load balancing across the multiple paths, which is the required outcome.

Why this answer

BGP multipath on PE routers enables the router to install multiple equal-cost paths into the routing table, allowing load balancing across them. In an MPLS L3VPN iBGP full mesh with an RR, duplicate routes are received from different PEs; BGP multipath selects and uses up to the configured number of equal-cost paths. Option A (BGP additional paths) allows advertising multiple paths but does not directly install them.

Option B (second RR) reduces sessions but does not affect duplicate routes. Option C (ignoring interior cost) modifies best-path selection but does not enable multipath.

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85
MCQhard

Refer to the exhibit. What is the role of this router for prefix 10.0.1.0/24?

A.Transit LSR.
B.Egress LSR.
C.Penultimate hop.
D.Ingress LSR.
AnswerC

Correct. The 'Pop tag' indicates this router is the penultimate hop, which removes the label before forwarding to the egress LSR.

Why this answer

The router is the penultimate hop for prefix 10.0.1.0/24 because it performs a 'Pop tag' operation (implicit null label). In MPLS, the penultimate hop pops the label before forwarding the unlabeled IP packet to the egress LSR. Therefore, this router is not the egress; it is the penultimate Label Switching Router (LSR).

Exam trap

A common trap is assuming that any router performing a label pop is the egress LSR. In MPLS with implicit null, the penultimate hop performs the pop, and the egress LSR receives an unlabeled IP packet.

How to eliminate wrong answers

Option A is wrong because a transit LSR swaps labels and forwards MPLS packets without removing the top label, but this router pops the label (implicit null), indicating it is not a transit node. Option C is wrong because the penultimate hop is the router immediately upstream that pops the label before forwarding to the egress; here, the router itself performs the pop, so it is the egress, not the penultimate hop. Option D is wrong because an ingress LSR imposes labels on incoming IP packets, but this router is receiving labeled packets and removing the label, not imposing one.

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86
MCQhard

An engineer is troubleshooting MPLS LSP connectivity. The ingress PE router has the label binding for the FEC 10.1.1.0/24, but no LSP is established. Which command should be checked on the P routers to verify the LSP path?

A.show mpls interface
B.show mpls ldp neighbor
C.show mpls forwarding-table
D.show ip route 10.1.1.0
AnswerC

Displays MPLS forwarding entries, revealing label path issues.

Why this answer

The correct command is 'show mpls forwarding-table' because it displays the LFIB (Label Forwarding Information Base) on each P router. Since the ingress PE has a label binding for FEC 10.1.1.0/24 but no LSP is established, the issue likely lies in label propagation or forwarding along the path. Checking the LFIB on P routers reveals whether they have installed the correct outgoing label and interface for that FEC, which is essential for LSP connectivity.

Exam trap

Cisco often tests the distinction between control plane (LDP neighbors, IP routes) and data plane (LFIB) verification, trapping candidates who assume LDP adjacency or IP reachability guarantees a working LSP.

How to eliminate wrong answers

Option A is wrong because 'show mpls interface' only verifies which interfaces are enabled for MPLS processing, not the label bindings or forwarding state for a specific FEC. Option B is wrong because 'show mpls ldp neighbor' checks LDP adjacency status, but if LDP neighbors are up, the problem could still be in label distribution or forwarding, so this command does not directly confirm LSP establishment. Option D is wrong because 'show ip route 10.1.1.0' shows the IP routing table entry, which may be present even if MPLS labels are not correctly propagated or the LSP is broken; MPLS LSPs depend on label switching, not just IP reachability.

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87
Multi-Selecteasy

An SP network is using IS-IS and wants to achieve fast convergence using BFD. Which two statements about BFD in IS-IS are correct?

Select 2 answers
A.BFD requires IS-IS to be configured with wide metrics.
B.BFD can provide sub-second failure detection for IS-IS adjacencies.
C.BFD can be used with both L1 and L2 adjacencies.
D.IS-IS BFD configuration is done under the IS-IS router process.
E.BFD replaces the need for IS-IS hello packets.
AnswersB, C

BFD provides fast detection, typically less than 1 second.

Why this answer

BFD provides fast failure detection (sub-second) and is configured under the interface. IS-IS does not require BFD for normal operation; it's an enhancement.

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88
Multi-Selectmedium

An engineer is configuring VPLS on a Cisco ASR 9000. After verifying the pseudowire status, they notice that the 'pw status' shows 'down' for one of the PWs. Which two conditions could cause this? (Choose two.)

Select 2 answers
A.Mismatched encapsulation type on the pseudowire (e.g., Ethernet vs. VLAN)
B.VPLS ID mismatch on the same bridge domain
C.SNMP MIB not loaded
D.MTU mismatch between local and remote PE
E.LDP session is missing
AnswersA, D

Encapsulation must match between peers.

Why this answer

A mismatched encapsulation type (e.g., Ethernet vs. VLAN) on the pseudowire causes the PW to fail to come up. In VPLS, the encapsulation must match between the local and remote PE for the pseudowire to be operational; otherwise, the PW status will show 'down' due to a negotiation failure.

Exam trap

Cisco often tests the distinction between conditions that cause a pseudowire to be 'down' versus conditions that affect VPLS forwarding but leave the PW 'up', leading candidates to incorrectly select VPLS ID mismatch (Option B) as a cause of PW failure.

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89
MCQmedium

Which BGP attribute is used to influence inbound traffic to an AS by indicating the preferred path into the AS?

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

Correct. MED is used to influence inbound traffic.

Why this answer

The MED (Multi-Exit Discriminator) is used to influence inbound traffic from neighboring ASes. It is advertised to eBGP peers and is compared when multiple paths to the same prefix exist from the same neighboring AS.

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90
MCQmedium

In IS-IS, what is the result of configuring wide metrics on all interfaces?

A.Higher metric values are supported for traffic engineering
B.Only L2 routers can use wide metrics
C.Metrics are limited to 63
D.IS-IS TLVs are backward compatible with narrow metrics
AnswerA

Wide metrics support higher values, essential for TE.

Why this answer

Wide metrics (24-bit or 32-bit) allow values up to 16,777,215 or higher, enabling TE and faster convergence with precise metric adjustments.

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91
MCQmedium

An SP network engineer configures IS-IS with wide metrics. Which statement is true about IS-IS wide metrics compared to narrow metrics?

A.Narrow metrics are required for IPv6 routing in IS-IS.
B.Wide metrics use Type-Length-Value (TLV) extensions to carry traffic engineering information.
C.Wide metrics are only supported in Level 1 routing.
D.Narrow metrics support up to 63 cost values, while wide metrics support up to 255.
AnswerB

Wide metrics use TLV 135 and 236 for TE.

Why this answer

Wide metrics use a 24-bit field, allowing values up to 16,777,215, and support TE extensions and segment routing.

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92
MCQmedium

An engineer is troubleshooting a BGP peering issue between two routers. The peering is established, but routes are not being exchanged. On router R1, 'show bgp neighbors 192.0.2.2' shows the neighbor state as 'Established' but the 'Prefixes received' counter is zero. What is most likely the cause?

A.An outbound route-map on the neighbor is filtering all routes.
B.The 'maximum-prefix' limit is exceeded, causing the session to reset.
C.The BGP session is in the Idle state due to a misconfigured update-source.
D.The 'next-hop-self' command is missing on R1.
AnswerA

An outbound route-map on R2 would filter routes sent to R1, causing zero prefixes received on R1.

Why this answer

The neighbor state is 'Established', confirming that the TCP session and BGP open messages have been successfully exchanged. However, zero prefixes received indicates that R1 is not receiving any routes from the neighbor. An outbound route-map applied on the neighbor (the router sending routes to R1) can filter all prefixes before they are advertised, resulting in zero received prefixes while the session remains up.

Exam trap

Cisco often tests the distinction between session state and route exchange; the trap here is that candidates assume an Established session guarantees route exchange, overlooking outbound filtering on the neighbor side.

How to eliminate wrong answers

Option B is wrong because if the 'maximum-prefix' limit were exceeded, the BGP session would reset or go into an Idle state, not remain Established with zero prefixes received. Option C is wrong because a misconfigured update-source would prevent the BGP session from reaching the Established state entirely; the session would be stuck in Idle or Active. Option D is wrong because the 'next-hop-self' command affects the next-hop attribute of advertised routes, not the reception of prefixes; missing it would not cause zero prefixes received.

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93
MCQmedium

An OSPF network uses point-to-point links. The engineer notices that LSAs are being flooded every 30 minutes even when no topology changes occur. What is the most likely reason?

