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Nokia MPLS practice questions

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Question 1IP routing, switching and aggregation

In an IP/MPLS service provider network, which device role is responsible for pushing the transport label onto a customer packet at the ingress edge?

  1. The route reflector serving the core iBGP sessions
  2. The provider edge router acting as ingress label edge router Correct answer.
  3. The penultimate hop router in the core
  4. The customer edge router connected to the access link

Explanation

The ingress label edge router sits at the boundary of the MPLS domain and imposes the label stack, mapping the incoming IP packet into an LSP. Customer edge routers do not participate in label distribution, and core routers only swap labels already present.

Question 2IP routing, switching and aggregation

Which forwarding behaviour describes an MPLS label swap operation on a core router?

  1. The router inserts an additional label beneath the existing top label
  2. The router copies the top label into the IP header and forwards using normal destination lookup
  3. The router removes the top label and forwards the remaining packet as unlabelled IP
  4. The router replaces the incoming top label with a new outgoing label and forwards on the corresponding interface Correct answer.

Explanation

Label swapping is the standard core operation: the incoming label is looked up in the incoming label map, and the packet leaves with the replacement label and next hop recorded there. No IP header analysis is needed at that hop.

Question 3IP routing, switching and aggregation

What is the primary purpose of the label distribution protocol used between adjacent routers inside an MPLS domain?

  1. To synchronise clocking for the underlying optical transport
  2. To carry the actual customer payload across the core
  3. To exchange label-to-prefix bindings so each router can build its incoming and outgoing label maps Correct answer.
  4. To advertise customer VPN routes between provider edge routers

Explanation

A label distribution protocol such as LDP or RSVP-TE signals bindings between a forwarding equivalence class and a label, letting each hop populate its label forwarding information base. VPN route advertisement is a separate BGP function.

Question 4IP routing, switching and aggregation

A provider edge router receives an IP packet destined for a remote site reachable through a VRF. What determines which label stack is imposed?

  1. The global routing table entry for the packet destination
  2. The DSCP value carried in the arriving IP header
  3. The source MAC address of the arriving frame
  4. The route lookup in the VRF, which returns a next hop with an associated label binding Correct answer.

Explanation

The VRF lookup yields the remote next hop and its label binding, so the ingress PE can push the correct stack for that VPN. The global table does not hold the customer prefix in a VRF-based design.

Question 5IP routing, switching and aggregation

Why is penultimate hop popping used in an MPLS network?

  1. To reduce the label stack depth for traffic engineering tunnels
  2. To force all traffic onto a backup path
  3. To remove the transport label one hop early so the egress router avoids a double lookup Correct answer.
  4. To allow the egress to forward based on the label only

Explanation

With penultimate hop popping, the last core router removes the transport label so the egress router receives a packet it can forward with a single IP lookup. This saves processing at the edge device.

Question 6IP routing, switching and aggregation

An engineer needs a fast convergence mechanism for protecting a primary label switched path without waiting for IGP reconvergence. Which approach fits?

  1. Reducing the label stack depth on the primary path
  2. Increasing the IGP hello interval on the protected links
  3. Disabling penultimate hop popping on the path
  4. Configuring a backup LSP with fast reroute facility protection Correct answer.

Explanation

Fast reroute pre-establishes a backup path so that the head end or a point of local repair can redirect traffic immediately on failure. IGP reconvergence is avoided for the protected traffic during the switchover.

Question 7IP routing, switching and aggregation

Which statement about an aggregated Ethernet bundle used for IP core uplinks is correct?

  1. The bundle uses a single member link at a time for active traffic
  2. Member links must have consistent speed and duplex and are hashed across by the bundle Correct answer.
  3. LACP is required for any bundle to forward traffic
  4. Member links must be configured with different IP addresses to distribute traffic

Explanation

Aggregated links require consistent member characteristics so the bundle operates predictably, and traffic is distributed by a hash over the members. LACP is recommended for negotiation but static bundles can forward without it.

