350-501 MPLS and Segment Routing Practice Test 6 — 15 Questions

Question 1

A network architect is designing an MPLS network that must provide fast convergence in case of a link failure. The network uses Segment Routing. Which mechanism provides sub-50ms protection by using a precomputed backup path?

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

Accepted Answer

TI-LFA (Topology-Independent Loop-Free Alternate). B is correct because TI-LFA (Topology-Independent Loop-Free Alternate) is a Segment Routing mechanism that precomputes a backup path using a segment list, enabling sub-50ms convergence upon link failure. It guarantees loop-free forwarding even in complex topologies where traditional LFA may fail, by leveraging SR's source routing capabilities.

Answer

BFD (Bidirectional Forwarding Detection)

Answer

LFA (Loop-Free Alternate)

Answer

LDP FRR

Question 2

A service provider is deploying MPLS L2VPN using Virtual Private Wire Service (VPWS). After configuration, the two CEs cannot ping each other. The PE routers show the xconnect interface is up, but no MAC addresses are learned. What is the most likely cause?

Accepted Answer

The VC ID mismatch between the two PEs. In MPLS L2VPN VPWS, the VC ID (Virtual Circuit ID) must match on both PE routers to establish the pseudowire between them. A mismatch prevents the pseudowire from coming up, so even though the local xconnect interface shows as up (because the attachment circuit is operational), no MAC learning can occur because the pseudowire control plane (e.g., LDP or BGP signaled) fails to bind the two endpoints. This directly explains why the CEs cannot ping and no MAC addresses are learned.

Answer

The MTU on the core links is less than 1500

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The control word is enabled on one PE but not the other

Answer

The encapsulation type on the AC is different on the two PEs

Question 3

A network engineer notices that BGP-LU prefixes learned from an eBGP neighbor are not being installed in the MPLS forwarding table (LFIB). The BGP table shows the prefixes with the correct label. The IGP route to the neighbor is reachable. What additional configuration is needed?

Accepted Answer

Set `label mode per-prefix` under the BGP neighbor or address-family. BGP-LU (BGP Labeled Unicast) installs labels into the BGP table, but for those labels to be used for MPLS forwarding, the router must be configured to allocate labels on a per-prefix basis. By default, BGP may use per-VRF or per-next-hop label allocation, which does not populate the LFIB with per-prefix entries. Setting `label mode per-prefix` under the BGP neighbor or address-family forces BGP to allocate a unique label for each prefix, enabling the LFIB installation required for MPLS forwarding.

Answer

Enable `no bgp default ipv4-unicast`

Answer

Configure `mpls ldp autoconfig` on the interface

Answer

Add `allowas-in` to accept routes with AS-path containing own AS

Question 4

A company is connecting two sites using MPLS L3VPN. The PE routers are running both LDP and Segment Routing (SR-MPLS) in the core. The CE router at Site A cannot reach the CE at Site B. On the PEs, the VRF routes are present. Which command would you use to verify the MPLS forwarding path for the VPN traffic?

Accepted Answer

show mpls forwarding-table vrf VPN-A 10.1.1.0. The 'show mpls forwarding-table vrf VPN-A 10.1.1.0' command displays the MPLS label stack that will be imposed on packets destined for the VPN prefix, including the inner VPN label and the outer transport label (LDP or SR-MPLS). This directly verifies the end-to-end MPLS forwarding path for VPN traffic, which is necessary when VRF routes are present but connectivity fails due to label issues.

Answer

show mpls ldp bindings

Answer

show ip route vrf VPN-A 10.1.1.0

Answer

show bgp vpnv4 unicast labels

Question 5

Which TWO are characteristics of Segment Routing (SR-MPLS)? (Choose two.)

Accepted Answer

It relies on source routing. Segment Routing (SR-MPLS) is a source routing paradigm where the ingress node encodes a list of segment identifiers (SIDs) into the packet header, specifying the path the packet must take through the network. This eliminates the need for per-hop signaling protocols like LDP or RSVP-TE, as each intermediate node simply forwards based on the top SID.

Answer

It requires a centralized controller for traffic engineering

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It forwards packets based on destination IP address at each hop

Answer

It requires MPLS LDP for label distribution

Question 6

Which THREE steps are required when configuring MPLS L3VPN on a PE router? (Choose three.)

Accepted Answer

Create a VRF definition. Creating a VRF definition on the PE router is required to separate customer routing tables and forwarding instances in an MPLS L3VPN. The VRF provides the logical isolation needed to keep each customer's routes and traffic distinct, and it is the foundation for associating interfaces, route targets, and VRF-specific routing processes.

Answer

Configure BGP on the CE router to advertise routes to PE

Answer

Configure route-target import/export using route-map

Question 7

Which TWO commands are most effective to verify that an SR-TE policy is active and forwarding traffic? (Choose two.)