A.The routers are using LSU packets incorrectly.
B.There is a flapping interface on the network.
C.The dead timer is set too low.
D.The LSA refresh interval has expired.
AnswerD

Correct. OSPF refreshes LSAs every 30 minutes.

Why this answer

In OSPF, LSAs have a default refresh interval of 30 minutes (1800 seconds). When this timer expires, the router that originated the LSA re-floods it to maintain database synchronization, even if no topology changes have occurred. This is normal OSPF behavior defined in RFC 2328, and it explains why LSAs are flooded every 30 minutes.

Exam trap

Cisco often tests the distinction between event-driven LSA flooding (due to topology changes) and periodic LSA refresh (due to the 30-minute timer), leading candidates to incorrectly attribute the behavior to a network issue like flapping interfaces or misconfigured timers.

How to eliminate wrong answers

Option A is wrong because LSU (Link State Update) packets are the correct mechanism for flooding LSAs; using them every 30 minutes for refresh is proper behavior, not incorrect usage. Option B is wrong because a flapping interface would cause frequent, irregular LSA updates (every few seconds or minutes), not a predictable 30-minute cycle. Option C is wrong because the dead timer (default 40 seconds on broadcast links) controls how long a neighbor is considered dead without receiving Hello packets; it does not trigger LSA flooding.

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94
MCQmedium

A service provider is deploying segment routing in their MPLS core. They want to use an IGP as the control plane for label distribution without running LDP or RSVP-TE. Which IGP is best suited for this purpose?

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

IS-IS supports segment routing extensions and is widely used in SP cores.

Why this answer

IS-IS is the best-suited IGP for segment routing in an MPLS core because it natively supports the Segment Routing (SR) extensions defined in RFC 8667. These extensions allow IS-IS to advertise Prefix-SIDs and Adjacency-SIDs directly within the link-state database, enabling label distribution without requiring LDP or RSVP-TE. This makes IS-IS a natural fit for service providers deploying SR-MPLS.

Exam trap

The trap here is that candidates may think OSPF is also a valid choice, but the question specifically asks for the IGP best suited for segment routing without LDP or RSVP-TE, and while OSPF does support SR (RFC 8665), IS-IS is historically more common in service provider cores due to its native support for CLNS and easier migration from LDP to SR.

How to eliminate wrong answers

Option A is wrong because BGP is not an IGP; it is an EGP used for inter-domain routing and, while it can carry SR policies via BGP-LS or BGP SR-TE, it does not function as the IGP control plane for label distribution within a single IGP domain. Option B is wrong because EIGRP is a Cisco-proprietary distance-vector protocol that does not support segment routing extensions; it relies on its own RIB-based label distribution and is not standardized for SR-MPLS. Option D is wrong because RIP is a legacy distance-vector protocol that lacks any support for MPLS or segment routing, and it cannot distribute labels or SIDs.

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95
Multi-Selectmedium

Which TWO tasks are required when implementing segment routing in an MPLS network?

Select 2 answers
A.Configure RSVP-TE to establish LSPs
B.Enable CEF on all routers
C.Configure the IGP (OSPF or IS-IS) with segment routing extensions
D.Enable MPLS on all interfaces that participate in segment routing forwarding
E.Enable LDP on all routers and interfaces
AnswersC, D

IGP must be configured to support segment routing and advertise labels (prefix-SIDs).

Why this answer

Options C and D are correct. Configuring the IGP (OSPF or IS-IS) with segment routing extensions is essential to advertise prefix-SIDs and adjacency-SIDs, enabling segment routing control plane. Enabling MPLS on all interfaces that participate in segment routing forwarding is required to enable MPLS forwarding for the SIDs.

Option A (RSVP-TE) is not required because segment routing uses source-routed paths instead of RSVP-TE LSPs. Option B (CEF) is generally enabled by default on Cisco routers and is a prerequisite for MPLS forwarding, but it is not a task specific to implementing segment routing. Option E (LDP) is not required because segment routing replaces LDP with SID-based label distribution.

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96
Multi-Selecthard

An SP engineer configures IS-IS fast convergence mechanisms. Which three techniques help achieve sub-second convergence in IS-IS?

Select 3 answers
A.Incremental SPF (iSPF)
B.Bidirectional Forwarding Detection (BFD)
C.Partial Route Computation (PRC)
D.Full SPF recalculation
E.LDP label distribution
AnswersA, B, C

iSPF reduces SPF recomputation.

Why this answer

BFD provides fast failure detection, PRC recalculates only affected routes, and SPF incremental (iSPF) reduces SPF computation. LFA provides fast reroute.

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97
Multi-Selecteasy

Which TWO statements correctly describe differences between PIM dense mode and PIM sparse mode? (Choose two.)

Select 2 answers
A.PIM-DM supports the use of a bootstrap router (BSR) for RP discovery.
B.PIM-SM is more bandwidth efficient for high density groups.
C.PIM-DM uses explicit join messages, while PIM-SM uses flood and prune.
D.PIM-DM assumes all downstream routers want to receive multicast traffic, so it initially floods traffic.
E.PIM-SM requires a Rendezvous Point (RP) to facilitate group membership.
AnswersD, E

Dense mode floods to all interfaces and then prunes where not wanted.

Why this answer

PIM-DM operates on the assumption that every downstream router wants to receive multicast traffic for a given group. It initially floods multicast traffic out all PIM-enabled interfaces, then prunes back branches that do not have interested receivers. This flood-and-prune behavior is fundamental to PIM-DM's design for dense multicast environments.

Exam trap

Cisco often tests the reversal of PIM-DM and PIM-SM behaviors, specifically confusing which mode uses explicit joins versus flood-and-prune, and which mode requires an RP.

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98
Multi-Selectmedium

A service provider is implementing BGP route reflectors to scale iBGP. Which THREE mechanisms are used to prevent routing loops in a route reflector topology? (Choose three.)

Select 3 answers
A.MED comparison
B.CLUSTER_LIST
C.ORIGINATOR_ID
D.AS_PATH loop detection
E.NEXT_HOP loop detection
AnswersB, C, D

Correct. CLUSTER_LIST tracks clusters to prevent loops.

Why this answer

BGP route reflectors use several attributes to prevent loops: CLUSTER_LIST (cluster-id list), ORIGINATOR_ID (original router's RID), and the standard AS_PATH loop detection. Also, the route reflector does not reflect routes back to the client that sent it.

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99
MCQhard

A service provider is deploying segment routing (SR) with MPLS data plane. The network uses OSPF as the IGP. Which configuration is required to enable SR-MPLS and ensure that routers advertise prefix-SIDs for their loopback interfaces?

A.Configure 'mpls ip' on all interfaces and 'router ospf 1 mpls ldp auto-config'.
B.Configure 'label mode per-prefix' under the OSPF process and assign a label to the loopback.
C.Configure 'segment-routing global-block 16000 23999' globally and enable 'mpls ip' on all interfaces.
D.Configure 'segment-routing mpls' globally and assign a prefix-SID to the loopback interface under OSPF.
AnswerD

This enables SR-MPLS and advertises a prefix-SID for the loopback.

Why this answer

To enable SR-MPLS with OSPF, you must globally enable segment routing with the 'segment-routing mpls' command, and then assign a prefix-SID to the loopback interface under the OSPF process using the 'prefix-sid' command. This ensures that routers advertise the prefix-SID for their loopback via OSPF extensions, which is the fundamental requirement for SR-MPLS operation without LDP.

Exam trap

Cisco often tests the distinction between enabling segment routing globally versus configuring the SRGB; candidates mistakenly think that setting the SRGB alone enables SR-MPLS, but the 'segment-routing mpls' command is the actual enabler.

How to eliminate wrong answers

Option A is wrong because it enables MPLS LDP via auto-config, which is not required for SR-MPLS and actually introduces a different label distribution protocol (LDP) that conflicts with the segment routing paradigm. Option B is wrong because 'label mode per-prefix' is a command used for MPLS LDP label allocation, not for SR-MPLS prefix-SID assignment; SR-MPLS uses the 'prefix-sid' command under OSPF, not label mode configuration. Option C is wrong because while configuring the segment-routing global block (SRGB) is important for SR-MPLS, it does not enable segment routing itself; the 'segment-routing mpls' global command is mandatory, and 'mpls ip' on interfaces is not required for SR-MPLS as it relies on IGP extensions, not LDP.

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100
MCQmedium

An SP is deploying IS-IS fast convergence. Which mechanism triggers an immediate SPF computation when a neighbor fails?

A.Partial Route Computation (PRC)
B.Event-driven SPF
C.IS-IS Hello multiplier
D.BFD (Bidirectional Forwarding Detection)
AnswerD

BFD detects failures quickly and triggers convergence.

