350-501 MPLS and Segment Routing Practice Test 4 — 15 Questions

Question 1

An engineer is troubleshooting MPLS traffic engineering tunnels and needs to verify path calculation and RSVP errors. Which command should be used?

Accepted Answer

show mpls traffic-eng tunnels. The 'show mpls traffic-eng tunnels' command displays detailed information about MPLS TE tunnels, including path calculation status, RSVP signaling errors, and tunnel state. This command is specifically designed to verify TE tunnel operations and troubleshoot issues such as path computation failures or RSVP resource reservation problems.

Answer

show ip route

Answer

show mpls ldp neighbor

Answer

debug mpls lfib errors

Question 2

In MPLS forwarding, what label operation does a transit LSR perform on the top label of a labeled packet?

Accepted Answer

Swap. A transit Label Switch Router (LSR) in an MPLS network receives a labeled packet and must forward it toward the egress LSR. The core operation on the top label is a swap: the incoming label is replaced with an outgoing label learned from the downstream LSR via Label Distribution Protocol (LDP) or other label distribution protocols. This maintains the label-switched path (LSP) and ensures the packet continues along the correct path.

Answer

None

Answer

Pop

Answer

Push

Question 3

Which BGP extended community is used to signal the OSPF domain ID between PE routers in an MPLS L3VPN when OSPF is the PE-CE protocol?

Accepted Answer

OSPF Domain ID. In an MPLS L3VPN where OSPF is the PE-CE protocol, the OSPF Domain ID extended community is used to signal the OSPF domain identifier between PE routers. This allows the receiving PE to determine whether the OSPF route originated from the same OSPF domain (and thus should be redistributed as an intra-area or inter-area route) or from a different domain (requiring a Type 5 LSA). The OSPF Domain ID is carried as a BGP extended community (type 0x0005 or 0x8005) and is critical for maintaining OSPF route type semantics across the MPLS backbone.

Answer

Site of Origin

Answer

OSPF Route Type

Answer

Route Target

Question 4

A service provider wants to use Segment Routing TE with Flex-Algo to engineer traffic around links with low bandwidth. Which configuration approach should be taken?

Accepted Answer

Define a Flex-Algo with a metric type of 'min-unidirectional-link-bandwidth'.. Flex-Algo allows the definition of a custom metric type, such as 'min-unidirectional-link-bandwidth', which enables Segment Routing TE to compute paths that avoid low-bandwidth links. This approach directly addresses the requirement to engineer traffic around links with low bandwidth by using bandwidth as the path selection metric.

Answer

Configure a policy-map to apply QoS marking on TE tunnels.

Answer

Use the 'min-delay' metric in Flex-Algo to prefer low-delay links.

Answer

Use standard IS-IS metric and adjust it on low-bandwidth links to a high value.

Question 5

Which tool can verify connectivity along an MPLS Label Switched Path (LSP) and detect label loops?

Accepted Answer

MPLS LSP ping. MPLS LSP ping is the correct tool because it specifically tests connectivity along an MPLS Label Switched Path by sending MPLS echo request packets that follow the label stack, and it can detect label loops by checking the TTL field in the MPLS header. Unlike regular ICMP-based tools, MPLS LSP ping operates at the MPLS forwarding plane, verifying that the LSP is intact and that labels are correctly swapped at each hop.

Answer

SNMP

Answer

traceroute

Answer

ping

Question 6

An engineer notices that MPLS VPN traffic is taking a suboptimal path because the MPLS label binding for the BGP next-hop loopback is missing. What is the most likely cause?

Accepted Answer

LDP is not enabled on the core-facing interfaces.. MPLS VPN traffic relies on LDP to distribute labels for the BGP next-hop loopback addresses used in the VPNv4 route exchange. If LDP is not enabled on the core-facing interfaces, the local PE cannot obtain a label binding for the remote PE's loopback, causing the traffic to be forwarded via IP (without a label) or taking a suboptimal path. Enabling LDP on all core-facing interfaces resolves the missing label binding and restores optimal label-switched forwarding.

Answer

The remote PE is configured with a different VPN ID.

Answer

The local PE does not have a route to its own loopback.

Answer

LDP is not enabled on the PE-CE interface.

Question 7

In an MPLS L3VPN network with route reflectors, what is the default behavior regarding the BGP next-hop attribute for reflected VPNv4 routes?

Accepted Answer

It leaves the next-hop unchanged from the originating PE.. In an MPLS L3VPN network with route reflectors, the default behavior for VPNv4 routes is that the route reflector does not modify the BGP next-hop attribute. This is because the next-hop should remain the address of the originating PE router to ensure proper MPLS label switching and reachability. Changing the next-hop would break the MPLS forwarding path, as the P routers rely on the IGP to reach the PE's loopback, not the route reflector's.

Answer

It sets the next-hop to 0.0.0.0.

Answer

It removes the next-hop attribute.

Answer

It sets the next-hop to the route reflector's loopback.