Question 8IP routing, switching and aggregation

What is the effect of configuring an implicit null label for a prefix on an LDP-enabled router?

  1. The prefix is excluded from label distribution entirely
  2. The upstream neighbour pops the label before forwarding to this router Correct answer.
  3. The router advertises the prefix with an explicit non-zero label
  4. The upstream neighbour pushes an additional label for this prefix

Explanation

Advertising implicit null tells the upstream neighbour to pop the label so the packet arrives unlabelled, which is the mechanism behind penultimate hop popping for that prefix. An explicit non-zero label would instead require the router to receive and process a label.

Question 9IP routing, switching and aggregation

Which IGP is most commonly used to distribute loopback addresses that serve as MPLS LSP endpoints in a service provider core?

  1. IS-IS with traffic engineering extensions Correct answer.
  2. EIGRP with stub routing
  3. RIP version 2
  4. BGP as the sole interior protocol

Explanation

IS-IS, often with traffic engineering extensions, is widely deployed in provider cores to advertise loopbacks and TE link attributes. The loopback address is the stable endpoint used for LSP signalling.

Question 10IP routing, switching and aggregation

A router receives a packet with a three-label stack. Which label is examined first for forwarding?

  1. The label with the highest numeric value
  2. The bottom label
  3. The top label Correct answer.
  4. The label associated with the largest prefix

Explanation

MPLS forwarding always operates on the top of the stack, which is the outermost label. Lower labels are exposed only after the top label is popped.

Question 11IP routing, switching and aggregation

What does the EXP field in an MPLS shim header traditionally carry?

  1. The label value for the next hop
  2. A class-of-service marking used for queuing decisions Correct answer.
  3. The protocol type of the encapsulated payload
  4. The TTL value for loop prevention

Explanation

The three-bit EXP field carries a class-of-service marking that routers map to per-hop behaviour such as queuing and drop precedence. TTL and label value occupy separate fields.

Question 12IP routing, switching and aggregation

Which mechanism allows a provider to carry multiple customer VRFs over a shared core while keeping their routes separate?

  1. OSPF areas mapped one-to-one to each customer
  2. Multiprotocol BGP with route targets and a per-VRF label Correct answer.
  3. A single global routing table shared by all customers
  4. Static routes configured identically on every provider edge

Explanation

Multiprotocol BGP distributes VPN routes with route targets that control import and export, and a per-VRF label identifies the customer context at the egress. This keeps customer addressing isolated across a shared core.

Question 13IP routing, switching and aggregation

A provider edge router must forward traffic for two customers that both use the prefix 10.0.0.0/24. What enables this?

  1. The prefix is rewritten with NAT at the ingress
  2. The prefix is advertised only to one customer
  3. Each customer prefix is installed in a separate VRF with its own label Correct answer.
  4. The prefix is summarised into a single global route

Explanation

VRF separation allows identical prefixes to coexist because each VRF has its own routing and forwarding table and its own label binding. The egress router uses the label to select the correct VRF.

Question 14IP routing, switching and aggregation

Which field in the MPLS shim header prevents packets from looping indefinitely when a label switched path is misconfigured?

  1. The bottom-of-stack bit
  2. The TTL field Correct answer.
  3. The label value
  4. The EXP field

Explanation

The TTL field is decremented at each label switch hop, mirroring IP TTL behaviour, so a looping packet eventually expires. The other fields do not perform loop suppression.

Question 15IP routing, switching and aggregation

What is the role of a route reflector in a large MPLS core running iBGP?

  1. To terminate customer LSPs at the core
  2. To impose labels on behalf of the provider edge routers
  3. To reflect VPN and IPv4 routes among clients so a full iBGP mesh is not required Correct answer.
  4. To perform penultimate hop popping for all prefixes

Explanation

A route reflector reduces the number of iBGP sessions by reflecting routes from clients to other clients. It distributes control-plane information rather than forwarding customer traffic.