Why this answer

BFD (Bidirectional Forwarding Detection) provides sub-second failure detection and triggers SPF/PRC.

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101
MCQmedium

A network operator wants to prefer a specific BGP route from a peer for a prefix. After applying a route-map to set local preference to 200, the route is still not preferred over a route from another peer with local preference 150. What could be the issue?

A.The prefix was received with a higher weight.
B.The neighbor address-family is not correct.
C.The route-map was applied on the wrong BGP neighbor direction.
D.The route is received via an IBGP session.
AnswerA

Correct. Weight is considered before local preference, so a higher weight overrides a lower local preference.

Why this answer

BGP uses multiple attributes in a specific order of precedence when selecting the best path. Weight (Cisco-proprietary) is checked first, before local preference. If the peer's route has a higher weight (e.g., 32768 for locally injected routes or a manually set value), it will be preferred over a route with a local preference of 200, even if the other route's local preference is only 150.

Exam trap

Cisco often tests the BGP best-path selection order, specifically that weight is evaluated before local preference, leading candidates to mistakenly believe local preference is the highest-priority attribute.

How to eliminate wrong answers

Option B is wrong because an incorrect neighbor address-family would prevent the route from being received or installed at all, not cause a preference issue with a specific attribute. Option C is wrong because applying the route-map on the wrong direction (e.g., inbound vs outbound) would affect whether the local preference is set on the received route; however, the question states the route-map was applied and the local preference was set to 200, so direction is not the issue. Option D is wrong because receiving the route via an IBGP session does not inherently override local preference; IBGP routes can have local preference set, and the BGP best-path selection process still follows the same attribute order (weight first).

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102
MCQeasy

A router is receiving a BGP prefix with community 100:100. The operator wants to modify the local preference to 200 for this prefix. Which configuration will achieve this?

A.route-map SET_LP permit 10, match community 100, set local-preference 200
B.route-map SET_LP permit 10, match ip address prefix-list, set weight 200
C.ip bgp-community new-format, route-map SET_LP permit 10, match community 100, set community 200
D.route-map SET_LP permit 10, match community 100, set metric 200
AnswerA

Correct. This sets local preference as desired.

Why this answer

The route-map uses `match community 100` to identify the BGP community value 100:100 (in Cisco's default format, the community is matched without the colon), and `set local-preference 200` modifies the local preference for that prefix to 200. This is the standard method to influence inbound BGP path selection by adjusting local preference based on community matching.

Exam trap

Cisco often tests the distinction between BGP attributes (local preference, weight, MED, community) and their respective `set` commands, leading candidates to confuse `set weight` or `set metric` with `set local-preference` when the goal is to influence path selection within the AS.

How to eliminate wrong answers

Option B is wrong because it uses `set weight 200` instead of `set local-preference 200`; weight is a Cisco-proprietary attribute that is only locally significant and not exchanged between BGP peers, whereas the question explicitly requires modifying local preference. Option C is wrong because `set community 200` changes the community value itself, not the local preference; this would replace the community 100:100 with a new community, failing to achieve the goal of setting local preference to 200. Option D is wrong because `set metric 200` modifies the MED (Multi-Exit Discriminator) attribute, which influences inbound path selection from a different AS, not the local preference; MED is used for different scenarios and does not affect local preference.

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103
MCQhard

An SP network uses MPLS to provide IPv6 connectivity over an IPv4 backbone using 6PE. Which BGP extension is required on the provider edge routers to exchange IPv6 prefixes with an IPv4 next-hop?

A.BGP multiprotocol extensions (MP-BGP) with IPv6 address family
B.BGP route reflection
C.BGP confederation
D.BGP prefix-independent convergence (PIC)
AnswerA

MP-BGP with IPv6 unicast address family allows carrying IPv6 routes with IPv4 next-hop.

Why this answer

6PE uses BGP multiprotocol extensions (MP-BGP) with an AFI of 2 (IPv6) and SAFI of 1 (unicast), and the next-hop is encoded as an IPv4 address using the 'next-hop' attribute.

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104
MCQmedium

In BGP confederation, which attribute is used to prevent loops between sub-ASes?

A.Confederation AS_PATH
B.CLUSTER_LIST
C.AS_PATH
D.ORIGINATOR_ID
AnswerA

Confederation AS_PATH prevents loops within confederation.

Why this answer

The confederation AS_PATH (AS_CONFED_SEQUENCE and AS_CONFED_SET) is used to track the path within the confederation, and loop detection checks this attribute.

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105
Multi-Selectmedium

An ISP is designing a new core network using MPLS-TE. They require very fast failure detection for link and node failures to minimize traffic loss. Which two technologies should they combine? (Choose two.)

Select 2 answers
A.OSPF with hello timers set to 1 second
B.MPLS-TE Fast Reroute (FRR)
C.Bidirectional Forwarding Detection (BFD)
D.LSP Ping and Traceroute
E.Label Distribution Protocol (LDP)
AnswersB, C

Provides local repair via backup tunnels.

Why this answer

MPLS-TE Fast Reroute (FRR) provides sub-50 ms protection switching by pre-computing backup paths (bypass tunnels) for link and node failures, ensuring minimal traffic loss. Bidirectional Forwarding Detection (BFD) provides sub-second failure detection by sending rapid, lightweight hello packets independent of routing protocols, enabling faster convergence when combined with FRR.

Exam trap

Cisco often tests the misconception that OSPF or IS-IS fast hello timers alone can achieve sub-second failure detection, but BFD is required for truly fast detection independent of routing protocol convergence.

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106
MCQmedium

In BGP, which attribute is used to prevent routing loops when route reflectors are deployed?

A.Cluster ID
B.AS_PATH
C.Originator ID
D.NEXT_HOP
AnswerC

Originator ID identifies the originator of a route within an AS.

Why this answer

Originator ID is used by route reflectors to prevent loops. If a router receives a route with its own router ID as originator, it discards it.

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107
MCQhard

An MPLS-TE tunnel is configured with Fast Reroute using link protection. The primary path traverses links A-B and B-C. If link B-C fails, which action does the head-end router take?

A.The router at node B switches traffic to a pre-computed backup tunnel around link B-C.
B.The head-end router immediately switches to a secondary explicit path.
C.RSVP-TE signals a new LSP from the head-end after detecting the failure.
D.Traffic is dropped until the IGP converges on the new topology.
AnswerA

Link protection works by having the PLR (point of local repair) at the upstream node.

Why this answer

MPLS-TE Fast Reroute (FRR) with link protection pre-computes a backup tunnel that bypasses the protected link. When link B-C fails, the router at node B (the Point of Local Repair, or PLR) immediately switches traffic to this pre-established backup tunnel, ensuring sub-50ms failover without involving the head-end router.

Exam trap

Cisco often tests the misconception that the head-end router handles all rerouting decisions in MPLS-TE, but FRR delegates local repair to the PLR, so candidates must remember that link protection is handled at the point of failure, not the head-end.

How to eliminate wrong answers

Option B is wrong because the head-end router does not immediately switch to a secondary explicit path; FRR is designed for local repair at the PLR, and head-end path switching would be slower and is not triggered by link protection. Option C is wrong because RSVP-TE does not signal a new LSP from the head-end upon failure detection; FRR uses pre-signaled backup tunnels, and new LSP signaling would exceed the sub-50ms recovery target. Option D is wrong because traffic is not dropped until IGP convergence; FRR provides fast reroute before IGP reconverges, preventing packet loss.

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108
MCQhard

In OSPFv3 (for IPv6), which of the following statements is true regarding OSPFv3 packet format and authentication?

A.OSPFv3 relies on IPsec for authentication and encryption.
B.OSPFv3 does not support authentication.
C.OSPFv3 authenticates using the Authentication field in the OSPF header.
D.OSPFv3 uses the same authentication mechanisms as OSPFv2.
AnswerA

OSPFv3 uses IPsec AH or ESP for authentication and confidentiality.

Why this answer

OSPFv3 uses IPsec for authentication and does not include authentication fields in the OSPF header. It runs per-link instead of per-subnet.

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109
MCQeasy

A service provider is experiencing high CPU usage on a router running BGP. Which action should be taken first to mitigate the issue without disrupting traffic?

A.Disable BGP on the router.
B.Increase the BGP keepalive timer.
C.Apply inbound route filtering using prefix lists.
D.Implement BGP route dampening.
AnswerC

Filtering unnecessary inbound routes reduces the number of prefixes the router must process, lowering CPU utilization.

Why this answer

High CPU usage on a BGP-speaking router is often caused by processing a large number of BGP updates. Applying inbound route filtering using prefix lists (option C) reduces the number of routes the router must process and store, directly lowering CPU load without disrupting existing traffic flows. This is a non-disruptive, targeted mitigation that addresses the root cause of excessive route processing.