Accepted Answer

show segment-routing traffic-eng policy. The 'show segment-routing traffic-eng policy' command displays the operational state of SR-TE policies, including whether the policy is 'up' and actively forwarding traffic. It shows the candidate path, binding SID, and forwarding status, making it the primary verification command for SR-TE policy activity.

Answer

show isis database verbose

Answer

show segment-routing traffic-eng segment-list

Answer

show bgp labels

Question 8

A service provider is troubleshooting an MPLS L3VPN scenario where a customer in VRF Blue cannot reach a server in VRF Blue at a remote site. The PEs are running MPLS with LDP. The VRF on both PEs shows the remote prefix. The PE at the local site shows the label from the remote PE for the prefix in the BGP table. However, when pinging from the CE, the packets are dropped. A packet capture on the core shows MPLS packets with the correct VPN label, but the transport label is missing. Further investigation shows that the LDP session between the two PEs is up, but the LDP label binding for the remote PE's loopback is not present. What is the most likely cause and correct action?

Accepted Answer

Check MPLS LDP interface configuration and enable LDP on all core interfaces. The LDP session is up, but the label binding for the remote loopback is missing. This could be because LDP is not enabled on the underlying interfaces between the PEs, or because the label space is configured incorrectly. Typically, if LDP is enabled on all core interfaces, label bindings for loopbacks are automatically exchanged. The missing binding suggests that either LDP is not configured on some interfaces, or an access-list is blocking LDP. The most direct action is to verify that LDP is enabled on all core interfaces using the command 'show mpls ldp interface'.

Answer

Increase the TTL on the MPLS packets

Answer

Change the transport address in LDP to the loopback IP

Answer

Configure the remote PE's loopback to be advertised via BGP

Question 9

A service provider is migrating from a legacy MPLS network using LDP to Segment Routing (SR-MPLS). They have enabled SR on all routers and are using IS-IS. The migration plan is to keep LDP running alongside SR during the transition. After enabling SR, some traffic is being forwarded using the SR path, but the network operator notices that some CEs in an L2VPN are experiencing packet loss during failover scenarios. Troubleshooting shows that the primary pseudowire is using SR labels, but the backup pseudowire is still using LDP labels. The backup path is not working correctly. What is the most likely cause?

Accepted Answer

The backup pseudowire is still using LDP labels and the LDP session for the backup path is down. During coexistence of LDP and SR, there might be a mismatch in label switching. The backup pseudowire might be using LDP labels that are not consistent with the SR forwarding plane. The most likely cause is that the backup path is not properly programmed to use SR labels, or the LDP session for the backup path is down. The correct action is to ensure that the pseudowire's backup signaling is using the same transport mechanisms as the primary, or to disable LDP gradually.

Answer

The backup path uses a different IGP metric

Answer

The MTU on the backup path is smaller than on the primary

Answer

The prefix-SID for the remote PE is missing in the SR-TE database

Question 10

A network engineer is configuring an MPLS L3VPN on a PE router. The VRF is configured with route-target import/export. The PE and CE are running eBGP. The CE advertises routes to the PE, and the PE installs them in the VRF routing table. However, the remote PE does not receive these routes via BGP VPNv4. The local PE's BGP table shows the VPNv4 prefixes with the correct next hop and label. The remote PE's BGP table shows no such prefixes. The IGP between the PEs is working, and MPLS LSPs are up. What is the most likely cause and correct action?

Accepted Answer

Check the route-target import/export configuration on both PEs and ensure they match. Despite the local PE having the routes in BGP VPNv4, the remote PE does not receive them. This often indicates that the route-target export on the local PE does not match the route-target import on the remote PE, or that the VPNv4 session is not correctly configured. Since the local PE shows the prefixes in its BGP VPNv4 table, they are being advertised to the BGP peer, but the remote PE's import policy filters them due to mismatched RT. The correct action is to verify the route-target configuration on both PEs.

Answer

Enable the 'soft-reconfiguration inbound' command

Answer

Reset the BGP session between the PEs

Answer

Check the MTU on the link between the PEs

Question 11

A service provider is migrating their MPLS core network to Segment Routing (SR-MPLS). All existing core routers run IOS-XR and are configured with an SRGB of 16000-23999 and OSPF as the IGP. A new router (R5) is added as a PE. The engineer configures R5 with the same SRGB and enables segment-routing under OSPF. However, when configuring an SR-TE policy from R5 to the remote loopback 10.0.0.1 on R1, the policy remains down. Show commands reveal that R5 is not learning the prefix-SID for 10.0.0.1. On R1, the prefix-SID is configured with index 100. The engineer verifies that OSPF adjacencies are up and that R5 can ping 10.0.0.1. What is the most likely cause of the issue?