Question 16IP routing, switching and aggregation

Which statement best describes the difference between LDP and RSVP-TE in an MPLS core?

  1. LDP and RSVP-TE are identical in path selection behaviour
  2. LDP reserves bandwidth, while RSVP-TE only distributes prefixes
  3. LDP follows the IGP shortest path, while RSVP-TE can signal explicit paths with reserved bandwidth Correct answer.
  4. LDP requires traffic engineering extensions, while RSVP-TE does not

Explanation

LDP builds LSPs that follow the IGP best path without resource reservation, whereas RSVP-TE can signal constraint-based explicit paths and reserve bandwidth. This makes RSVP-TE suitable for traffic engineering.

Question 17IP routing, switching and aggregation

An operator wants to reduce the label stack depth for a VPN service by using a single label to identify both the egress PE and the VRF. Which approach achieves this?

  1. Using two transport labels for redundancy
  2. Using per-VRF label allocation with a single label to identify the VRF at the egress Correct answer.
  3. Using implicit null for the VPN label
  4. Using penultimate hop popping on the VPN label

Explanation

Allocating a label per VRF lets the egress identify the customer context from that label alone, so the transport label plus one VPN label is sufficient. This is the standard single-label VPN forwarding model.

Question 18IP routing, switching and aggregation

A provider edge router receives a labelled packet whose top label maps to a VRF. What does the router do next?

  1. It swaps the label and forwards the packet into the core
  2. It pushes an additional label for the customer site
  3. It pops the label and performs an IP lookup in the identified VRF Correct answer.
  4. It drops the packet because labels cannot map to VRFs

Explanation

At the egress, the VPN label identifies the VRF, so the router removes the label and forwards the inner IP packet using that VRF's table. This is the final step of VPN label processing.

Question 19IP routing, switching and aggregation

Which factor most affects the distribution of traffic across member links in a link aggregation bundle?

  1. The hashing algorithm and the variety of flow header values Correct answer.
  2. The number of configured VRFs on the bundle
  3. The label stack depth of the MPLS packets
  4. The TTL value in the IP header

Explanation

Link aggregation uses a hash over selected header fields to choose a member link, so the algorithm and the entropy in those fields determine balance. Few flows with identical headers can still concentrate on one member.

Question 20IP routing, switching and aggregation

A network designer wants to advertise a summary route for customer prefixes into the core while keeping individual VPN routes hidden. Which technique applies?

  1. Disabling route reflection for the customer
  2. Using implicit null on each individual prefix
  3. Aggregating the prefixes and advertising the summary with the appropriate route target Correct answer.
  4. Advertising every prefix with a unique route target

Explanation

Aggregation reduces the number of routes advertised while the summary carries the route target needed for import by the correct VRFs. Individual prefixes remain internal to the edge.

Question 21IP routing, switching and aggregation

Which statement about the bottom-of-stack bit in the MPLS shim header is correct?

  1. It is set to one to indicate that the label is implicit null
  2. It is set to one to indicate that the following label is the last in the stack Correct answer.
  3. It is set to one to indicate that the packet is expired
  4. It is set to one to indicate that the EXP field is valid

Explanation

The bottom-of-stack bit marks the final label in the stack, telling the router that the next header is the payload. It has no relation to expiry, implicit null, or EXP validity.

Question 22IP routing, switching and aggregation

What is the main benefit of segment routing with MPLS data plane compared to LDP in a provider core?

  1. It forces all traffic onto a single path
  2. It eliminates the need for any IGP in the core
  3. It encodes the path in the packet header, reducing per-flow state in the core Correct answer.
  4. It requires more label distribution sessions than LDP

Explanation

Segment routing places the path as a list of segments in the packet header, so core routers need less per-path state. This simplifies traffic engineering and improves scalability.