Exam trap

Cisco often tests the misconception that route dampening is a first-line CPU mitigation tool, when in fact it is a stability mechanism for flapping routes and can itself be CPU-intensive; the correct first step is to filter unwanted routes at the point of entry.

How to eliminate wrong answers

Option A is wrong because disabling BGP entirely would drop all BGP sessions and disrupt traffic, which violates the requirement to not disrupt traffic. Option B is wrong because increasing the BGP keepalive timer reduces the frequency of keepalive messages but does not address the CPU load caused by processing BGP updates or route churn; it may even delay failure detection. Option D is wrong because BGP route dampening is designed to suppress flapping routes over time, not to reduce immediate CPU usage from a high volume of updates; it can actually increase CPU load during the dampening calculation phase and does not filter routes.

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110
MCQmedium

A service provider is deploying MPLS L3VPN to connect multiple customer sites. The PE router receives a route from a CE router via MP-BGP. Which attribute must the PE router add to the route before advertising it to the route reflector?

A.MPLS label
B.Route distinguisher (RD)
C.Route target (RT)
D.IGP metric
AnswerB

RD makes the customer prefix unique across the MPLS VPN backbone.

Why this answer

When a PE router receives a customer route from a CE router via MP-BGP, it must first make the route unique within the MPLS VPN context by adding a Route Distinguisher (RD). The RD prepended to the IPv4 prefix creates a VPNv4 address (per RFC 4364), which is the format required for advertisement to a route reflector. Without the RD, the route would not be distinguishable from other customers' overlapping IP prefixes in the BGP table.

Exam trap

Cisco often tests the distinction between RD and RT, where candidates mistakenly think RT is required for route advertisement to the route reflector, but the RD is the attribute that makes the route globally unique in the VPNv4 address family.

How to eliminate wrong answers

Option A is wrong because the MPLS label is added by the PE router during label allocation for the VPN route, but it is not an attribute added before advertising to the route reflector; the label is part of the NLRI in the MP-BGP update, not a separate attribute. Option C is wrong because the Route Target (RT) controls route import/export between VRF instances and is attached to the route, but it is not required for the route to be advertised to the route reflector; the RD is the mandatory attribute for VPNv4 address uniqueness. Option D is wrong because the IGP metric is a routing metric used within the IGP (e.g., OSPF or IS-IS) and is not added by the PE router to MP-BGP VPNv4 updates; BGP uses MED or local preference for path selection, not IGP metrics.

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111
Multi-Selecteasy

Which TWO MPLS VPN features are used to provide Layer 3 VPN services in a service provider network?

Select 2 answers
A.LDP
B.VRF
C.MPLS labels
D.RSVP-TE
E.MP-BGP
AnswersC, E

MPLS labels are used to encapsulate and forward VPN traffic across the provider core.

Why this answer

MP-BGP (Option E) is used to exchange VPNv4 routes between Provider Edge (PE) routers, carrying both the route and its corresponding MPLS label. MPLS labels (Option C) are used to forward packets across the provider core via label-switched paths (LSPs), enabling the separation of customer traffic. Together, MP-BGP and MPLS labels form the foundation of Layer 3 MPLS VPN services.

Exam trap

Cisco often tests the distinction between the control plane protocols (MP-BGP) and data plane mechanisms (MPLS labels) versus supporting protocols like LDP or RSVP-TE, leading candidates to mistakenly include LDP as a VPN service feature.

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112
MCQeasy

Which statement about the use of MTU in an MPLS network is correct?

A.MPLS adds a label stack to packets, reducing the payload MTU.
B.MPLS does not affect the MTU because labels are part of the header.
C.MTU must be increased on all MPLS interfaces.
D.MPLS eliminates the need for IP fragmentation.
AnswerA

The label overhead reduces available MTU for data.

Why this answer

MPLS adds a label stack (typically 4 bytes per label) between the Layer 2 header and the IP packet. This additional overhead reduces the maximum payload size that can be carried without exceeding the interface MTU, effectively lowering the payload MTU available for the IP packet.

Exam trap

The trap here is that candidates often assume MPLS labels are part of the IP header (like an option) or that MPLS inherently handles fragmentation, when in fact the label is a separate shim that increases frame size and can cause MTU-related drops.

How to eliminate wrong answers

Option B is wrong because MPLS labels are inserted as a shim header between the Layer 2 header and the IP packet, not as part of the IP header; this increases the total frame size, which can cause fragmentation or drop if the MTU is not adjusted. Option C is wrong because MTU does not need to be increased on all MPLS interfaces; only the core-facing interfaces may require an MTU increase (e.g., to 1504 bytes) to accommodate the label stack, while edge interfaces often remain at standard MTU. Option D is wrong because MPLS does not eliminate the need for IP fragmentation; if the packet plus label stack exceeds the path MTU, fragmentation may still occur at the IP layer or the packet may be dropped if the DF bit is set.

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113
MCQmedium

In BGP route selection, if two routes have the same weight, local preference, and are both locally originated, which attribute is considered next?

A.ORIGIN code
B.NEXT_HOP metric
C.AS_PATH length
D.MED
AnswerC

Shorter AS_PATH is preferred.

Why this answer

After weight, local pref, and locally originated, BGP compares AS_PATH length (shorter is preferred).

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114
MCQhard

An ISP is implementing BGP PIC (Prefix Independent Convergence) for edge routers. Which BGP feature must be enabled to support PIC for core-facing failures?

A.BGP additional-paths install
B.BGP deterministic-med
C.BGP route refresh
D.BGP bestpath as-path multipath-relax
AnswerA

This allows installation of backup paths for PIC.

Why this answer

BGP PIC relies on installing a backup path in the FIB. For core-facing failures, BGP Fast External Fallover and BGP PIC with backup paths are used; specifically, 'bgp additional-paths install' is needed to install backup paths.

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115
MCQhard

An engineer is configuring an MPLS L3VPN and needs to ensure that the PE router installs VPNv4 routes from a remote PE into the VRF of a customer. The remote PE sends a VPNv4 route with route-target 100:1. Which configuration on the local PE causes the route to be imported into the VRF?

A.router bgp 100 address-family ipv4 vrf CUSTOMER route-target import 100:1
B.vrf definition CUSTOMER rd 100:1 route-target both 100:1 route-map IMPORT
C.vrf definition CUSTOMER rd 100:1 route-target import 100:1
D.vrf definition CUSTOMER rd 100:1 route-target export 100:1
AnswerC

This imports routes with RT 100:1 into the VRF.

Why this answer

The `route-target import 100:1` command under the VRF definition configures the local PE to accept VPNv4 routes that carry the specified route-target (100:1) from the remote PE. This import RT must match the export RT of the remote PE for the route to be installed into the VRF's routing table. The `rd 100:1` defines the route distinguisher, which is separate from the RT and ensures uniqueness of the VPNv4 prefix.

Exam trap

Cisco often tests the distinction between `route-target import` and `route-target export`, and the trap here is that candidates may select the export-only option (D) or misplace the RT command under BGP (A), failing to recognize that import must be explicitly configured under the VRF definition to receive routes from a remote PE.

How to eliminate wrong answers

Option A is wrong because the `route-target import 100:1` command is placed under `address-family ipv4 vrf CUSTOMER` within BGP, which is not a valid configuration; route-target import/export is configured under the VRF definition, not under the BGP address-family for the VRF. Option B is wrong because it uses the `route-target both 100:1 route-map IMPORT` syntax; while `route-target both` is valid, appending a route-map to the import/export RT statement is not supported in standard IOS/IOS-XE — route-maps can only be applied to `import` or `export` individually, not to `both`, and the syntax is incorrect. Option D is wrong because `route-target export 100:1` only configures the local PE to attach that RT to outgoing VPNv4 routes; it does not cause the import of incoming routes from the remote PE, which requires the `import` keyword.

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116
MCQhard

An SP is deploying IS-IS fast convergence using Partial Route Computation (PRC). Under which condition does PRC occur?

A.When the IS-IS metric changes
B.When the system ID changes
C.When a new adjacency is formed
D.When a prefix is added or removed without topology change
AnswerD

PRC handles prefix changes only.

Why this answer

PRC is triggered when a prefix changes but the topology remains unchanged. It recalculates only the affected prefixes, not the entire SPF tree.

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117
Multi-Selectmedium

A service provider is configuring BGP route reflectors to scale iBGP. Which two mechanisms are used to prevent routing loops in a route reflector environment? (Choose two.)

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

Correct. CLUSTER_LIST contains the cluster-ids and prevents loops.

