350-501 MPLS and Segment Routing Practice Test 9 — 15 Questions

Question 1

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

Accepted Answer

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

Answer

Push

Answer

Pop

Answer

Replace

Question 2

Which LDP label distribution mode is characterized by a router advertising labels for all prefixes in its routing table to all neighbors, regardless of whether they are needed?

Accepted Answer

Downstream unsolicited with liberal label retention. Downstream unsolicited means the router sends label bindings without being asked; liberal label retention means it keeps all bindings even if not from the next hop.

Answer

Downstream on demand with conservative label retention

Answer

Downstream on demand with liberal label retention

Answer

Downstream unsolicited with conservative label retention

Question 3

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 4

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

Answer

Bypass tunnel protecting multiple LSPs

Answer

Node protection bypass tunnel

Question 5

In Segment Routing over MPLS (SR-MPLS), how are Prefix-SIDs advertised in IS-IS?

Accepted Answer

Via IS-IS TLV 135 and TLV 149. In SR-MPLS, Prefix-SIDs are advertised in IS-IS using TLV 135 (Extended IP Reachability) to carry the prefix and its metric, and TLV 149 (IS-IS Router Capability) to carry the SID/Label Binding sub-TLV, which maps the prefix to a specific label (Prefix-SID). This allows IS-IS to distribute both the prefix reachability and its associated segment identifier in a single protocol update, enabling source routing without LDP or RSVP-TE.

Answer

Via LDP label mapping messages

Answer

Via RSVP-TE PATH messages

Answer

Via BGP VPN labels

Question 6

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

Accepted Answer

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

Answer

15000-25000

Answer

16000-24000

Answer

16-23999

Question 7

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 8

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. Each SID in the list is a node SID (Prefix-SID) of intermediate routers, forming a strict hop-by-hop path.

Answer

A list of labels for an RSVP-TE LSP

Answer

A list of adjacency SIDs for each link

Answer

A list of anycast SIDs for redundancy

Question 9

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 10

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

Accepted Answer

End.DT4. End.DT2U is for L2VPN; for IRB, the router uses End.DT4/DT6 for IP forwarding and then bridges to the segment; however, the question asks for the SID behavior that enables IRB. Usually, IRB is achieved by combining a Layer 2 VPN SID and a Layer 3 VPN SID, but for the specific behavior that forwards between IP and MAC, it's often End.DT4 or End.DT6 plus a MAC lookup. But standard SRv6 IRB uses a dedicated SID like End.DT2U? Actually, IRB requires both routing and bridging; the common approach is to use a Layer 3 VPN SID for the routed part and an EVPN Ethernet segment for bridging. In SRv6, the endpoint behavior for IRB is typically End.DT4 or End.DT6 for the routing side, and for bridging, it's End.DT2U. But the question expects a single behavior? Perhaps End.DT2U is for L2VPN only. Let's think: Cisco documentation often describes 'End.DT2U' for L2VPN, and 'End.DT4' for L3VPN. For IRB, the service is provided by a combination. However, a specific SID behavior called 'End.DT2U' can be used for EVPN VPWS. To align with common SIDs, the answer is End.DT2U for L2VPN IRB? Actually, IRB typically uses a Layer 3 gateway SID. I'll choose End.DT4 as it routes IP, and then the router bridges locally. But the question says 'forward packets between the IP fabric and the Ethernet segment' - that is routing. So End.DT4 is correct.

Answer

End

Answer

End.DT2U

Answer

End.X

Question 11

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 incoming labels to outgoing labels and interfaces.

Answer

FIB

Answer

LIB

Answer

RIB

Question 12

Which RSVP reservation style is most commonly used in MPLS-TE to allow sharing of resources among multiple LSPs of the same session?

Accepted Answer

Shared Explicit (SE). Shared Explicit (SE) style allows multiple senders to share a reservation, which is typical for MPLS-TE where the headend may have multiple LSPs to the same destination.

Answer

Wildcard Filter (WF)

Answer

Distinct Filter (DF)

Answer

Fixed Filter (FF)

Question 13

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

Accepted Answer

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

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.

Question 14

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

Accepted Answer

Aliasing allows other PEs to send traffic to any PE in the multi-homed group.. EVPN multi-homing uses ESI, DF election for BUM traffic, and aliasing for load balancing.

Answer

Split horizon prevents loops by blocking traffic from the same ES.

Answer

MAC mobility is provided by multi-homing.

Question 15

Which TWO statements about SR-MPLS compared to LDP are correct? (Choose two.)

Accepted Answer

SR-MPLS uses a global label block (SRGB) that is consistent across the domain.. SR-MPLS uses a Segment Routing Global Block (SRGB), which is a reserved label range (e.g., 16000–23999 per RFC 8660) that is configured consistently across all routers in the SR domain. This global consistency allows any router to interpret a label from the SRGB as the same topological or service instruction, eliminating the need for per-hop label negotiation.

Answer

SR-MPLS requires LDP for label distribution.

Answer

SR-MPLS supports only dynamic label allocation.

Answer

SR-MPLS requires RSVP-TE for TE tunnels.