Question 23IP routing, switching and aggregation

Which mechanism ensures that a labelled packet is not forwarded out of the interface it arrived on in a looped topology?

  1. The bottom-of-stack bit
  2. The TTL decrement in the shim header Correct answer.
  3. The EXP field marking
  4. The route target attached to the prefix

Explanation

TTL is decremented at each label switch hop, so a looping packet eventually reaches zero and is discarded. This mirrors the loop suppression provided by IP TTL.

Question 24IP routing, switching and aggregation

A provider edge router must forward traffic for a customer site reachable through two equal-cost paths in the core. Which behaviour is expected with per-prefix label distribution?

  1. The router installs only one next hop and discards the other
  2. The router advertises the prefix with implicit null
  3. The router installs both next hops and load-balances using the label forwarding entries Correct answer.
  4. The router disables penultimate hop popping for the prefix

Explanation

When multiple equal-cost next hops exist, the label forwarding table can hold multiple entries and the router load-balances across them. This is analogous to equal-cost multipath in IP forwarding.

Question 25IP routing, switching and aggregation

Which statement about the label forwarding information base on a label switch router is correct?

  1. It maps route targets to VRFs
  2. It maps incoming labels to outgoing labels and next-hop interfaces Correct answer.
  3. It stores the customer MAC address table
  4. It stores only IP prefixes and their next hops

Explanation

The label forwarding information base is the data-plane structure that maps an incoming label to the outgoing label and interface. It is populated by the label distribution protocol.

Question 26IP routing, switching and aggregation

In an MPLS-enabled IP/MPLS network, which protocol is used to distribute labels for interior gateway protocol (IGP) prefixes between directly connected label switch routers?

  1. RSVP-TE
  2. PIM
  3. LDP Correct answer.
  4. BGP

Explanation

Label Distribution Protocol (LDP) is the standard protocol for distributing labels that map to IGP prefixes, such as loopback addresses, between neighboring label switch routers. It binds a label to each prefix in the routing table and advertises that binding to LDP peers. BGP distributes labels for VPN routes, RSVP-TE signals labels for explicitly routed traffic-engineered tunnels, and PIM is a multicast routing protocol.

Question 27IP routing, switching and aggregation

Which statement describes the purpose of the label forwarding information base (LFIB) on a label switch router?

  1. It maintains the list of LDP peers and their keepalive timers.
  2. It maps IP prefixes to MPLS labels for label imposition at the ingress.
  3. It stores the IP routing table used for unlabelled packet forwarding.
  4. It maps incoming labels to outgoing labels and next-hop interfaces for labelled packet forwarding. Correct answer.

Explanation

The LFIB is the forwarding table consulted for labelled packets; each entry maps an incoming label to an outgoing label and next-hop interface. The IP routing table (RIB) and forwarding table (FIB) handle unlabelled IP forwarding, while the label information base (LIB) holds label bindings received from peers. LDP peer and timer information is maintained by the LDP session state, not the LFIB.

Question 28IP routing, switching and aggregation

What is the primary function of an interior gateway protocol (IGP) such as IS-IS or OSPF in an MPLS service provider core?

  1. To compute the shortest paths and advertise loopback addresses that serve as MPLS LSP endpoints. Correct answer.
  2. To signal traffic-engineered label switched paths with bandwidth reservations.
  3. To distribute customer VPN routes between provider edge routers.
  4. To carry MPLS labels between directly connected routers.

Explanation

The IGP provides intra-domain IP reachability, including the loopback addresses used as the endpoints of MPLS label switched paths. It computes shortest paths and populates the routing table that LDP and RSVP-TE rely on. Customer VPN routes are distributed by multiprotocol BGP, RSVP-TE performs traffic-engineered signalling, and LDP distributes labels.

Question 29IP routing, switching and aggregation

Which field in the MPLS shim header identifies the label value used for forwarding decisions?

