350-501 MPLS and Segment Routing Practice Test 3 — 15 Questions

Question 1

A network engineer is designing an MPLS core using Segment Routing. They want to minimize label stack depth while still providing fast convergence using TI-LFA. Which prefix-SID assignment strategy should be used?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Advertise prefix-SIDs for all IGP prefixes. Advertising prefix-SIDs for all IGP prefixes allows TI-LFA to compute backup paths using any prefix in the network, not just loopbacks. This minimizes label stack depth by enabling TI-LFA to use a single prefix-SID for the repair path, avoiding the need for additional labels or explicit path constructs. Fast convergence is achieved because TI-LFA precomputes a backup next-hop that can be activated immediately upon failure, and having all prefixes as SIDs ensures the backup path can be expressed with minimal label imposition.

Answer

Assign a node-SID to each loopback interface

Answer

Use per-interface label mode from SRGB

Answer

Allocate labels per-VRF on PE routers

Question 2

Refer to the exhibit. An engineer notices that R1 has an LDP neighbor but 'show mpls forwarding-table' on R1 shows no label bindings for prefixes learned from R2. What is the most likely cause?

Accepted Answer

R1 is configured for 'label distribution on-demand'. R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.

Answer

The LDP discovery source is incorrect

Answer

The peer LDP identifier is not reachable

Answer

LDP session is not established

Question 3

A service provider is migrating from LDP to Segment Routing in an IS-IS network. After enabling IS-IS with segment-routing on all routers, they observe that some prefixes still receive labels from LDP instead of from SR. Which configuration is most likely missing on these routers?

Accepted Answer

Missing 'segment-routing mpls' command under IS-IS. When migrating from LDP to Segment Routing in an IS-IS network, the 'segment-routing mpls' command must be explicitly enabled under the IS-IS routing process. Without this command, IS-IS will not allocate MPLS labels for prefixes using the Segment Routing (SR) extension, causing the router to fall back to LDP for label distribution. This is the most common missing configuration when SR labels are not being assigned.

Answer

OSPF segment-routing is still configured

Answer

TI-LFA is not enabled

Answer

SRGB range conflicts with MPLS label range

Question 4

An engineer is configuring an MPLS Traffic Engineering tunnel using RSVP-TE. The tunnel is intended to carry voice traffic with strict delay requirements. Which RSVP-TE object should be used to enforce a maximum delay path?

Accepted Answer

Use the 'path-option' with 'metric delay' under the tunnel. The 'path-option with metric delay' command instructs RSVP-TE to compute a path that minimizes cumulative delay rather than IGP cost or hop count. This directly enforces a maximum delay constraint for voice traffic by selecting the path with the lowest measured or configured delay metric across the MPLS network.

Answer

Enable 'record-route' on the tunnel

Answer

Configure 'fast-reroute' protection

Answer

Specify an explicit-path (ERO) manually

Question 5

A network operator is deploying Segment Routing with TI-LFA across an OSPF network. After configuring OSPF with 'segment-routing mpls' and 'fast-reroute per-prefix', they notice that backup paths are not being installed for some prefixes. 'Show ip ospf segment-routing protected' shows 'No FRR backup' for those prefixes. What is a possible reason?

Accepted Answer

Adjacency-SIDs are not configured on neighboring routers. TI-LFA (Topology Independent Loop-Free Alternate) for per-prefix fast-reroute in OSPF requires that adjacency-SIDs be configured on neighboring routers. Without adjacency-SIDs, OSPF cannot compute the necessary repair paths to provide backup protection, resulting in 'No FRR backup' for those prefixes.

Answer

LDP label distribution is still active

Answer

The prefix-SID index is not globally unique

Answer

OSPF does not support TI-LFA for per-prefix prefixes

Question 6

A large enterprise uses MPLS L3VPN to connect multiple sites. They want to implement inter-AS option B (ASBR-to-ASBR MP-eBGP) for scalability. Which statement about this model is correct?

Accepted Answer

ASBRs exchange VPNv4 routes using MP-eBGP. In Inter-AS Option B, ASBRs use MP-eBGP to exchange VPNv4 routes. They do not maintain VRFs (Option A incorrect), and sessions are per address family, not per VRF (Option C incorrect). Option B is incorrect because a full mesh of MP-iBGP between PEs is not required. Option D correctly describes the VPNv4 route exchange using MP-eBGP.

Answer

Each ASB router maintains VRFs for all attached customers

Answer

Inter-AS option B requires a full mesh of MP-iBGP between all PEs

Answer

MP-eBGP sessions are established per VRF between ASBRs

Question 7

An MPLS core network uses LDP to distribute labels. An engineer wants to verify that the LDP session between two routers is up and exchanging labels. Which command should be used?

Accepted Answer

show mpls ldp neighbor. The command 'show mpls ldp neighbor' displays the status of LDP sessions, including the neighbor's IP address, session state (e.g., Operational), and label exchange activity. Since LDP is the protocol used to distribute labels in this MPLS core network, this command directly verifies that the session is up and exchanging labels between the two routers.

Answer

show mpls interfaces

Answer

show mpls l2transport binding

Answer

show mpls forwarding-table

Question 8

Which TWO statements correctly describe Segment Routing characteristics? (Select two.)

