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MPLS practice questions

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Question 1MPLS Fundamentals and Architecture

In an MPLS network, which device is responsible for imposing and removing labels at the edge of the provider network?

  1. Label Switching Router (LSR)
  2. Provider Core Router
  3. Route Reflector
  4. Label Edge Router (LER) Correct answer.

Explanation

An LER sits at the boundary between a label-switched domain and adjacent networks, pushing labels on ingress and popping them on egress. Core LSRs only swap labels between interfaces and do not perform edge imposition or disposition.

Question 2MPLS Fundamentals and Architecture

Which field of the MPLS shim header identifies the forwarding equivalence class to which a packet belongs?

  1. The TTL field
  2. The EXP bits
  3. The 20-bit Label field Correct answer.
  4. The S bit

Explanation

The 20-bit Label field carries the value that a router uses as its index into the label forwarding table, thereby identifying the FEC. The other fields carry class-of-service, stack-depth, and loop-prevention information rather than the forwarding identity.

Question 3MPLS Fundamentals and Architecture

What is the primary purpose of the label stack in MPLS?

  1. To replace IP routing tables entirely within the provider core
  2. To encrypt payloads between provider edge routers
  3. To allow multiple labels to be pushed so that hierarchical forwarding, such as VPN or TE, can be applied Correct answer.
  4. To carry the original IP header unchanged across the core

Explanation

The label stack lets an outer label steer the packet through the core while inner labels preserve service context such as a VPN or a traffic-engineered path. Each label is processed in order, enabling layered forwarding decisions.

Question 4MPLS Fundamentals and Architecture

Which protocol is the most common label distribution protocol used in service provider MPLS backbones?

  1. IGMP
  2. LDP Correct answer.
  3. RIPv2
  4. OSPFv2

Explanation

LDP is the standard protocol for distributing label bindings for IGP-learned prefixes in an MPLS domain. It maps FECs to labels and is widely deployed for basic MPLS forwarding.

Question 5MPLS Fundamentals and Architecture

What does the S bit in an MPLS label stack entry indicate?

  1. The label is a special reserved value
  2. The packet should be silently dropped
  3. The label is the bottom of the stack Correct answer.
  4. The EXP bits should be ignored

Explanation

The S bit, set to 1, marks the bottom-of-stack entry, telling the router that no further labels follow. This is essential for correct disposition of the packet at the egress LER.

Question 6MPLS Fundamentals and Architecture

What is a Forwarding Equivalence Class (FEC)?

  1. A BGP community used for route filtering
  2. A group of packets that receive the same forwarding treatment through the MPLS domain Correct answer.
  3. A classification of MPLS routers by hardware capacity
  4. A QoS queue assigned to EXP bits

Explanation

A FEC is a set of packets that share the same forwarding path and treatment, and each FEC is bound to a label. Packets are mapped to a FEC at ingress based on destination prefix or other policy.

Question 7MPLS Fundamentals and Architecture

Which MPLS label value is reserved to represent the IPv4 explicit null label?

  1. 3
  2. 2 Correct answer.
  3. 0
  4. 1

Explanation

Label value 2 is the IPv4 explicit null label, which signals to the egress LER that the packet should be forwarded based on the IPv4 header and that the EXP bits should be preserved. Label 0 is the IPv4 implicit null, 3 is the IPv6 implicit null, and 1 is the router alert label.

Question 8MPLS Fundamentals and Architecture

A provider edge router receives an unlabeled IPv4 packet destined for a remote VPN site and must forward it into the MPLS core. What is the correct ingress action?

  1. Push the appropriate label stack and forward out the MPLS interface Correct answer.
  2. Drop the packet because no label is present
  3. Pop the top label and forward based on the inner IP header
  4. Swap the label with the next-hop label and forward

Explanation

At ingress, the LER classifies the packet into a FEC and pushes the label or stack that the egress LER or next hop expects. Only after imposition is the packet forwarded as a labeled packet into the core.