  1. EXP
  2. TTL
  3. Label Correct answer.
  4. S

Explanation

The 20-bit Label field carries the label value that the LFIB uses to determine the outgoing label and next hop. The EXP field is used for class of service, the S bit indicates the bottom of the label stack, and the TTL field supports loop prevention and traceroute. Only the Label field directly drives the forwarding lookup.

Question 30IP routing, switching and aggregation

Which statement about the penultimate hop popping (PHP) behavior in an MPLS network is correct?

  1. The penultimate hop router pops the top label before forwarding the packet to the egress router. Correct answer.
  2. The ingress router pops the top label before forwarding the packet into the core.
  3. The penultimate hop router pushes an additional label before forwarding the packet to the egress router.
  4. The egress router pops the top label before forwarding the packet to the customer.

Explanation

With penultimate hop popping, the router before the egress LSR removes the top label so that the egress receives an unlabelled packet or a packet with only the remaining labels. This reduces the label processing burden on the egress router. The egress router may still pop a label in other configurations, but PHP specifically refers to the penultimate hop performing the pop.

Question 31IP routing, switching and aggregation

Which protocol is used to distribute labels for VPN routes between provider edge routers in an MPLS Layer 3 VPN?

  1. OSPF
  2. LDP
  3. MP-BGP Correct answer.
  4. RSVP-TE

Explanation

Multiprotocol BGP (MP-BGP) is used to distribute VPN-IPv4 routes, including their route targets and MPLS labels, between provider edge routers. LDP distributes labels for IGP prefixes, RSVP-TE signals traffic-engineered LSPs, and OSPF is an IGP for intra-domain routing. The VPN label is carried in the MP-BGP update.

Question 32IP routing, switching and aggregation

What is the purpose of the route target extended community in an MPLS Layer 3 VPN?

  1. It identifies the label used for forwarding VPN traffic.
  2. It specifies the next-hop address for VPN routes.
  3. It defines the quality of service treatment for VPN traffic.
  4. It controls which VRFs import and export VPN routes. Correct answer.

Explanation

Route targets are BGP extended communities attached to VPN routes; a VRF imports routes whose route target matches its import policy and exports routes with its configured export route target. This controls the distribution of routes between VRFs. The label is carried separately in the MP-BGP update, the next hop is a BGP attribute, and QoS treatment is not defined by route targets.

Question 33IP routing, switching and aggregation

Which statement about the route distinguisher (RD) in an MPLS Layer 3 VPN is correct?

  1. It selects the label switch path used to reach the remote provider edge router.
  2. It is prepended to the customer IPv4 prefix to create a unique VPN-IPv4 route. Correct answer.
  3. It determines which customer sites can communicate with each other.
  4. It defines the label value assigned to the customer prefix.

Explanation

The route distinguisher is an 8-byte value prepended to the IPv4 prefix to form a 12-byte VPN-IPv4 address, making otherwise identical customer prefixes unique in BGP. Route targets, not the RD, control which sites can communicate. The RD does not select an LSP or define a label value.

Question 34IP routing, switching and aggregation

A network engineer is configuring a link aggregation bundle between two routers in an MPLS core. Which statement about the hashing used for load balancing across member links is correct?

  1. The hash is based only on the destination MAC address of the frame.
  2. The hash is based on the BGP next-hop attribute of the packet.
  3. The hash typically includes source and destination IP addresses and may include MPLS label information. Correct answer.
  4. The hash always uses the MPLS EXP field as the sole input.

Explanation

Link aggregation load balancing typically uses a hash of source and destination IP addresses, and on MPLS-enabled interfaces the hash may also include label stack information to improve distribution. Using only the destination MAC address would be insufficient for IP traffic. The EXP field and BGP attributes are not the primary inputs to the member-link hash.

Question 35IP routing, switching and aggregation

A router receives an MPLS packet with a two-label stack. The top label is 100 and the bottom label is 200. The LFIB entry for label 100 specifies a swap to label 300 and an outgoing interface. What happens to the packet?