Why this answer

BGP route reflectors use the CLUSTER_LIST attribute (which includes the cluster-id) and the ORIGINATOR_ID attribute to prevent loops. CLUSTER_LIST lists the cluster-ids that a route has passed through, and ORIGINATOR_ID identifies the originator of the route to prevent it from being reflected back.

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118
MCQmedium

Which BGP attribute is used to prefer a specific entry point into an AS when multiple ASBRs are advertising the same prefix to a neighboring AS?

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

MED is exchanged between ASes to influence inbound traffic.

Why this answer

MED (Multi-Exit Discriminator) is used to influence the inbound traffic by suggesting the preferred entry point.

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119
Multi-Selectmedium

A service provider is designing an OSPF network with multiple areas. Which TWO area types allow external routes (Type 5 LSAs) to be injected into the area? (Choose two.)

Select 2 answers
A.NSSA
B.Standard area
C.Stub area
D.Backbone area (Area 0)
E.Totally stubby area
AnswersB, D

Standard area (non-stub) accepts Type 5 LSAs.

Why this answer

Standard areas and the backbone area (Area 0) allow Type 5 LSAs. Stub, totally stubby, and NSSA do not accept Type 5 LSAs (NSSA converts Type 7 to Type 5 at ABR, but internally uses Type 7).

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120
MCQmedium

An engineer is designing an MPLS L3VPN for a customer with multiple sites. The customer requires overlapping IP addresses between sites. Which method allows the provider to support overlapping customer addresses?

A.Implement VPLS instead of L3VPN.
B.Use separate VRF per site with route distinguisher.
C.Use BGP communities to control route distribution.
D.Use the same VRF for all sites with different route targets.
AnswerB

Each VRF has its own routing table, and the RD makes routes globally unique even with overlapping IPs.

Why this answer

B is correct because a separate VRF per site with a unique route distinguisher (RD) allows the provider to maintain isolated routing tables for each customer site. This isolation enables overlapping IP addresses between sites, as each VRF treats its prefixes as unique within the MPLS L3VPN backbone, regardless of address duplication.

Exam trap

Cisco often tests the misconception that route targets alone solve overlapping address issues, but the trap here is that route targets control route propagation, not address uniqueness—only the route distinguisher (RD) within a VRF provides the necessary prefix uniqueness.

How to eliminate wrong answers

Option A is wrong because VPLS is a Layer 2 VPN technology that provides Ethernet multipoint connectivity, not IP routing; it does not inherently support overlapping IP addresses without additional mechanisms like VLAN segmentation. Option C is wrong because BGP communities control route distribution and policy (e.g., filtering or preference), but they do not create separate routing tables or address space isolation required for overlapping IPs. Option D is wrong because using the same VRF for all sites with different route targets would merge routes into a single routing table, causing conflicts with overlapping addresses; route targets control import/export policies, not address uniqueness.

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121
MCQeasy

A service provider wants to ensure that customer traffic is not impacted during a planned maintenance on a core LSR in an MPLS network. Which MPLS feature should be used?

A.MPLS TE Fast Reroute
B.MPLS TTL propagation
C.MPLS LDP synchronization
D.MPLS OAM
AnswerA

FRR provides sub-50ms protection by pre-computing backup paths.

Why this answer

MPLS TE Fast Reroute (FRR) is the correct feature because it provides local protection against link or node failures by pre-computing backup paths (bypass tunnels) that are activated within 50 milliseconds of a failure. This ensures that customer traffic is not impacted during planned maintenance on a core LSR, as the backup path is already in place and can be triggered by a manual administrative action (e.g., shutting down the interface) to seamlessly redirect traffic before the maintenance begins.

Exam trap

The trap here is that candidates often confuse MPLS TE FRR with MPLS LDP synchronization or MPLS OAM, mistakenly thinking that any 'protection' or 'monitoring' feature can handle planned maintenance, when only FRR provides the sub-50 ms local repair capability required for hitless maintenance.

How to eliminate wrong answers

Option B (MPLS TTL propagation) is wrong because it controls how the TTL field is copied between the IP and MPLS headers for traceroute and hop-count visibility, and it has no role in traffic protection or maintenance scenarios. Option C (MPLS LDP synchronization) is wrong because it ensures that IGP and LDP are synchronized to prevent black-holing during link restoration, but it does not provide fast local protection or pre-computed backup paths for planned maintenance. Option D (MPLS OAM) is wrong because it is a set of tools for fault detection, connectivity verification, and performance monitoring (e.g., LSP ping/traceroute, VCCV), not a mechanism to reroute traffic during maintenance.

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122
MCQhard

An engineer is troubleshooting BGP convergence. The router has multiple paths for a prefix, but it selects a path with a lower local preference over a path with a higher local preference. The higher local preference path is from an EBGP peer. What could cause this?

A.The router is configured with 'bgp deterministic-med'.
B.The path with lower local preference has a higher weight.
C.The path with lower local preference has a higher router ID.
D.The path with lower local preference has a lower MED.
AnswerB

Correct. Weight is checked before local preference.

Why this answer

B is correct because BGP selects the path with the highest weight first, before considering local preference. Weight is a Cisco-proprietary attribute that is local to the router and takes precedence over all other BGP attributes. Therefore, a path with a lower local preference but a higher weight will be preferred over a path with a higher local preference but a lower weight, even if the higher local preference path comes from an EBGP peer.

Exam trap

Cisco often tests the BGP path selection order by presenting a scenario where a lower-priority attribute (like local preference) is overridden by a higher-priority attribute (like weight), and candidates mistakenly assume that local preference is always the deciding factor because it is the first global attribute in the algorithm.

How to eliminate wrong answers

Option A is wrong because 'bgp deterministic-med' ensures that MED is compared only among paths from the same neighboring AS, but it does not override the weight or local preference comparison order; it affects MED handling, not the local preference selection issue described. Option C is wrong because router ID is used as a tie-breaker only after comparing weight, local preference, AS-path length, origin code, and MED; it cannot override a higher local preference unless all higher-priority attributes are equal. Option D is wrong because MED is compared after local preference in the BGP best-path selection algorithm; a lower MED would only matter if local preference were equal, so it cannot cause a path with lower local preference to be selected over one with higher local preference.

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123
MCQhard

In a VXLAN EVPN deployment, a host sends a broadcast ARP request. Which component in the fabric is responsible for replying on behalf of the target host to reduce flooding?

A.The VTEP that receives the broadcast
B.The spine switch
C.The VTEP that has the target host's MAC address in its local table (ARP suppression)
D.The default gateway (anycast IP)
AnswerC

ARP suppression allows VTEP to proxy-reply.

Why this answer

In VXLAN EVPN, ARP suppression is a feature implemented on the ingress VTEP (the VTEP that receives the broadcast ARP request). The ingress VTEP maintains a local ARP/ND cache populated via EVPN Type-2 routes (MAC/IP advertisement routes). When a broadcast ARP request arrives, the ingress VTEP checks its local cache for the target IP; if found, it replies directly on behalf of the target host, suppressing the broadcast and preventing unnecessary flooding across the fabric.

Option C correctly identifies this VTEP as the component responsible for the reply.

Exam trap

Cisco often tests the misconception that the spine switch or the default gateway handles ARP suppression, when in fact it is the ingress VTEP that performs this function using its locally cached EVPN-learned MAC/IP entries.

How to eliminate wrong answers

Option A is wrong because the VTEP that receives the broadcast is the ingress VTEP, which performs ARP suppression only if it has the target host's MAC address in its local table; it does not automatically reply simply because it received the broadcast. Option B is wrong because spine switches in a VXLAN EVPN fabric operate as pure IP underlay routers (typically running an IGP like OSPF or IS-IS) and do not participate in the overlay control plane or maintain ARP caches for tenant hosts. Option D is wrong because the default gateway (anycast IP) is used for routing traffic between subnets, not for replying to intra-subnet ARP requests; ARP suppression is a function of the VTEP, not the gateway.

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124
MCQhard

An SP is deploying 6VPE (IPv6 L3VPN). Which BGP extensions are used to carry IPv6 VPN prefixes?

A.AFI=1 (IPv4), SAFI=128 with IPv6 next-hop.
B.AFI=2, SAFI=1 (unicast) with MPLS labels.
C.AFI=2 (IPv6), SAFI=128 with IPv4 next-hop encoded as IPv6-mapped.
D.AFI=1, SAFI=128 with IPv6 VPN prefixes.
AnswerC

Correct. 6VPE uses AFI=2, SAFI=128, and the next-hop is an IPv4 address mapped to IPv6.

