350-501 MPLS and Segment Routing Practice Test 12 — 15 Questions

Question 1

In SRv6, what is the function of the 'End.DT4' behavior?

Accepted Answer

Decapsulation and IPv4 lookup in a VRF table. End.DT4 is an SRv6 endpoint behavior that decapsulates an IPv4 packet from an SRv6 header and performs an IPv4 lookup in a specified VRF table. It is used for IPv4 VPN services, allowing IPv4 payloads to be forwarded based on the VRF routing table. This behavior is defined in RFC 8986 and is essential for interworking between SRv6 and IPv4 VPNs.

Answer

Decapsulation and IPv6 lookup in a VRF table

Answer

Decapsulation and Ethernet switching

Answer

Encapsulation and forwarding to an IPv6 destination

Question 2

In MPLS-TE Fast Reroute (FRR), what is a 'detour'?

Accepted Answer

A backup LSP that protects a single primary LSP. In MPLS-TE Fast Reroute (FRR), a detour is a pre-established backup Label Switched Path (LSP) that protects a single primary LSP. It is created by the Point of Local Repair (PLR) and merges back into the primary LSP downstream of the protected link or node. Detours are 1:1 protection, meaning each primary LSP has its own dedicated detour, which allows for sub-50ms recovery upon failure.

Answer

A secondary path for load balancing

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A bypass tunnel that protects multiple LSPs

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A mechanism to reroute traffic after failure detection

Question 3

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

Accepted Answer

DF Election. Designated Forwarder (DF) Election is the EVPN mechanism defined in RFC 7432 that elects one PE per Ethernet Segment (identified by ESI) to forward broadcast, unknown unicast, and multicast (BUM) traffic toward the CE. This prevents duplicate frame delivery to a multi-homed CE when multiple PEs are attached to the same segment. The election is based on a deterministic algorithm using the ESI and PE addresses, and the DF role can be per-VLAN or per-EVI.

Answer

Split Horizon

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Aliasing

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ESI (Ethernet Segment Identifier)

Question 4

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

Accepted Answer

To provide a global label space for prefix SIDs. The SRGB (Segment Routing Global Block) is a reserved range of MPLS labels that a node uses for global segments, specifically prefix SIDs. Each node advertises its SRGB, and prefix SIDs are computed as SRGB base + index, ensuring globally unique label values across the SR domain. This provides a consistent global label space for prefix segments.

Answer

To define the range of labels used for LDP

Answer

To reserve labels for MPLS-TE tunnels

Answer

To allocate labels for adjacency SIDs

Question 5

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

Accepted Answer

It allows faster convergence after a topology change.. Option B is correct because liberal label retention keeps labels received from all LDP neighbors in the LIB, so if a topology change causes a new next hop to be selected, the label for that neighbor is already available and forwarding can be restored without waiting for a new label mapping, enabling faster convergence. Option D is correct because liberal label retention is defined by retaining all received label bindings from every LDP neighbor, even if those labels are not currently used in the LFIB for forwarding. Option A is incorrect because liberal label retention is the default LDP behavior in Cisco IOS and does not require explicit configuration of the retention mode. Option C is incorrect because conserving memory by storing labels only for next-hop neighbors describes conservative label retention, not liberal retention. Option E is incorrect because manual label filtering is not required for liberal label retention; it is an optional control mechanism and not a defining requirement.

Answer

It requires explicit configuration of the label retention mode in Cisco IOS.

Answer

It conserves memory by only storing labels for next-hop neighbors.

Answer

It requires manual configuration of label filtering.

Question 6

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

Accepted Answer

They are defined at the headend router with a destination and color.. Option A is correct because an SR-TE policy is instantiated at the headend router, where it is uniquely identified by a tuple of destination (the endpoint/prefix) and color (an extended community used to map the policy to a intent/constraint), which together select the candidate path. Option B is correct because SR-TE encodes the forwarding path as an ordered segment list (SID list) of MPLS labels or SRv6 SIDs carried in the packet header, rather than relying on hop-by-hop signaling. Option D is correct because SR-TE policies are used precisely to steer selected traffic onto a path that satisfies SLA requirements such as low latency, jitter, or bandwidth by binding a color to a constrained candidate path. Option C is incorrect because SR-TE does not require RSVP-TE signaling; it leverages source routing with segment lists and can use PCEP or BGP-LS for path computation, avoiding RSVP-TE state maintenance. Option E is incorrect because SR-TE keeps state only at the headend (the policy and SID list); intermediate routers simply process the segment list in the packet header without maintaining per-flow state.

Answer

They require RSVP-TE signaling to maintain state.

Answer

They require per-flow state at intermediate routers.