  1. The packet is dropped because the bottom label is not in the LFIB.
  2. Both labels are swapped and the packet is forwarded with labels 300 and 400.
  3. The top label is swapped to 300 and the packet is forwarded with labels 300 and 200. Correct answer.
  4. The top label is popped and the packet is forwarded with label 200 only.

Explanation

The LFIB lookup uses the top label (100). The swap operation replaces the top label with 300 while the remaining label stack, including label 200, is preserved. The bottom label is not inspected at this hop, and only the top label is swapped according to the LFIB entry.

Question 36IP routing, switching and aggregation

Which mechanism allows an MPLS network to support multiple customer VRFs over a shared core while keeping their routes separate?

  1. OSPF with multiple areas per customer
  2. LDP with per-VRF label spaces on the P routers
  3. MP-BGP with route targets and VRFs on the PE routers Correct answer.
  4. RSVP-TE with per-customer bandwidth reservations

Explanation

MP-BGP distributes VPN-IPv4 routes with route targets, and each PE router maintains separate VRFs that import and export routes based on those targets. This keeps customer routing tables separate over a shared core. LDP, OSPF, and RSVP-TE do not provide per-VRF route separation by themselves.

Question 37IP routing, switching and aggregation

Which statement about the label information base (LIB) on an LSR is correct?

  1. It stores label bindings received from LDP peers and locally assigned labels before they are selected for forwarding. Correct answer.
  2. It is the forwarding table used to forward labelled packets.
  3. It holds the RSVP-TE reservation state for traffic-engineered tunnels.
  4. It contains the IP routing table for unlabelled traffic.

Explanation

The LIB holds label bindings learned from LDP and locally assigned labels; the best binding is then installed in the LFIB for forwarding. The LFIB is the forwarding table for labelled packets, the IP routing table handles unlabelled traffic, and RSVP-TE state is maintained separately.

Question 38IP routing, switching and aggregation

A network operator wants to reduce the label stack depth for a VPN service by using a single label to identify both the egress PE and the customer VRF. Which mechanism achieves this?

  1. Segment routing with a single segment identifier
  2. Label stacking with two labels
  3. Per-VRF label allocation with implicit null for the egress PE Correct answer.
  4. Penultimate hop popping with a single label

Explanation

When the egress PE allocates a per-VRF label and the penultimate hop pops the transport label, the egress receives a single label that identifies both the PE and the VRF. This is sometimes called a single-label VPN or per-VRF label allocation. PHP alone does not reduce the stack to one label unless the VPN label is used, and segment routing uses a different label stack model.

Question 39IP routing, switching and aggregation

Which factor most affects the distribution of traffic across member links in a link aggregation bundle carrying MPLS traffic?

  1. The hash algorithm and the fields it includes, such as IP addresses and label stack information. Correct answer.
  2. The OSPF cost of the bundle interface.
  3. The number of LDP sessions configured on the bundle.
  4. The BGP local preference of the routes.

Explanation

Traffic distribution across member links depends on the hash algorithm and which packet fields are included in the hash, such as source and destination IP addresses and, for MPLS, label information. LDP session count, OSPF cost, and BGP local preference affect routing and label distribution but not per-flow distribution across member links.

Question 40IP routing, switching and aggregation

A network designer wants to advertise a summary route for customer prefixes into the core while keeping individual VPN routes hidden. Which approach is appropriate?

  1. Use route targets to summarize VPN routes automatically.
  2. Configure LDP to aggregate labels for customer prefixes.
  3. Use RSVP-TE to summarize VPN routes.
  4. Configure a static route to a summary prefix and redistribute it into the IGP, while keeping VPN routes in MP-BGP. Correct answer.

Explanation

A summary route can be advertised into the IGP using a static route or an IGP summary, while the individual VPN routes remain in MP-BGP and are not leaked into the core IGP. Route targets do not summarize routes, LDP does not aggregate labels, and RSVP-TE does not summarize routes.