Why this answer

6VPE uses MP-BGP with the IPv6 VPN AFI (AFI=2) and SAFI=128 (MPLS-labeled VPN). The next-hop is an IPv4 address (the PE's loopback) encoded as a mapped IPv6 address, but the VPNv6 address family carries the prefixes.

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125
MCQmedium

Which BGP attribute is used to influence inbound traffic to an AS by advertising a preferred path to a neighbor AS?

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

MED is used to influence inbound traffic from a neighbor AS.

Why this answer

MED (Multi-Exit Discriminator) is sent to a neighboring AS to suggest the best entry point into the local AS. Lower MED is preferred.

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126
MCQhard

A service provider is experiencing intermittent multicast issues in their core network. They use PIM-SM with a static RP at 10.1.1.1. The multicast traffic originates from a source connected to PE1 and is received by customers connected to PE2. Recently, after a firewall upgrade between the PE routers and the core, some multicast streams stopped working, while others continue. The network team notices that 'show ip mroute' on PE2 shows the (*, G) entry but not the (S, G) entry for the affected groups. The RP is reachable via OSPF. The firewall logs show no dropped packets for known multicast addresses. Which action should the engineer take to restore full multicast forwarding?

A.Increase the PIM register suppression interval on the source's first-hop router
B.Configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic
C.Change the multicast mode from PIM-SM to PIM-DM on all interfaces
D.Enable Auto-RP on the network to dynamically learn the RP
AnswerB

A static RP on PE2 ensures that the source's registration reaches the RP even if the firewall blocks unicast PIM register messages. This allows the (S,G) to be formed.

Why this answer

The firewall upgrade likely blocked PIM register messages from PE1 to the RP, preventing the RP from learning about the source. By configuring a static RP on the customer-facing interface or using a different RP that can receive registration, the (S,G) state can be built. Option A is wrong because PIM-SM is correct for sparse-mode groups.

Option C is wrong because adjusting timers would not fix the absence of (S,G). Option D is wrong because Auto-RP would add complexity and might be blocked by the firewall as well.

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127
MCQhard

A service provider deploys BGP route reflectors in the core. Which mechanism prevents routing loops within the route reflector cluster?

A.Next-hop-self command
B.Originator-id attribute
C.Cluster-list attribute and cluster-id
D.AS_PATH loop detection
AnswerB

Originator-id identifies the originator; if a router receives its own originator-id, it discards the route.

Why this answer

The originator-id attribute is set by the route reflector to the router-id of the originating router, preventing loops by checking if the originator-id matches the local router-id.

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128
MCQhard

A service provider operates a large MPLS VPN network using OSPF as the IGP and LDP for label distribution. The PE routers (PE1, PE2, PE3) are connected to a core of P routers. Recently, a new link was added between P2 and P3. After the link came up, the engineering team noticed that several VPN routes that were previously reachable via PE2 are now being blackholed when traffic is sent from PE1 to those prefixes. The teams verify that the VPNv4 routes are present in the BGP table on PE1 with valid next-hops, but traffic fails. The traceroute from PE1 to the CE behind PE2 stops at P2. The show mpls forwarding-table on P2 shows the correct label for the VPN prefix, but the outgoing interface is null. Which action should the engineer take to resolve the issue without causing additional disruption?

A.Clear LDP neighbor sessions on P2 to re-initiate label exchange.
B.Add a static route on P2 for the BGP next-hop pointing to Null0.
C.Shut down the new link between P2 and P3.
D.Clear BGP sessions on PE1 to force re-advertisement of VPN routes.
AnswerA

Correct. This forces LDP to re-establish and exchange labels, likely resolving the missing label.

Why this answer

The issue is that P2 has a label for the VPN prefix but a null outgoing interface, indicating an LDP label mapping problem. Clearing LDP neighbor sessions on P2 forces re-establishment of LDP sessions and re-exchange of label bindings, which should resolve the missing or incorrect label mapping for the BGP next-hop without disrupting other services.

Exam trap

Cisco often tests the misconception that clearing BGP sessions (Option D) fixes MPLS forwarding issues, but the real problem is at the LDP label distribution layer, not the BGP VPN route advertisement.

How to eliminate wrong answers

Option B is wrong because adding a static route to Null0 for the BGP next-hop would blackhole all traffic to that next-hop, worsening the issue. Option C is wrong because shutting down the new link between P2 and P3 is a disruptive workaround that does not address the root cause (LDP label inconsistency) and may cause routing loops or suboptimal paths. Option D is wrong because clearing BGP sessions on PE1 would only re-advertise VPN routes but does not fix the underlying MPLS label forwarding issue on P2; the VPNv4 routes are already present with valid next-hops in BGP.

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129
MCQhard

A service provider wants to provide IPv6 connectivity over an existing MPLS IPv4 core using BGP extensions. Which technology should be used?

A.6PE
B.NAT64
C.Dual-stack
D.6VPE
AnswerA

6PE uses BGP to carry IPv6 over MPLS with IPv4 next-hop.

Why this answer

6PE (IPv6 Provider Edge) uses BGP to carry IPv6 prefixes with an IPv4 next-hop over an MPLS core, avoiding the need for IPv6 in the core.

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130
MCQeasy

Which OSPF network type elects a Designated Router (DR) and Backup Designated Router (BDR) to reduce the number of adjacencies on a multi-access segment?

A.Point-to-point
B.Loopback
C.Point-to-multipoint
D.Broadcast
AnswerD

Broadcast elects DR and BDR.

Why this answer

Broadcast network type uses DR/BDR election. Point-to-point and NBMA do not elect DR/BDR (NBMA does elect DR/BDR but is less common).

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131
MCQhard

An engineer configures MPLS TE tunnels. After configuration, the tunnel remains down. The 'show mpls traffic-eng tunnels' output shows 'Tunnel is down - path computation failed'. What is the most likely cause?

A.IGP TE extensions are not enabled on the head-end router.
B.RSVP is not enabled on the head-end router.
C.MPLS LDP is not enabled on the head-end router.
D.There is an MTU mismatch along the path.
AnswerA

Correct. Without TE extensions, the router cannot compute a path.

Why this answer

The 'path computation failed' error indicates that the head-end router cannot find a valid path for the MPLS TE tunnel. IGP TE extensions (OSPF TE or IS-IS TE) must be enabled on the head-end router to flood link attributes (bandwidth, TE metrics) and build the TED (Traffic Engineering Database). Without the TED, CSPF (Constrained Shortest Path First) cannot compute a path, leaving the tunnel down.

Exam trap

Cisco often tests the distinction between control-plane failures (path computation, TED) and signaling failures (RSVP, label distribution); the trap here is that candidates assume RSVP is the root cause because MPLS TE relies on RSVP, but the specific error message points to a missing TED, not a signaling issue.

How to eliminate wrong answers

Option B is wrong because RSVP is required for signaling the MPLS TE tunnel (to reserve bandwidth and distribute labels), but the error 'path computation failed' occurs before signaling even begins, so RSVP not being enabled would cause a different failure (e.g., 'RSVP neighbor not reachable'). Option C is wrong because MPLS LDP is used for label distribution in non-TE MPLS networks; MPLS TE uses RSVP-TE for label distribution, not LDP, so LDP being disabled has no impact on TE tunnel path computation. Option D is wrong because an MTU mismatch along the path would cause packet fragmentation or drops, not a path computation failure; path computation is a control-plane function that does not involve MTU checks.

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132
MCQhard

During a network migration from EIGRP to OSPF, you notice that some routes are being redistributed incorrectly, causing routing loops. The OSPF domain uses area 0 and area 1. The EIGRP domain uses AS 100. Which configuration change would best prevent loops during the migration?

A.Implement OSPF stub areas to limit external routes.
B.Use route-maps to tag EIGRP routes and filter them on OSPF routers.
C.Use distribute-list in EIGRP to block OSPF routes.
D.Set a high administrative distance on redistributed routes in OSPF.
AnswerB

Tags allow conditional redistribution filtering, preventing routes from being sent back to EIGRP.

Why this answer

Route-maps allow you to tag redistributed EIGRP routes with a specific tag value (e.g., 'tag 100') and then filter those tagged routes on OSPF routers using a distribute-list in or prefix-list combined with the route-map. This prevents the redistributed routes from being re-injected back into EIGRP, breaking the redistribution loop. Without such tagging and filtering, mutual redistribution between EIGRP and OSPF can cause routing loops due to the two-way redistribution of routes.

Exam trap

Cisco often tests the misconception that simply adjusting administrative distance or using stub areas can prevent redistribution loops, when in fact only explicit tagging and filtering (or route-map-based control) can break the two-way redistribution cycle.