Question 9MPLS Fundamentals and Architecture

An LSR receives a labeled packet whose top label maps to a next hop that is also an MPLS interface. Which forwarding operation does it perform?

  1. Swap the incoming label for the outgoing label and forward Correct answer.
  2. Push an additional label and forward
  3. Pop the label and forward an unlabeled packet
  4. Flood the packet to all MPLS interfaces

Explanation

A transit LSR performs a label swap, replacing the incoming label with the outgoing label associated with the next hop and forwarding the packet. This is the standard core forwarding operation in an MPLS domain.

Question 10MPLS Fundamentals and Architecture

Which mechanism allows a penultimate hop router to remove the top label before delivering the packet to the egress LER?

  1. Explicit null signaling
  2. Label stacking
  3. Label Distribution Protocol
  4. Penultimate Hop Popping (PHP) Correct answer.

Explanation

PHP is the practice in which the penultimate hop pops the top label so the egress LER receives an unlabeled packet, reducing processing load at the edge. It is signaled by the egress advertising the implicit null label.

Question 11MPLS Fundamentals and Architecture

A PE router advertises the implicit null label for a connected prefix to its upstream neighbor. What behavior does this trigger?

  1. The upstream neighbor drops the packet
  2. The upstream neighbor pushes two labels
  3. The upstream neighbor forwards the packet with the explicit null label
  4. The upstream neighbor pops the top label before sending to the PE Correct answer.

Explanation

Advertising the implicit null label requests that the upstream neighbor perform PHP, sending the packet to the egress PE without the top label. This is the standard signal for penultimate hop popping.

Question 12MPLS Fundamentals and Architecture

Which statement correctly describes the relationship between an MPLS label and an IP destination prefix?

  1. A label is bound to a FEC that may correspond to an IP prefix or another forwarding class Correct answer.
  2. A label is bound to a MAC address
  3. A label is always identical to the IP destination address
  4. A label is bound to a TCP port number

Explanation

Label bindings are associated with FECs, and a FEC can be an IP prefix, a VPN route, or another classification. The label itself is a local value and need not match the IP address.

Question 13MPLS Fundamentals and Architecture

Which of the following best describes the control plane function of LDP?

  1. It distributes label bindings for FECs using TLV-encoded messages over a TCP session Correct answer.
  2. It negotiates encryption keys for MPLS payloads
  3. It performs hardware forwarding of labeled packets
  4. It distributes IP routing information using link-state advertisements

Explanation

LDP uses TLV-encoded messages over a TCP session to exchange label bindings for FECs. It relies on the IGP for reachability and does not itself compute routes.

Question 14MPLS Fundamentals and Architecture

A packet arrives at an LSR with three labels in the stack, and the S bit is set on the third entry. How should the LSR interpret the stack?

  1. The stack is malformed because only one S bit may be set
  2. The third label is the bottom of the stack and no further labels follow Correct answer.
  3. The third label is a reserved value
  4. The third label is the top label and should be processed first

Explanation

The S bit is set only on the bottom-of-stack entry, so a stack of three labels has S set on the third entry. This confirms to the router that the label stack ends there.

Question 15MPLS Fundamentals and Architecture

Why does an MPLS domain typically use an IGP such as OSPF or IS-IS in addition to LDP?

  1. The IGP assigns EXP bits based on traffic class
  2. The IGP provides reachability to the next hop so that LDP can bind labels to reachable prefixes Correct answer.
  3. The IGP replaces the need for label distribution
  4. The IGP encrypts label distribution messages

Explanation

LDP depends on the routing table to know the next hop for each FEC, so an IGP is needed to build reachability. LDP then distributes labels for those IGP-learned prefixes.

Question 16MPLS Fundamentals and Architecture

What happens to the TTL field of an IP packet when it enters an MPLS domain?

  1. It is copied into the label TTL field and decremented at each LSR Correct answer.
  2. It is reset to 255 and ignored in the MPLS domain
  3. It is removed and restored only at egress
  4. It is used as the label value

Explanation

By default the IP TTL is copied into the TTL field of the imposed label, and each LSR decrements it to preserve loop detection. The value is copied back to the IP header at egress.