Question 41IP routing, switching and aggregation

Which statement about the bottom-of-stack (S) bit in the MPLS shim header is correct?

  1. It is set to 1 in every label entry to indicate that the stack is valid.
  2. It is set to 1 in the label entry that is at the bottom of the stack, indicating that no more labels follow. Correct answer.
  3. It is used to indicate the TTL value of the packet.
  4. It is used to indicate the EXP value of the packet.

Explanation

The S bit is set to 1 in the bottom label entry to indicate that it is the last label in the stack. The EXP and TTL fields are separate fields in the shim header. The S bit is not set in every entry; it is only set in the bottom entry.

Question 42IP routing, switching and aggregation

Which statement about the label forwarding information base (LFIB) on a label switch router is correct?

  1. It contains the LDP session state and keepalive timers.
  2. It contains entries for incoming labels, outgoing labels, and next-hop interfaces, and is used for labelled packet forwarding. Correct answer.
  3. It contains the BGP routing table for VPN routes.
  4. It contains only IP prefixes and is used for unlabelled packet forwarding.

Explanation

The LFIB is the forwarding table for labelled packets; each entry maps an incoming label to an outgoing label and next-hop interface. IP prefixes are in the IP routing table, LDP session state is maintained by the LDP process, and the BGP routing table is separate from the LFIB.

Question 43IP routing, switching and aggregation

A network engineer is troubleshooting an MPLS core where LDP sessions are up but labels are not being installed for a loopback prefix. Which step is most useful to verify the label binding?

  1. Check the LIB for the prefix and confirm that a label binding has been received and selected. Correct answer.
  2. Check the BGP table for the prefix and confirm the next-hop attribute.
  3. Check the RSVP-TE reservation state for the prefix.
  4. Check the OSPF link-state database for the prefix.

Explanation

The LIB holds label bindings received from LDP peers; verifying that a binding exists and has been selected confirms whether label installation should occur. BGP and RSVP-TE are not involved in LDP label distribution for IGP prefixes, and the OSPF database confirms reachability but not label binding.

Question 44IP routing, switching and aggregation

A router receives an MPLS packet with a three-label stack. Which label is examined first for forwarding?

  1. The bottom label of the stack.
  2. The top label of the stack. Correct answer.
  3. All three labels are examined simultaneously.
  4. The middle label of the stack.

Explanation

MPLS forwarding always examines the top label of the stack first; the LFIB lookup uses the top label to determine the next operation. Lower labels are not examined until the top label is popped. This is a fundamental MPLS forwarding rule.

Question 45IP routing, switching and aggregation

In an IP/MPLS core, a router uses an equal-cost multipath (ECMP) next-hop set for a loopback prefix reachable through four parallel links. Which statement describes how MPLS-labeled traffic for that prefix is normally load-shared across the next hops?

  1. The router transmits every labeled packet on all next hops simultaneously and lets the egress router discard duplicates.
  2. The router hashes on fields of the exposed IP header, or on the label stack when no IP header is available, and selects one next hop per flow. Correct answer.
  3. The router assigns each label value to exactly one next hop and never uses the other next hops for that label.
  4. The router distributes packets strictly in round-robin order regardless of the label stack or payload contents.

Explanation

MPLS forwarding entries can inherit the ECMP next-hop set of the underlying IGP route, and per-packet selection is done with a hash over header fields so that a given flow stays on one path. Because the top label may not be the IP header, the hash uses the label stack when the payload is not inspectable, which preserves packet ordering within a flow.

Question 46IP routing, switching and aggregation

Which statement describes the role of a loopback interface address on a router in an IP/MPLS core when that address is advertised by the IGP?