Question 8

Which two statements about MPLS label distribution are correct? (Choose two)

Accepted Answer

LDP can establish sessions with non-directly connected neighbors using targeted LDP.. LDP can establish sessions with non-directly connected neighbors using targeted LDP, which uses UDP discovery (port 646) and TCP (port 646) to form a session even when the peers are not directly connected at Layer 3. This is defined in RFC 5036 and is commonly used in MPLS VPN inter-AS options and AToM scenarios.

Answer

RSVP-TE allocates labels based on LDP.

Answer

LDP uses the BGP next-hop as the label FEC.

Answer

TDP is the Cisco proprietary version of LDP.

Question 9

Which two MPLS OAM tools can be used to troubleshoot label switching paths? (Choose two)

Accepted Answer

MPLS LSP ping. MPLS LSP ping (option C) is an OAM tool specifically designed to verify the data plane of an MPLS label switched path (LSP) by sending an MPLS echo request packet with a target FEC stack, which is forwarded along the LSP and validated by the egress LSR. This tests the actual label forwarding path, not just the control plane, making it a correct choice for troubleshooting LSPs.

Answer

traceroute

Answer

show ip route

Answer

ping

Question 10

Based on the exhibit, which prefix is missing a label binding and likely causing traffic to be dropped?

Accepted Answer

10.4.4.0/24. The exhibit shows an MPLS LDP label binding table. Prefix 10.4.4.0/24 has no local label assigned (shown as 'no label' or missing), meaning LDP has not established a label binding for this FEC. Without a label, traffic destined for 10.4.4.0/24 cannot be label-switched and will be dropped or forwarded via IP lookup (if enabled), but in an MPLS core, this typically results in packet loss.

Answer

10.2.2.0/24

Answer

10.1.1.0/24

Answer

10.3.3.0/24

Answer

10.5.5.0/24

Question 11

The CE router behind the VRF interface is unable to reach the remote CE. Which configuration error is most likely the cause?

Accepted Answer

The BGP neighbor is configured with a directly connected interface address but uses update-source Loopback0.. When BGP is configured with a neighbor using a directly connected interface address (e.g., 10.0.0.2) but the update-source is set to Loopback0, the BGP session will attempt to establish using the loopback IP as the source. If the loopback address is not reachable via the directly connected link (e.g., no IGP route for the loopback across the MPLS/VPN core), the BGP session will fail, preventing the CE router behind the VRF from reaching the remote CE. This mismatch between the neighbor address and the update-source is a common misconfiguration in MPLS L3VPN environments.

Answer

The VRF is missing the 'rd' command.

Answer

The 'mpls ip' command is missing on the core interface.

Answer

The VRF interface does not have an IP address.

Question 12

What does the output indicate about the TI-LFA protection on R1?

Accepted Answer

Only 1 prefix has a backup path.. The output shows that for the protected prefix 10.1.1.1/32, the backup path is listed as 'N/A', meaning no backup path was computed. However, for the other prefix (likely a different /32), a backup path is present. This indicates that only one prefix has a backup path, making option B correct. TI-LFA computes a backup path only when a viable post-convergence path exists without microloops; if the topology doesn't allow it, the backup is marked as not available.

Answer

All prefixes have backup paths.

Answer

0 prefixes have backup paths.

Answer

2 prefixes have backup paths.

Question 13

A service provider is deploying MPLS in their core network and wants to ensure that all routers in the MPLS domain can dynamically exchange label bindings. Which protocol should be enabled on all routers to meet this requirement?

Accepted Answer

LDP. LDP (Label Distribution Protocol) is the correct choice because it is the standard protocol used in MPLS networks to dynamically distribute label bindings for all routes in the routing table. It operates by establishing LDP sessions between directly connected LSRs and exchanging label mappings for each prefix, ensuring every router in the MPLS domain has the necessary labels to forward labeled packets.

Answer

OSPF

Answer

iBGP

Answer

RSVP-TE

Question 14

A network engineer is configuring Segment Routing on an IOS-XR router. They want to use the preferred algorithm for automatically computing paths based on IGP metrics. Which algorithm should be configured under the SR segment list?

Accepted Answer

preferred. The 'preferred' algorithm is the correct choice because it instructs the router to automatically select the best algorithm for path computation based on IGP metrics. In Segment Routing, the 'preferred' algorithm uses the default SPF (Shortest Path First) algorithm, which computes paths based solely on IGP link metrics, ensuring optimal routing without additional constraints.

Answer

strict-spf

Answer

delay

Answer

te

Question 15

During MPLS LDP operation, a router receives a label mapping for a prefix from its neighbor. What is the correct behavior when the receiving router has a route to that prefix?

Accepted Answer

It installs the label as the outgoing label for the prefix.. When a router receives a label mapping from an LDP neighbor for a prefix, it installs the received label as the outgoing label for that prefix in its Label Forwarding Information Base (LFIB). This label is used when forwarding packets toward the neighbor. The router must have a route to the prefix in its routing table to accept the mapping.

Answer

It discards the label mapping unless it has a corresponding route.

Answer

It uses UDP to send a notification back.

Answer

It installs the label as the incoming label for the prefix.