Question 17MPLS Fundamentals and Architecture

Which statement best explains why MPLS labels have only local significance?

  1. Labels are derived from the destination IP address
  2. Each LSR assigns labels independently and the binding is meaningful only to that LSR and its neighbors Correct answer.
  3. Labels are assigned by a central server and distributed to all LSRs
  4. Labels are globally unique across all MPLS domains

Explanation

Labels are locally assigned by each LSR and advertised to its neighbors, so the same label value can mean different things on different routers. Forwarding is based on the incoming label and interface.

Question 18MPLS Fundamentals and Architecture

A PE router must forward traffic for two different VPNs over the same core link. Which MPLS feature makes this possible?

  1. Use of the S bit to identify the VPN
  2. Use of the EXP bits to identify the VPN
  3. A two-label stack with an outer transport label and an inner VPN label Correct answer.
  4. A single label shared by both VPNs

Explanation

VPN traffic is typically encapsulated with an inner label identifying the VPN and an outer label used for transport across the core. The outer label is swapped at each LSR while the inner label is preserved for the egress PE.

Question 19MPLS Fundamentals and Architecture

An LSR receives a labeled packet but has no label binding for the incoming label. What is the standard behavior?

  1. Push a new label and forward
  2. Drop the packet Correct answer.
  3. Flood it to all neighbors
  4. Forward it based on the IP header

Explanation

If an LSR has no label binding for an incoming label, it cannot determine the correct forwarding action and drops the packet. This is a normal consequence of inconsistent label distribution or a stale binding.

Question 20MPLS Fundamentals and Architecture

Which statement correctly distinguishes the MPLS data plane from its control plane?

  1. The data plane and control plane are the same in MPLS
  2. The data plane runs LDP, while the control plane runs OSPF
  3. The data plane forwards labeled packets using the label forwarding table, while the control plane builds that table via routing and label distribution Correct answer.
  4. The data plane distributes labels, while the control plane forwards packets

Explanation

The MPLS data plane performs label lookup and forwarding, while the control plane runs routing protocols and label distribution to populate the forwarding table. Separating the two allows forwarding to remain fast and simple.

Question 21MPLS Fundamentals and Architecture

A network engineer observes that the egress PE receives packets with an explicit null label instead of having the label popped by the penultimate hop. What is the most likely explanation?

  1. The penultimate hop is misconfigured with an incorrect label stack
  2. The ingress PE is pushing too many labels
  3. The LDP session is down
  4. The egress PE advertised the explicit null label rather than the implicit null label Correct answer.

Explanation

Explicit null (label 0 for IPv4) instructs the penultimate hop to keep the label in place so the egress PE can preserve EXP bits. If the egress advertises explicit null instead of implicit null, PHP does not occur.

Question 22MPLS Fundamentals and Architecture

Which of the following is a valid reason to use MPLS instead of pure IP forwarding in a service provider network?

  1. To increase the MTU of core links
  2. To support traffic engineering and VPN services through label-based forwarding Correct answer.
  3. To encrypt all customer traffic by default
  4. To eliminate the need for any routing protocol

Explanation

MPLS enables traffic engineering and Layer 3 VPN services by allowing forwarding decisions based on labels rather than only destination prefixes. This provides greater control over path selection and service separation.

Question 23MPLS Fundamentals and Architecture

Which statement accurately describes the forwarding behavior of an MPLS LSR when the incoming label maps to a next hop that is the egress LER?

  1. The LSR may pop the label if PHP is in effect and forward the packet unlabeled Correct answer.
  2. The LSR must swap the label and forward it labeled
  3. The LSR must push an additional label before forwarding
  4. The LSR must drop the packet

Explanation

When the next hop is the egress LER and PHP is in effect, the penultimate LSR pops the top label and forwards an unlabeled packet. This reduces the processing burden at the egress.

