350-501 MPLS and Segment Routing Practice Test 2 — 15 Questions

Question 1

A network operator is migrating from traditional MPLS LDP to Segment Routing (SR) with IS-IS. The network consists of four routers (R1-R4) in a square topology. The engineer has enabled SR on all routers and configured prefix-SIDs for loopbacks. However, when checking the MPLS forwarding table on R1, the engineer sees that some prefixes have label values that are not the prefix-SIDs. For example, the prefix for R4's loopback shows label 16004 instead of the expected 16004 (which is correct). But for another prefix, the label is 16003 instead of 16003 (correct). The engineer does not see any labels for some external routes. What is the most likely reason that some labels are missing?

Accepted Answer

External routes are not covered by prefix-SIDs; they require LDP or another label distribution mechanism.. Option D is correct because Segment Routing (SR) with IS-IS only assigns prefix-SIDs to prefixes that are part of the IS-IS domain and explicitly configured with a SID. External routes, such as those redistributed from another protocol (e.g., BGP or OSPF), are not covered by prefix-SIDs and require a separate label distribution mechanism like LDP or a manually configured explicit-null label to be forwarded with MPLS encapsulation. The engineer's observation that some labels are missing for external routes directly points to this limitation.

Answer

The IS-IS wide metrics are not enabled on all interfaces.

Answer

Route redistribution from another protocol is not configured.

Answer

The prefix-SIDs are inconsistent across routers.

Question 2

A service provider is deploying MPLS-TE with RSVP-TE in their core network. They notice that some LSPs are not being established due to resource contention. Which action would best address this issue without redesigning the entire traffic engineering deployment?

Accepted Answer

Enable preemption on RSVP-TE LSPs with appropriate priority levels.. Enabling preemption on RSVP-TE LSPs with appropriate setup and hold priorities allows higher-priority LSPs to tear down lower-priority LSPs to free up bandwidth, resolving resource contention without redesigning the entire TE deployment. This is the standard mechanism defined in RFC 3209 for managing bandwidth contention in MPLS-TE networks.

Answer

Increase the bandwidth of all core links.

Answer

Configure LSP path-option explicit paths with strict hops.

Answer

Disable RSVP-TE and use LDP for label distribution.

Question 3

Refer to the exhibit. An engineer is configuring Segment Routing for BGP (BGP-SR) on a PE router to assign labels to prefixes learned from a CE. The route-policy SET-LABEL is applied to the neighbor under the address-family ipv4 unicast. However, the CE prefix 10.1.1.0/24 is not receiving the label. What is the most likely reason?

Accepted Answer

The neighbor is missing 'send-community extended' under address-family ipv4 unicast.. For BGP-SR (Segment Routing for BGP) to advertise labels for prefixes learned from a CE, the neighbor must be configured with 'send-community extended' under the address-family. This is because BGP-SR uses the BGP Prefix-SID attribute, which is carried as an extended community. Without this command, the PE will not send the label information to the CE, even if the route-policy is correctly applied.

Answer

The route-policy syntax is incorrect; 'destination' should be 'ip prefix-list'.

Answer

The update-source should be the interface facing the CE, not Loopback0.

Answer

The 'set label-index' command should be 'set label' for BGP-SR.

Question 4

A service provider is designing a greenfield MPLS core and wants to minimize control-plane complexity while still supporting traffic engineering. They plan to use Segment Routing with MPLS. Which statement about Segment Routing in this context is accurate?

Accepted Answer

Segment Routing reduces the number of protocols required in the core.. Option C is correct because Segment Routing (SR-MPLS) eliminates the need for a separate label distribution protocol like LDP or RSVP-TE. The MPLS labels are derived directly from the IGP (IS-IS or OSPF) extensions, reducing control-plane complexity while still enabling traffic engineering via SR-TE policies and flexible path computation.

Answer

Segment Routing only works with IPv6 data plane.

Answer

Segment Routing uses RSVP-TE for label distribution.

Answer

Segment Routing requires TI-LFA to be enabled for traffic engineering.

Question 5

Drag and drop the steps for the BGP route selection process (best path selection) in order.

Question 6

Drag and drop the steps to configure a PPPoE client on a Cisco router into the correct order.

Question 7

Match each IS-IS term to its definition.

Question 8

Match each Ethernet OAM protocol to its function.

Question 9

A service provider is implementing MPLS in their core network. They want to ensure that labeled packets for BGP prefixes are forwarded correctly without requiring an IGP label for every BGP prefix. Which technique should be used to achieve this?

Accepted Answer

Use BGP-free core with LDP or Segment Routing. A BGP-free core with LDP or Segment Routing allows the service provider to forward labeled packets for BGP prefixes without requiring an IGP label for every BGP prefix. In this design, edge routers (PEs) impose a label stack where the outer label (IGP label) is distributed by LDP or Segment Routing for the BGP next-hop, and the inner label is the BGP label. Core routers (P routers) only need to swap the outer label based on the IGP label, not the BGP prefix, thus eliminating the need for IGP labels per BGP prefix.

Answer

Enable MPLS on all interfaces and rely on BGP label distribution

Answer

Implement LDP for BGP prefixes

Answer

Configure static labels for BGP prefixes

Question 10

An engineer is troubleshooting an MPLS VPN issue where CE1 cannot ping CE2 across the provider network. The PE routers are configured with MP-BGP and LDP. On PE1, 'show ip bgp vpnv4 vrf CUST' shows the route to CE2's loopback as valid, but 'show mpls forwarding-table' does not list a label for that prefix. What is the most likely cause?

Accepted Answer

Route target mismatch between PE1 and PE2. The route is present in the VRF BGP table on PE1 (valid), but no MPLS label is assigned for that prefix in the forwarding table. This indicates that PE1 has learned the route via MP-BGP but cannot install it into the MPLS forwarding table because the route target (RT) import policy on PE1 does not match the RT export policy on PE2. Without a matching RT, PE1 does not consider the VPNv4 route as belonging to the CUST VRF, so it cannot resolve the next hop or assign a label for forwarding.

Answer

MTU mismatch on the MPLS core interfaces

Answer

LDP is not enabled on the core link between PE1 and P

Answer

BGP session between PE1 and PE2 is not established

Question 11

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?

Accepted Answer

Advertise prefix-SIDs for all IGP prefixes. Option D is correct because 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 12

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 13

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 14

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 15

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. Option D is correct because 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