Accepted Answer

The OSPF process on R5 is not configured with segment-routing mpls.. The most likely cause is that the OSPF process on R5 is not configured with the 'segment-routing mpls' subcommand. Without this configuration, OSPF does not advertise or learn prefix-SIDs, so R5 cannot obtain the prefix-SID for 10.0.0.1. Option B is incorrect: while a color attribute is often used in SR-TE policies, the immediate issue is the lack of prefix-SID learning, not the absence of color. Option C is incorrect: all routers share the same SRGB (16000-23999), so there is no mismatch. Option D is incorrect: index 100 maps to label 16100 (16000+100), which falls within the SRGB range, so this is not the problem.

Answer

The SR-TE policy on R5 is missing the color attribute.

Answer

R1 is not configured with the same SRGB as R5.

Answer

The prefix-SID index 100 is not within the SRGB range of R5 (16000-23999).

Question 12

A service provider is migrating from LDP to Segment Routing in its MPLS core. The team has enabled IS-IS as the IGP and configured segment routing under the IS-IS process on all core routers. However, after the migration, some LSPs are not being signaled correctly and traffic is blackholing. Which action should be taken to ensure seamless interworking between LDP and SR during the migration?

Accepted Answer

Configure 'segment-routing mpls sr-prefer' under the IS-IS process.. The 'segment-routing mpls sr-prefer' command under IS-IS configures the router to prefer SR labels for forwarding while maintaining LDP forwarding entries for fallback. This ensures seamless interworking between LDP and SR during migration, preventing blackholing when some routers are not fully SR-capable. Option B is incorrect because changing the IGP to OSPF is unnecessary and disruptive; IS-IS can support both LDP and SR simultaneously. Option C is incorrect because removing LDP from all routers would break LDP signaling for any router not yet migrated to SR, causing traffic loss. Option D is incorrect because 'mpls ldp igp sync' synchronizes LDP with IGP convergence, not SR-LDP interworking, and does not address label preference during coexistence.

Answer

Change the IGP to OSPF with segment routing enabled.

Answer

Remove the 'mpls ldp' configuration from all routers that have SR enabled.

Answer

Enable 'mpls ldp igp sync' on the IGP interfaces.

Question 13

An SP network engineer is designing a new segment routing traffic engineering deployment within a single IGP area. The network consists of 50 core routers running IS-IS and MPLS. The engineer needs to steer traffic from Router A to Router D over a path that avoids high latency links. Which technology should be used to define and instantiate the explicit path?

Accepted Answer

Configure an SR-TE policy with an explicit path using segment lists.. SR-TE policies with explicit paths using segment lists allow steering traffic over a specific sequence of nodes or links, avoiding high latency links. Option A is wrong because LDP distributes labels for IGP shortest path forwarding, not explicit path control. Option B is wrong because while PCE can assist with path computation, the question asks for the technology to define and instantiate the explicit path, which is the SR-TE policy itself. Option C is wrong because BGP-LU distributes labels for BGP prefixes, not for traffic engineering paths.

Answer

Use LDP to distribute labels and rely on IGP shortest path.

Answer

Set up a Path Computation Element (PCE) and delegate path computation.

Answer

Implement BGP LU to create an explicit path via local policies.

Question 14

In an MPLS L3VPN network, a route reflector (RR) is used to distribute VPNv4 routes between PE routers. After a new PE router (PE4) is added, some VPN routes are not being received by other PEs. Which TWO actions should be investigated to resolve the issue? (Choose two.)

Accepted Answer

Ensure the VRF route-target import/export values are correctly configured on the new PE.. The new PE must have matching VRF route-target import/export values to receive and advertise VPN routes. Option C is correct because a BGP VPNv4 neighbor relationship between the new PE and the RR is necessary for VPNv4 route exchange. Option B is incorrect: OSPF as an additional IGP is not required for VPN route distribution. Option D is incorrect: while physical connectivity and IGP adjacency are prerequisites, the specific issue with missing VPN routes points to BGP session or RT misconfiguration. Option E is incorrect: a route refresh does not fix missing route-target or BGP session problems.

Answer

Configure OSPF as an additional IGP to redistribute VPN routes.

Answer

Check physical connectivity and IGP adjacency between the new PE and the RR.

Answer

Issue a 'clear ip bgp * refresh' on the RR.

Question 15

Refer to the exhibit. The router has three LDP neighbors established. A network administrator notices that MPLS labels for the prefix 10.4.4.0/24 are not being advertised from this router to its neighbors, although the prefix is present in the routing table. Which is the most likely cause?

Accepted Answer

The route to 10.4.4.0/24 is not the best path in the routing table.. The LDP neighbor for 10.4.4.4 (via 10.3.3.3) is up, but the router does not have a label for 10.4.4.0/24. This suggests the route is not being used for forwarding because of the IGP metric or administrative distance. Option A is wrong because the neighbor is up. Option C is wrong because LDP has been configured globally. Option D is wrong because there is no indication of an mroute issue.

Answer

The LDP session to 10.3.3.3 is down.

Answer

The 'mpls ldp' command has not been applied globally.

Answer

The multicast routing table is empty for the prefix.