Question 24MPLS Fundamentals and Architecture

Which field in the MPLS shim header is used to carry class-of-service information for per-hop behavior?

  1. The TTL field
  2. The 20-bit Label field
  3. The 3-bit EXP field Correct answer.
  4. The S bit

Explanation

The 3-bit EXP field, also called the traffic class field, carries class-of-service marking used by routers to apply per-hop queuing and scheduling behavior. It is preserved across label swaps.

Question 25Label Distribution and Signalling Protocols (LDP, RSVP-TE)

In LDP, which protocol and port are used to establish and maintain the session over which label mappings are exchanged?

  1. TCP port 646 Correct answer.
  2. TCP port 179
  3. UDP port 711
  4. UDP port 646

Explanation

LDP uses TCP as its transport because label advertisement and session state must be reliable and ordered; the well-known port assigned to LDP is 646. Discovery is performed with UDP 646, but the session itself is TCP.

Question 26Label Distribution and Signalling Protocols (LDP, RSVP-TE)

Two LSRs are directly connected and both are configured with LDP on the interface. Which discovery mechanism allows them to learn each other's loopback-based LSR ID before the TCP session is formed?

  1. Targeted hello messages sent unicast to the peer's loopback address
  2. BGP Update messages carrying the label distribution capability
  3. Basic hello messages sent to UDP 646 on the all-routers multicast address Correct answer.
  4. RSVP Path messages exchanged over IP protocol 46

Explanation

Link hello messages are sent to the multicast address 224.0.0.2 on UDP port 646 and carry the sender's LSR ID and label space. They are the standard mechanism for discovering directly connected LDP peers.

Question 27Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An LSR receives a Label Mapping for prefix 10.10.10.0/24 from its LDP peer. What does the label in that mapping identify?

  1. The label-value range reserved for the peer's per-interface label space
  2. The label the receiving LSR must impose when forwarding traffic toward that prefix
  3. The label the advertising LSR expects to receive on packets destined to that prefix Correct answer.
  4. The label used only for the LDP session's own control traffic

Explanation

In LDP, the label in a mapping is the label the advertising router expects to see on packets it receives for that FEC, so the downstream router allocates the label. The upstream router installs it as the outgoing label for that prefix.

Question 28Label Distribution and Signalling Protocols (LDP, RSVP-TE)

Which LDP message is used by an LSR to withdraw a previously advertised label mapping for a FEC?

  1. Notification
  2. Label Withdraw Correct answer.
  3. Initialization
  4. Label Request

Explanation

The Label Withdraw message tells the peer to stop using a previously advertised mapping for a specific FEC. The peer normally responds with a Label Release to confirm.

Question 29Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An LSR is configured for downstream unsolicited label distribution. What behavior does this produce?

  1. It forwards labelled packets only after RSVP reserves bandwidth for the FEC
  2. It advertises mappings only after receiving an explicit Label Request from the peer
  3. It advertises mappings only to peers that have requested a specific label range
  4. It advertises mappings for its FECs to LDP peers without waiting for a request Correct answer.

Explanation

With downstream unsolicited mode, an LSR distributes label mappings for its local FECs to all discovered peers as soon as it is ready, without a prior request. This is the default mode on most implementations.

Question 30Label Distribution and Signalling Protocols (LDP, RSVP-TE)

Which statement describes the purpose of the Liberal Label Retention mode in LDP?

  1. Labels are retained only for prefixes installed in the routing table
  2. Labels are retained only after a Label Request has been acknowledged
  3. Labels received from non-next-hop peers are discarded immediately to conserve label space
  4. Labels received from peers that are not currently the next hop are kept for possible future use Correct answer.

Explanation

Liberal retention keeps mappings from all peers, including those that are not currently selected as the next hop, so that a routing change can be accommodated without waiting for new advertisements. Conservative retention discards them.

Question 31Label Distribution and Signalling Protocols (LDP, RSVP-TE)

In RSVP-TE, which object in a Path message carries the explicit route that the LSP must follow?