How to eliminate wrong answers

Option A is wrong because OSPF stub areas limit the injection of external routes (Type 5 LSAs) into the area, but they do not prevent redistribution loops between EIGRP and OSPF; loops occur due to mutual redistribution, not the presence of external routes in non-stub areas. Option C is wrong because using a distribute-list in EIGRP to block OSPF routes only prevents OSPF-learned routes from entering the EIGRP domain, but it does not address the reverse direction where EIGRP routes are redistributed into OSPF and then potentially re-redistributed back into EIGRP; a one-way filter is insufficient to break the loop. Option D is wrong because setting a high administrative distance on redistributed routes in OSPF (e.g., to 170) does not prevent the routes from being redistributed back into EIGRP; administrative distance affects route preference within a single routing table, not the redistribution process itself, so loops can still occur.

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133
Multi-Selecteasy

A service provider is deploying IS-IS with wide metrics to support traffic engineering. Which two statements about IS-IS wide metrics are correct?

Select 2 answers
A.Wide metrics are required for segment routing.
B.Wide metrics are incompatible with narrow metrics and require a full migration.
C.Wide metrics use TLV 135 for extended IS reachability.
D.Narrow metrics can only be used with L1 routing.
E.Wide metrics support values up to 16,777,215.
AnswersC, E

TLV 135 carries wide metrics for IS-IS.

Why this answer

Wide metrics use a 24-bit or 32-bit field, allowing values up to 16,777,215 or more, and are backward compatible with narrow metrics via apportioning.

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134
Multi-Selectmedium

Which TWO BGP attributes are considered during the best path selection process and can be used to influence outbound traffic from an AS? (Choose two.)

Select 2 answers
A.LOCAL_PREF
B.NEXT_HOP
C.MED
D.AS_PATH
E.Weight
AnswersA, E

Correct. LOCAL_PREF influences outbound traffic within the AS.

Why this answer

Weight (Cisco proprietary) and LOCAL_PREF are the first two criteria in BGP path selection. They influence outbound traffic by choosing the preferred path for routes learned from multiple sources.

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135
Multi-Selectmedium

Which TWO conditions must be met for a BGP route to be considered valid and used for forwarding?

Select 2 answers
A.The BGP synchronization must be enabled.
B.The AS path must not contain the router's own AS.
C.The route must be the best path selected by BGP.
D.The prefix must be in the BGP table.
E.The next-hop IP must be reachable via an IGP route.
AnswersC, E

Correct. Only the best path is installed in the routing table.

Why this answer

BGP only installs the best path into the routing table for forwarding. BGP selects a single best path from all available paths for a given prefix based on its path selection algorithm (e.g., highest weight, local preference, shortest AS path). Only this best path is considered valid for forwarding, even if multiple paths exist in the BGP table.

Exam trap

Cisco often tests the distinction between a route being in the BGP table (valid) and being installed in the routing table (best path + reachable next hop), leading candidates to mistakenly think that any BGP table entry is automatically used for forwarding.

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136
MCQhard

A service provider uses MP-BGP with IPv6 address family. They notice that routes redistributed from OSPFv3 are not being advertised to iBGP peers. The OSPF routes are internal. What is a likely reason?

A.The network command is missing under IPv6 address family.
B.The bgp default ipv4-unicast command is disabled.
C.The next-hop is not resolved for IPv6.
D.The routes are not in the IPv6 unicast table.
AnswerC

If the BGP next-hop for the redistributed routes is not reachable via the IPv6 routing table, BGP will not advertise them to iBGP peers.

Why this answer

In MP-BGP for IPv6, the next-hop address for iBGP peers must be reachable via an IPv6 route in the global routing table or the appropriate VRF. When OSPFv3 redistributes internal routes into BGP, the next-hop is often set to the OSPFv3 router's own IPv6 address; if that address is not reachable (e.g., because the interface is not in the IPv6 unicast routing table or the next-hop is link-local), iBGP peers will not install the routes. This is a common cause of routes being learned but not advertised to iBGP peers.

Exam trap

The trap here is that candidates often assume the issue is with the network command or the IPv4 unicast default, but the real problem is the IPv6 next-hop reachability, which is a subtle but critical requirement for MP-BGP IPv6 route propagation.

How to eliminate wrong answers

Option A is wrong because the network command is not used under the IPv6 address family in MP-BGP; instead, the network command is used under the IPv4 unicast address family, and for IPv6, you use the network command under the IPv6 unicast address family, but the issue here is about redistribution from OSPFv3, not about originating a network. Option B is wrong because disabling bgp default ipv4-unicast only affects IPv4 unicast sessions and does not impact IPv6 address family advertisements; it prevents automatic activation of IPv4 unicast for new peers but does not block IPv6 route propagation. Option D is wrong because the OSPFv3 routes are internal and are present in the IPv6 unicast table (OSPFv3 populates the IPv6 unicast RIB); the problem is that they are not being advertised to iBGP peers, not that they are missing from the table.

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137
MCQeasy

An engineer is troubleshooting MPLS forwarding. On a router, the 'show mpls forwarding-table' command displays that for a specific FEC, the outgoing label is 'Untagged'. What does this indicate?

A.The label has been explicitly null.
B.The router is the penultimate hop, performing PHP.
C.The router is the egress LSR.
D.The next-hop router does not support MPLS.
AnswerB

Correct. 'Untagged' means the label is removed before forwarding.

Why this answer

When the 'show mpls forwarding-table' command shows 'Untagged' for a specific FEC, it indicates that the router is performing Penultimate Hop Popping (PHP). In PHP, the penultimate router (the router before the egress LSR) pops the MPLS label and forwards the packet as an IP packet (untagged) to the egress LSR. This is standard behavior to avoid the egress LSR having to perform a double lookup.

Exam trap

The trap here is that candidates often confuse 'Untagged' with the egress LSR behavior or with an implicit null label, but Cisco specifically uses 'Untagged' to denote the penultimate hop's PHP action, not the egress router's label disposition.

How to eliminate wrong answers

Option A is wrong because 'Untagged' is not the same as 'explicitly null'; an explicit null label (label value 0 or 2) is a valid MPLS label that is still tagged, whereas 'Untagged' means no label is present. Option C is wrong because the egress LSR would show 'Pop Label' or 'No Label' in its forwarding table, not 'Untagged', as it is the router that removes the label. Option D is wrong because 'Untagged' in this context is a normal PHP operation and does not indicate that the next-hop router lacks MPLS support; if the next hop did not support MPLS, the router would typically not have an MPLS forwarding entry for that FEC.

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138
MCQhard

An ISP is implementing MAP-T to transition to IPv6. Which of the following best describes MAP-T?

A.It encapsulates IPv6 packets in IPv4 headers for transport over an IPv4 core
B.It requires both IPv4 and IPv6 addresses on every router
C.It uses stateless translation between IPv4 and IPv6 at the network edge
D.It tunnels IPv4 packets over IPv6 using GRE
AnswerC

MAP-T uses stateless NAT64-like translation.

Why this answer

MAP-T (Mapping of Address and Port using Translation) uses IPv4-IPv6 translation (stateless NAT64) to allow IPv4 devices to communicate over an IPv6 network. It is an IPv6 transition mechanism.

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139
MCQhard

A service provider wants to provide IPv6 connectivity over an MPLS IPv4 core using 6PE. Which BGP extension is required on the provider edge routers?

A.Labeled unicast IPv4 address family
B.VPNv4 address family with route-target extended community
C.IPv6 address family with next-hop encoded as an IPv4-mapped IPv6 address
D.IPv6 address family with next-hop encoded as an IPv6 address
AnswerC

This allows IPv4 next-hop in IPv6 AF.

Why this answer

6PE uses MP-BGP with the IPv6 address family and an IPv4 next-hop (the PE's IPv4 address), encoded via the IPv4-mapped IPv6 address format.

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140
MCQmedium

Refer to the exhibit. Which of the following is true about the BGP table?

A.The route to 192.168.3.0/24 with path 300 400 has an origin of IGP.
B.The prefix 192.168.3.0/24 has two paths, with the best path selected based on some attribute.
C.The prefix 192.168.2.0/24 is not the best path because it has a lower local preference.
D.The route to 192.168.3.0/24 via 10.4.4.4 is the best path because it has a shorter AS path.
AnswerB

Correct. There are two entries, one is best.

Why this answer

The exhibit shows two paths for prefix 192.168.3.0/24: one via 10.4.4.4 with AS path 300 400 and one via 10.5.5.5 with AS path 300 500. BGP selects the best path based on a sequence of attributes (e.g., weight, local preference, AS path length, origin code, MED, etc.). Option B is correct because the prefix has two paths and BGP will choose one as best based on these attributes.