  1. It is assigned to every physical interface so that each link shares the same label space.
  2. It is advertised only to external peers through eBGP and is hidden from the interior gateway protocol.
  3. It provides a stable, always-up address used as the LDP transport address and as the BGP next hop for services. Correct answer.
  4. It is used only as the OSPF or IS-IS router identifier and is never placed in the forwarding table.

Explanation

A loopback is logically always up, so advertising it in the IGP gives the router a stable address that survives link failures. LDP commonly uses that address as its transport address, and iBGP commonly uses it as the next hop, keeping label-switched paths and service routing resilient.

Question 47IP routing, switching and aggregation

Two provider edge routers are connected by a bundle of four 10 Gb/s member links. A single large video flow traverses the bundle and is hashed to one member link, which becomes congested while the other members are lightly loaded. Which statement explains this behavior?

  1. MPLS encapsulation forces all labeled traffic onto the first member link of the bundle.
  2. Link aggregation hashing is per flow, so a single high-rate flow cannot be spread across multiple member links. Correct answer.
  3. The hash uses only the destination MAC address, which is identical for every packet in the bundle.
  4. The bundle is misconfigured because a correctly formed bundle always splits each flow across all members.

Explanation

Link aggregation distributes traffic by hashing flow-identifying fields onto member links, so each flow is pinned to one member for its lifetime. A single flow that exceeds one member's capacity will congest that link even when other members have spare bandwidth. Spreading one flow requires higher-rate members or a multipath scheme at Layer 3, not the bundle itself.

Question 48IP routing, switching and aggregation

An engineer is planning a link aggregation bundle between two core routers that carry MPLS-encapsulated traffic. Which statement describes the requirement for the member links of the bundle?

  1. Each member link must be placed in a different OSPF area to prevent routing loops across the bundle.
  2. The member links must use different MPLS label values for the same destination to keep traffic separated.
  3. Only one member link may carry MPLS traffic; the others are restricted to untagged IP forwarding.
  4. The member links must be configured identically for speed, duplex, and MTU so that hashing and forwarding remain consistent. Correct answer.

Explanation

Member links of an aggregation bundle must match in link characteristics such as speed, duplex, and MTU, because traffic is hashed across them and inconsistent parameters cause forwarding and MTU problems. The bundle is treated as one logical interface by the IGP and by MPLS, so per-member differences are undesirable.

Question 49IP routing, switching and aggregation

A router receives an MPLS packet whose top label maps in the LFIB to an outgoing interface and a new top label, with no IP header visible. Which statement describes what the router does with the packet?

  1. It forwards the packet unchanged on all interfaces that have a route to the label's origin.
  2. It discards the packet because an IP header must always be present for forwarding decisions.
  3. It swaps the top label for the outgoing label and forwards the packet on the indicated interface without examining an IP header. Correct answer.
  4. It pops the top label and performs an IP lookup on the destination address in the payload.

Explanation

An LFIB entry maps an incoming label to an outgoing interface and a replacement label, so the router performs a label swap and forwards without IP lookup. This label-swapping behavior is what allows the core to forward solely on the shim header. The IP header is examined only at the edges or where the label stack is exhausted.

Question 50IP routing, switching and aggregation

A service provider wants the IGP to carry only the loopback addresses of core routers while still allowing BGP to resolve next hops for VPN and internet prefixes. Which design choice supports this requirement?

  1. Disable next-hop resolution on the provider edge routers so that BGP prefixes are installed without recursion.
  2. Advertise customer prefixes in the IGP and use BGP only to carry the loopback addresses between peers.
  3. Enable BGP next-hop resolution against IGP routes for the loopbacks and advertise customer prefixes only in BGP. Correct answer.
  4. Redistribute all customer prefixes into IS-IS or OSPF so that every prefix has an IGP label.

Explanation

Keeping customer prefixes out of the IGP limits IGP scale and churn, while BGP next hops remain resolvable because the loopbacks are still carried by the IGP. Labeled forwarding for the BGP next hop then relies on the IGP's label binding for the loopback.