Question 7

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

Accepted Answer

Eliminates the need for pseudowire configuration on each PE.. Option B is correct because EVPN-VPWS uses BGP EVPN to auto-discover and signal the pseudowire (Ethernet A-D route with the VPWS service ID), so the service is provisioned once and the remote PE/PW endpoint is learned dynamically rather than manually configuring a pseudowire on every PE as with traditional LDP-based VPWS. Option C is correct because EVPN-VPWS supports Ethernet Segment (ESI) based active/active multi-homing, using DF election and aliasing to load-balance across multiple access links without needing spanning tree to prevent loops. Option A is not a defining benefit of EVPN-VPWS, since it does not mandate MPLS-TE for path protection; transport can be plain MPLS LSPs or SR. Option D is wrong because EVPN-VPWS is specifically a point-to-point service, so 'supports only point-to-point' is a limitation, not a benefit over traditional VPWS, which is also point-to-point. Option E is wrong because EVPN-VPWS uses BGP EVPN for label/signaling rather than LDP, so requiring LDP is not a benefit.

Answer

Uses MPLS-TE tunnels for path protection.

Answer

Supports only point-to-point topologies.

Answer

Requires LDP for label distribution.

Question 8

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

Accepted Answer

Swap. In MPLS label operations, 'swap' is the term for replacing the top label in the stack with a new label as a packet is forwarded. The incoming label is looked up in the LFIB, and the outgoing label replaces it before the packet is sent to the next hop. This is the standard operation performed by transit Label Switch Routers (LSRs) along an LSP.

Answer

Push

Answer

Pop

Answer

Replace

Question 9

In RSVP-TE, which message is sent by the headend router along the explicit route to reserve resources and establish an LSP?

Accepted Answer

PATH. In RSVP-TE, the headend router sends a PATH message along the explicit route to signal the desired LSP. This message carries the label request, traffic parameters, and the explicit route object (ERO) to each downstream router, which then forwards it hop-by-hop toward the destination. Only after the PATH message reaches the tailend does the tailend respond with a RESV message to actually reserve resources and establish the LSP.

Answer

RESV

Answer

RSVP Hello

Answer

RSVP Tear

Question 10

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

Accepted Answer

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

Answer

Shared mesh protection

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Bypass tunnel protecting multiple LSPs

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Node protection bypass tunnel

Question 11

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

Accepted Answer

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

Answer

End.DT2U

Answer

End

Answer

End.X

Question 12

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

Accepted Answer

A list of node SIDs representing the path through routers with those SIDs. In Segment Routing Traffic Engineering (SR-TE), an SR Policy's explicit candidate path contains a segment list composed of SIDs. The values 16001, 16003, and 16005 fall in the typical SRGB range for node SIDs (Prefix-SIDs), meaning each represents a router along the intended path. The headend prepends this ordered list as an MPLS label stack so the packet traverses exactly those nodes in sequence.

Answer

A list of labels for an RSVP-TE LSP

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A list of adjacency SIDs for each link

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A list of anycast SIDs for redundancy

Question 13

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

Accepted Answer

It prunes links that do not satisfy constraints and then runs SPF on the remaining topology.. CSPF computes a path that meets constraints (bandwidth, affinity, etc.) and avoids links/ nodes not meeting them.

Answer

It distributes labels to LDP neighbors.

Answer

It calculates the shortest path based solely on IGP metric.

Answer

It signals RSVP-TE PATH messages.

Question 14

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

Accepted Answer

LFIB. The LFIB (Label Forwarding Information Base) is the MPLS forwarding table that maps an incoming label to an outgoing label and outgoing interface. It is populated from the LIB (control plane) and used by the data plane to forward labeled packets. The FIB is for IP forwarding, and the RIB is the routing table, so LFIB is the correct answer.

Answer

FIB

Answer

LIB

Answer

RIB

Question 15

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

Accepted Answer

At the ingress LSR, a label is pushed onto the packet.. Option B is correct because the ingress LSR (ingress edge router of the MPLS domain) imposes a label on the incoming IP packet, an operation known as a label push, before forwarding it into the MPLS core. Option E is correct because an intermediate LSR (transit router) performs a label swap, replacing the incoming label with the outgoing label found in its LFIB (Label Forwarding Information Base) entry for that FEC. Options A, C, and D are incorrect: label operations in the MPLS core are driven by the incoming label value, not the IP destination address; the ingress LSR pushes rather than pops labels; and the egress LSR pops (removes) the label rather than swapping it.

Answer

Label operations are performed based on the IP destination address.

Answer

At the ingress LSR, the label is popped.

Answer

At the egress LSR, a label is swapped.