Accepted Answer

The path is encoded as a label stack at the source router. Segment Routing (SR) encodes the forwarding path as an ordered list of segment identifiers (SIDs) pushed onto a label stack at the source router. This source-routing paradigm allows the ingress node to specify the exact path through the network without requiring intermediate routers to maintain per-flow state.

Answer

All routers must be configured with the same SRGB value

Answer

SR eliminates all per-prefix state from core routers

Answer

Traffic engineering policies are distributed via BGP without any IGP extension

Question 9

Which THREE are valid methods to provide fast convergence in an MPLS/Segment Routing network? (Select three.)

Accepted Answer

Topology-Independent LFA (TI-LFA). B is correct because Topology-Independent LFA (TI-LFA) provides fast convergence in Segment Routing networks by computing a backup path that is guaranteed to be loop-free regardless of the network topology. It leverages the Segment Routing data plane to pre-install a repair path using a segment list, enabling sub-50ms failover without relying on IGP convergence.

Answer

BGP Prefix Independent Convergence (PIC)

Answer

IP Fast Reroute (IPFRR)

Question 10

Which TWO types of SIDs are defined in Segment Routing? (Select two.)

Accepted Answer

Adjacency-SID. Segment Routing (SR) defines two primary types of Segment Identifiers (SIDs): the Prefix-SID and the Adjacency-SID. A Prefix-SID is a global or local label associated with an IGP prefix (e.g., a loopback), enabling shortest-path forwarding toward that prefix. An Adjacency-SID is a local label associated with a specific adjacency (link) between two routers, forcing traffic out that exact interface regardless of the IGP shortest path.

Answer

TI-LFA SID

Answer

Service-SID

Answer

Node-SID

Question 11

Refer to the exhibit. An engineer is troubleshooting packet loss in the MPLS core. For prefix 10.3.3.0/24, the outgoing label is 'Untagged'. What does this indicate?

Accepted Answer

No MPLS label is being imposed on outgoing packets for this prefix. When the outgoing label for a prefix in the MPLS forwarding table shows 'Untagged', it means that for packets destined to that prefix, no MPLS label is imposed. The router will forward the packet as a standard IP packet (without an MPLS header) out the egress interface. This typically occurs when the next-hop router has signaled an implicit null label (label 3) via LDP, instructing the upstream router to pop the label stack and send the packet as IP.

Answer

The packet will have an implicit null label (label 3) imposed

Answer

The prefix is not reachable and packets will be dropped

Answer

The outgoing label is the same as the local label (label 18)

Question 12

Refer to the exhibit. An operator configures an SR-TE policy on a headend router. Which statement is true about the traffic steered into this policy?

Accepted Answer

The headend will impose a label stack containing labels 16000, 16001, and 16002. The SR-TE policy uses a segment-list with three labels (16000, 16001, 16002), which are MPLS labels assigned to prefix-SIDs for specific nodes. The headend imposes this label stack to steer traffic along the explicit path defined by the segment-list, ensuring strict source routing without relying on dynamic CSPF computation or LDP.

Answer

The segment-list consists of adjacency-SIDs

Answer

The path is automatically computed using CSPF based on IGP metrics

Answer

Traffic is forwarded using MPLS LDP labels

Question 13

A service provider is troubleshooting an MPLS L3VPN issue where a CE router is receiving the VPN route from the PE but cannot ping the remote CE's loopback. The PE shows that the VPN label is assigned but cannot route the packets. Which command would help determine if the remote PE is correctly resolving the BGP next-hop via IGP?

Accepted Answer

show ip route bgp. 'show ip route bgp', is correct because the issue is that the remote PE cannot route packets to the BGP next-hop of the VPN route. The CE receives the route, but the PE cannot forward packets, indicating a missing or incorrect IGP route to the next-hop address. This command displays BGP routes in the global routing table, allowing you to verify if the next-hop is reachable via IGP (e.g., OSPF or IS-IS) and if the recursive routing is successful.

Answer

show mpls ldp neighbor

Answer

show ip route vrf CUST

Answer

show mpls forwarding-table vrf CUST

Question 14

Which protocol is typically used to distribute MPLS labels between directly connected LSRs?

Accepted Answer

LDP. LDP (Label Distribution Protocol) is the standard protocol used to distribute MPLS labels between directly connected LSRs (Label Switch Routers) in a non-traffic-engineered MPLS network. It operates by establishing LDP sessions over TCP (port 646) and exchanging label mappings for each FEC (Forwarding Equivalence Class) based on the underlying IGP routing table, enabling hop-by-hop label switching without requiring explicit path setup.

Answer

RSVP

Answer

BGP

Answer

OSPF

Question 15

Which BGP address family must be used to exchange VPNv4 routes between PE routers in an MPLS L3VPN?

Accepted Answer

VPNv4 unicast. In an MPLS L3VPN, VPNv4 routes carry both the IPv4 prefix and the Route Distinguisher (RD) to ensure uniqueness across overlapping customer address spaces. The VPNv4 unicast address family (address-family ipv4 vpn) is the mandatory BGP address family used between PE routers to exchange these VPNv4 routes, enabling MPLS-based VPN reachability.

Answer

RT constraint

Answer

IPv4 unicast

Answer

L2VPN VPLS