  1. RECORD_ROUTE
  2. EXPLICIT_ROUTE Correct answer.
  3. SESSION_ATTRIBUTE
  4. LABEL_REQUEST

Explanation

The EXPLICIT_ROUTE object contains the list of hops, or loose and strict subobjects, that the Path message must traverse. It is what makes the LSP traffic-engineered rather than simply following IGP shortest paths.

Question 32Label Distribution and Signalling Protocols (LDP, RSVP-TE)

Which RSVP message is sent by a transit or egress node back toward the ingress to confirm the reservation and provide the label binding for an LSP?

  1. Path
  2. PathErr
  3. Resv Correct answer.
  4. PathTear

Explanation

The Resv message travels hop by hop upstream from the egress and carries the LABEL object that the upstream node must use when sending traffic. Successful receipt of Resv confirms the reservation was installed.

Question 33Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An LDP session between two directly connected peers is up, but the peer's loopback prefix is missing from the LFIB. Which condition most plausibly explains this?

  1. The LDP router ID is configured as a physical interface address
  2. The peer advertised the prefix with a label value of 3
  3. The session uses TCP rather than UDP for label messages
  4. The prefix is not present in the routing table, so no FEC exists for LDP to bind Correct answer.

Explanation

LDP binds labels to prefixes that exist in the IP routing table; if the loopback is not reachable or not installed, no FEC is created and no mapping is installed. Verifying the RIB entry is the first troubleshooting step.

Question 34Label Distribution and Signalling Protocols (LDP, RSVP-TE)

A router receives two Label Mappings for the same prefix, one from peer A with a lower IGP metric path and one from peer B with a higher metric. In the default independent control mode, what does the router install?

  1. The mapping from peer A, because the next hop with the lowest IGP cost is preferred Correct answer.
  2. The mapping from peer B, because higher-metric paths are retained for backup
  3. Both mappings are installed and traffic is load-shared across the two peers
  4. Neither mapping, until both peers withdraw and re-advertise

Explanation

LDP selects the label mapping associated with the best next hop for the prefix, which is determined by the routing table. The non-selected mapping may still be retained in liberal retention mode but is not used for forwarding.

Question 35Label Distribution and Signalling Protocols (LDP, RSVP-TE)

What is the effect of configuring LDP-IGP synchronization on a link between two LSRs?

  1. The link is advertised into the IGP with maximum metric until the LDP session is fully established Correct answer.
  2. The LDP session is torn down whenever the IGP adjacency flaps
  3. The IGP is disabled on the link until a targeted LDP session is formed
  4. LDP labels are assigned before the IGP adjacency is formed

Explanation

LDP-IGP synchronization prevents traffic from being routed over a link before LDP is ready, avoiding black-holing; the link is advertised with the maximum metric until the LDP session is up. Once the session is established, the normal metric is restored.

Question 36Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An operator wants LDP to advertise labels only for a specific set of prefixes rather than the whole routing table. Which mechanism accomplishes this?

  1. Enabling downstream on-demand distribution
  2. Applying an outbound label advertisement filter for the selected prefixes Correct answer.
  3. Increasing the LDP session hold time
  4. Configuring a targeted LDP session to the peer's loopback

Explanation

Outbound label filters restrict which FECs are advertised to a peer, reducing label consumption and LFIB size. Inbound filters can additionally limit which received mappings are installed.

Question 37Label Distribution and Signalling Protocols (LDP, RSVP-TE)

What is the purpose of the LDP MD5 authentication configuration on a session?

  1. It authenticates each TCP segment of the LDP session using a shared key Correct answer.
  2. It signs the label mappings with a public key certificate
  3. It encrypts all labelled data traffic between the peers
  4. It restricts label advertisement to prefixes matching an ACL

Explanation

LDP MD5 authentication protects the TCP connection by computing a digest over each segment with a shared secret, so an attacker cannot inject or spoof LDP messages. It does not encrypt data-plane traffic.