Exam trap

Cisco often tests the misconception that a shorter AS path always determines the best path, but here both paths have equal AS path length, so candidates must look at other attributes like origin code or IGP metric to determine why one path is preferred.

How to eliminate wrong answers

Option A is wrong because the origin code for the route to 192.168.3.0/24 with path 300 400 is shown as '?' (incomplete), not 'i' (IGP). Option C is wrong because the prefix 192.168.2.0/24 is not shown in the exhibit at all, so no comparison of local preference can be made. Option D is wrong because the route via 10.4.4.4 has an AS path length of 2 (300 400) while the route via 10.5.5.5 also has an AS path length of 2 (300 500); they are equal in AS path length, so a shorter AS path cannot be the reason for best path selection.

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141
MCQeasy

What is the purpose of the SEL (NSAP Selector) field in an IS-IS NET address?

A.It identifies the system ID of the router.
B.It is always set to 0x00 for the device itself.
C.It is used to select the routing protocol.
D.It identifies the area within the routing domain.
AnswerB

Correct. SEL is 0x00 for the network layer entity.

Why this answer

In IS-IS, the NET (Network Entity Title) address includes an SEL byte. The SEL is always set to 0x00 for the device itself, as it identifies the network layer entity, not a specific service.

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142
MCQhard

In IS-IS, which TLV is used to carry traffic engineering information for MPLS-TE?

A.TLV 135
B.TLV 229
C.TLV 22
D.TLV 128
AnswerC

TLV 22 (Extended IS Reachability) carries TE information for MPLS-TE, including link attributes like bandwidth and admin group.

Why this answer

In IS-IS, TLV 22 (Extended IS Reachability) is used to carry traffic engineering information, such as link attributes (bandwidth, admin group, TE metric), for MPLS-TE. TLV 135 (Extended IP Reachability) carries IP prefix information, not TE link attributes.

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143
Multi-Selectmedium

A service provider is designing an OSPF network with multiple areas. Which two area types prevent Type 5 LSAs from being flooded? (Choose two.)

Select 2 answers
A.Standard area
B.Stub area
C.Totally stubby area
D.Backbone area
E.NSSA
AnswersB, E

Stub areas block Type 5 LSAs.

Why this answer

Stub and NSSA areas both block Type 5 LSAs; totally stubby also blocks Type 3/4.

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144
MCQeasy

Which technology allows an IPv6-only customer edge router to connect to an MPLS provider edge router using IPv4 transport, encapsulating IPv6 packets in MPLS with a labeled IPv4 next-hop?

A.6VPE
B.NAT64
C.MAP-T
D.6PE
AnswerD

6PE provides IPv6 over MPLS with IPv4 next-hop.

Why this answer

6PE (IPv6 Provider Edge) uses MP-BGP to exchange IPv6 prefixes with IPv4 next-hop addresses over MPLS. It does not require IPv6 in the core.

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145
Drag & Dropmedium

Drag and drop the steps to configure a VLAN on a Cisco switch 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

VLAN configuration involves creating the VLAN, optionally naming it, and then assigning ports to it.

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146
Multi-Selecthard

An SP is deploying IPv6 using 6VPE. Which THREE components are required? (Choose three.)

Select 3 answers
A.Customer edge (CE) routers with IPv6 connectivity
B.6rd tunneling
C.MPLS-enabled core network
D.NAT64 translation
E.MP-BGP with IPv6 VPN address family (VPNv6)
AnswersA, C, E

CE routers must support IPv6.

Why this answer

6VPE requires MPLS core, MP-BGP with VPNv6 address family, and CE routers running IPv6. LDP is used for label distribution in the core.

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147
MCQmedium

An OSPF network is configured with RSVP-TE. Which OSPF extension is used to advertise TE link attributes such as bandwidth and administrative group?

A.OSPF Type 10 Opaque LSA
B.OSPF Type 1 LSA
C.OSPF Type 5 LSA
D.OSPF Type 7 LSA
AnswerA

Type 10 Opaque LSA carries TE information.

Why this answer

OSPF TE extensions use Opaque LSAs (Type 9, 10, 11) to carry TE information. Type 10 (area-local) is commonly used for TE.

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148
Multi-Selectmedium

Which TWO of the following are characteristics of MPLS-TE (Traffic Engineering)?

Select 2 answers
A.Uses explicit paths to route traffic away from shortest-path IGP.
B.Uses LDP for label distribution along the TE tunnel.
C.Allows bandwidth reservation and priority.
D.Requires per-platform label space for TE tunnels.
E.Requires all routers in the TE tunnel to be in the same OSPF area.
AnswersA, C

MPLS-TE can specify explicit paths for traffic engineering.

Why this answer

MPLS-TE uses explicit paths (either strict or loose) to direct traffic away from the shortest path determined by the IGP (e.g., OSPF or IS-IS). This allows network operators to engineer traffic flows based on administrative policies, such as load balancing or avoiding congested links, rather than relying solely on the IGP's metric-based shortest path.

Exam trap

Cisco often tests the distinction between LDP and RSVP-TE, so the trap here is that candidates mistakenly associate MPLS-TE with LDP because both are label distribution protocols, but TE explicitly requires RSVP-TE for constraint-based path setup.

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149
MCQmedium

A service provider is deploying a new MPLS core network. The network has four routers: P1, P2, PE1, and PE2. OSPF is used as the IGP. The engineer configures MPLS LDP on all interfaces. After enabling LDP, the engineer notices that the LDP session between P1 and P2 is established, but no labels are exchanged for the loopback0 interfaces of PE1 and PE2. The loopback0 addresses are advertised in OSPF. The engineer verifies that the OSPF routes are present in the routing table of all routers. What is the most likely reason for the missing labels?

A.An access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes.
B.LDP is using UDP for label exchange.
C.The OSPF cost to the loopbacks is too high.
D.The loopback interfaces are not enabled with 'mpls ip'.
AnswerA

Label advertisement can be filtered; this is a common issue.

Why this answer

The most likely reason is that an access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes. LDP by default advertises labels for all prefixes in the routing table, but the 'advertise-labels' command can filter which prefixes receive labels. If the loopback0 prefixes of PE1 and PE2 are denied by such an access-list, no labels will be advertised for them, even though OSPF routes are present.

Exam trap

Cisco often tests the misconception that 'mpls ip' must be enabled on the loopback interface itself for its prefix to receive a label, when in fact LDP advertises labels for any prefix in the routing table as long as the outgoing interface has 'mpls ip' enabled.

How to eliminate wrong answers

Option B is wrong because LDP uses TCP (port 646) for session establishment and label exchange, not UDP; UDP is used only for LDP discovery (hello messages). Option C is wrong because OSPF cost does not affect LDP label advertisement; LDP advertises labels for all reachable prefixes regardless of metric, as long as they are in the routing table. Option D is wrong because 'mpls ip' must be enabled on interfaces for LDP to form adjacencies and exchange labels, but the loopback interfaces themselves do not need 'mpls ip' for their prefixes to be advertised with labels; the issue is about label advertisement for the loopback prefixes, not LDP session establishment.

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150
MCQhard

An engineer is deploying Segment Routing in an MPLS network. To ensure that routers can forward packets based on SR-MPLS labels without requiring LDP, which requirement must be met?

A.IS-IS or OSPF must have the SR extension enabled.
B.MPLS LDP must be enabled on all interfaces.
C.All routers must run BGP-LU.
D.The IGP must have the overload bit set.
AnswerA

Correct. The IGP distributes prefix-SIDs for SR-MPLS label forwarding.

Why this answer

Segment Routing (SR) can operate without LDP by encoding the forwarding path as a list of MPLS labels derived from the IGP. For routers to allocate and advertise these SR-MPLS labels, the IGP (IS-IS or OSPF) must have the Segment Routing extension enabled. This allows the IGP to distribute prefix-SIDs (labels) that routers use to forward packets, eliminating the need for LDP.

Exam trap

Cisco often tests the misconception that LDP is always required for MPLS forwarding, but SR-MPLS bypasses LDP by using the IGP to distribute labels, so candidates must recognize that the IGP SR extension is the enabling requirement.

How to eliminate wrong answers

Option B is wrong because MPLS LDP is specifically not required when SR-MPLS is used; SR replaces LDP by using IGP-distributed labels. Option C is wrong because BGP-LU (BGP Labeled Unicast) is used for inter-domain label distribution, not for the basic intra-domain label allocation needed for SR-MPLS forwarding. Option D is wrong because the IGP overload bit is a mechanism to prevent transit traffic through a router during convergence or maintenance, and it has no role in enabling SR-MPLS label-based forwarding.

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