Question 38Label Distribution and Signalling Protocols (LDP, RSVP-TE)

In RSVP-TE, what does the LABEL_REQUEST object in a Path message specify?

  1. The bandwidth and service class required for the LSP
  2. The explicit list of hops the LSP must traverse
  3. The preemption priority of the LSP relative to other LSPs
  4. The label range and encoding the LSP requires from downstream nodes Correct answer.

Explanation

LABEL_REQUEST tells downstream routers that a label binding is needed and indicates the label range and encoding type, such as ATM, frame relay, or generic. It is mandatory for establishing a labelled LSP.

Question 39Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An RSVP-TE LSP is configured with setup priority 7 and holding priority 0. What behavior results when bandwidth is scarce?

  1. The LSP can preempt other LSPs but cannot itself be preempted Correct answer.
  2. The LSP is treated as best effort and bypasses admission control
  3. The LSP is never admitted because holding priority exceeds setup priority
  4. The LSP cannot preempt other LSPs but can be preempted by them

Explanation

Lower numeric values indicate higher priority in RSVP-TE. A setup priority of 7 is the lowest, so the LSP cannot preempt others, while a holding priority of 0 is the highest, so it is protected from preemption once established.

Question 40Label Distribution and Signalling Protocols (LDP, RSVP-TE)

Which RSVP-TE object is used to request that a backup path be established for local protection of an LSP?

  1. SESSION_ATTRIBUTE
  2. ADMIN_STATUS
  3. FAST_REROUTE Correct answer.
  4. SENDER_TEMPLATE

Explanation

The FAST_REROUTE object in the Path message requests fast reroute and carries parameters such as the desired protection mode and bandwidth for the backup. It enables local repair at the point of failure.

Question 41Label Distribution and Signalling Protocols (LDP, RSVP-TE)

A Path message reaches a router that cannot satisfy the requested bandwidth on the outgoing interface. Which message is generated toward the ingress?

  1. PathErr with an admission-control failure error code Correct answer.
  2. ResvErr with a label allocation failure
  3. ResvConf confirming partial reservation
  4. PathTear with a shutdown reason

Explanation

When a transit or egress node cannot admit the reservation, it returns a PathErr identifying the error, such as admission control failure, and the ingress can then try an alternate path. The failed LSP is not established.

Question 42Label Distribution and Signalling Protocols (LDP, RSVP-TE)

What is the role of the SENDER_TEMPLATE object in an RSVP-TE Path message?

  1. It lists the hops that the LSP must traverse
  2. It records the actual path taken for loop detection
  3. It identifies the LSP by sender address and LSP ID so state can be matched Correct answer.
  4. It specifies the label to be used on the first hop

Explanation

SENDER_TEMPLATE, together with SESSION, uniquely identifies the LSP instance using the sender's address and LSP ID. Intermediate routers use these identifiers to match Path and Resv messages to the correct state.

Question 43Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An LSR has two LDP peers advertising the same FEC, and the routing table installs both next hops as equal cost. What does LDP do by default?

  1. It installs only the mapping from the most recently established session
  2. It installs only the mapping from the peer with the lowest LDP router ID
  3. It installs both label mappings and performs label-based load balancing Correct answer.
  4. It rejects both mappings because of the tie

Explanation

When the IGP installs multiple equal-cost next hops for a prefix, LDP installs the corresponding label mappings and traffic is load-shared across them. The forwarding plane then hashes flows across the outgoing labels.

Question 44Label Distribution and Signalling Protocols (LDP, RSVP-TE)

A targeted LDP session is configured between two PE routers that are not directly connected. Which statement is true about this session?

  1. It requires the two routers to share a common broadcast segment
  2. It cannot carry label mappings, only keepalive messages
  3. It uses UDP instead of TCP for the label exchange
  4. It is established using unicast targeted hellos and is used for features such as pseudowire signalling Correct answer.

Explanation

Targeted LDP extends the session beyond a single link by sending unicast hellos to the peer's address, commonly the loopback. It is widely used to signal pseudowires and other services between PEs.

Question 45Label Distribution and Signalling Protocols (LDP, RSVP-TE)

Which LDP message carries the FEC TLV and the label TLV when an LSR advertises a binding to a peer?

  1. Hello
  2. Label Release
  3. Keepalive
  4. Label Mapping Correct answer.

Explanation

Label Mapping carries both the FEC TLV identifying the prefix and the Label TLV carrying the binding. It is the core advertisement message in LDP.

Question 46Label Distribution and Signalling Protocols (LDP, RSVP-TE)

In RSVP-TE, what is the function of the RECORD_ROUTE object in a Path message?

  1. It forces the LSP to follow a strict hop list
  2. It requests that each node record its address so the actual path can be reported to the ingress Correct answer.
  3. It carries the label binding from the egress to the ingress
  4. It reserves bandwidth for the backup path

Explanation

RECORD_ROUTE asks each router along the LSP to append its address, allowing the ingress to verify the actual path, detect loops, and diagnose misrouting. It is informational and does not constrain the path.

Question 47Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An operator observes that LDP labels are installed for a prefix but traffic is forwarded unlabelled on the link to the peer. Which explanation is most consistent with this observation?

  1. The LDP router ID has changed and the session is re-establishing
  2. The peer advertised the implicit-null label, so the peer expects unlabelled IP packets Correct answer.
  3. The prefix is not present in the routing table
  4. The LDP session has gone down and the LFIB entry is stale

Explanation

When a router advertises label 3, the implicit-null label, it is asking its upstream neighbor to send unlabelled packets because it will perform the lookup itself, typically for directly connected or summarised prefixes. This is normal penultimate-hop behavior.

Question 48Label Distribution and Signalling Protocols (LDP, RSVP-TE)

Two LSRs are connected over a link where the IGP adjacency is up but LDP has failed to establish. LDP-IGP synchronization is enabled. What is the expected forwarding consequence?

  1. Labeled traffic is forwarded over the link using a static label
  2. The link is removed from the IGP topology entirely
  3. The link is advertised with maximum metric, so traffic avoids it until LDP recovers Correct answer.
  4. The IGP adjacency is torn down and re-formed

Explanation

With synchronization enabled, the IGP holds the link at maximum metric while LDP is not converged, keeping traffic away from a link that cannot forward labelled packets. Once LDP comes up, the normal metric is advertised.

Question 49Label Distribution and Signalling Protocols (LDP, RSVP-TE)

In RSVP-TE, a loose subobject in the EXPLICIT_ROUTE object differs from a strict subobject in what way?

  1. A loose hop may be reached through intermediate routers, while a strict hop must be directly adjacent Correct answer.
  2. A loose hop is used only for backup paths, while a strict hop is used only for primary paths
  3. A loose hop requires no label, while a strict hop requires a label
  4. A loose hop is recorded in RECORD_ROUTE, while a strict hop is not

Explanation

A strict subobject requires the previous hop to be directly connected to the specified node, whereas a loose subobject allows the path to traverse other routers to reach it. This lets an operator constrain key hops while leaving local routing decisions to the network.

Question 50Label Distribution and Signalling Protocols (LDP, RSVP-TE)

An LSR receives a Label Request message from a downstream LDP peer for a FEC it can resolve in its routing table. Which message does the LSR send back to the peer to supply the label binding?

  1. A Label Request message echoed back with the label inserted
  2. A Notification message with a label-release status code
  3. An Initialization message containing the label in the common session parameters
  4. A Label Mapping message carrying the FEC TLV and the assigned label TLV Correct answer.

Explanation

Under downstream unsolicited or downstream-on-demand operation, the LSR that owns the binding answers a Label Request with a Label Mapping message. That message contains the FEC TLV identifying the prefix and a label TLV carrying the locally allocated incoming label. The peer then uses this label when it forwards traffic for that FEC upstream.