350-501 MPLS and Segment Routing Practice Test 5 — 15 Questions

Question 1

An engineer is troubleshooting MPLS label switching in a service provider core. They notice that packets are being forwarded correctly between provider edge routers, but when they check the MPLS forwarding table on a P router, they see only implicit-null labels for some destinations. What is the most likely reason for this?

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

Accepted Answer

The router has a directly connected neighbor that is the egress LER.. When a P router sees only implicit-null labels (label 3) for some destinations in its MPLS forwarding table, it indicates that the router is performing penultimate hop popping (PHP). This occurs when the P router is the penultimate hop before the egress Label Edge Router (LER). The egress LER advertises an implicit-null label to its directly connected neighbor, causing the P router to pop the label and forward the packet using only the IP header. Option B is correct because the P router has a directly connected neighbor that is the egress LER, which triggers PHP.

Answer

The router is using explicit-null label due to security policies.

Answer

The router is misconfigured to use implicit-null for all labels.

Answer

The router is performing penultimate hop popping (PHP) incorrectly.

Question 2

A service provider is using Segment Routing with TI-LFA for fast convergence. During a link failure, the router performing the local repair must compute a backup path that avoids the failed link. Which type of Adjacency Segment Identifier (Adj-SID) is required for the backup path to be loop-free?

Accepted Answer

Protected Adj-SID. In Segment Routing with TI-LFA, the router performing local repair must compute a backup path that is guaranteed loop-free. A Protected Adj-SID (Adj-SID with the B-flag set) is required because it indicates that the adjacency segment is protected by a fast-reroute mechanism, allowing the router to use a precomputed backup path that avoids the failed link while ensuring no forwarding loops occur.

Answer

Anycast Adj-SID

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Unprotected Adj-SID

Answer

Backup Adj-SID

Question 3

A network operator is configuring RSVP-TE tunnels for traffic engineering in an MPLS core. They want to enforce that the tunnel path strictly follows a predefined set of hops. Which explicit path option should be used?

Accepted Answer

Strict. (Strict) is correct because RSVP-TE explicit paths with the 'strict' keyword require each hop in the path to be directly adjacent (connected) to the previous hop. This enforces that the tunnel path follows exactly the predefined set of hops without any intermediate routers being inserted, which is the requirement in the question.

Answer

Exact

Answer

Loose

Answer

Dynamic

Question 4

In an MPLS L3VPN network, a customer has overlapping IP addresses between two VPNs. The provider edge routers are configured with VRF instances. Which method ensures that traffic from one VPN does not leak into the other when using MPLS labels?

Accepted Answer

Use distinct route targets for import and export per VRF.. Route targets (RTs) control the import and export of VPNv4 routes between VRFs on a PE router. By assigning distinct import and export RTs per VRF, the PE ensures that routes from one VPN are not installed into the routing table of another VPN, even if the customer uses overlapping IP addresses. MPLS labels are allocated per VRF forwarding instance, but the isolation is enforced by the RT-based filtering of BGP VPNv4 routes, not by label ranges or route distinguishers alone.

Answer

Assign different MPLS label ranges to each VRF.

Answer

Apply BGP community strings to filter routes.

Answer

Configure different route distinguishers for each VPN.

Question 5

A service provider is implementing Segment Routing over IPv6 (SRv6) in their core. They want to provide path isolation for different services using SRv6 SIDs. Which SID structure is used to encode both the locator and the function?

Accepted Answer

The SID is divided into locator and function (and optionally arguments).. In SRv6, the SID is encoded as a 128-bit IPv6 address that is explicitly divided into three parts: the Locator (identifies the node or prefix), the Function (identifies the specific behavior or instruction), and optionally Arguments (additional parameters). This structure allows path isolation by assigning unique locator/function combinations per service, enabling per-service steering without additional label stacks.

Answer

The SID uses a separate label stack for function encoding.

Answer

The SID is an IPv6 address without any encoding.

Answer

The SID consists of a prefix only.

Question 6

An MPLS network is experiencing micro-loops during convergence after a link failure. The network uses LDP and IS-IS as IGP. Which of the following solutions can prevent micro-loops during IGP convergence without requiring additional protocols?

Accepted Answer

Configure LFA (Loop-Free Alternate) on the IGP.. LFA (Loop-Free Alternate) is a feature built directly into IGPs like IS-IS and OSPF that precomputes a backup next-hop that does not loop back to the failed link. This prevents micro-loops during IGP convergence without requiring any additional protocols such as RSVP-TE or BGP, making it a lightweight, protocol-integrated solution.

Answer

Implement MPLS-TE FRR using bypass tunnels.

Answer

Enable BGP PIC (Prefix Independent Convergence).

Answer

Deploy RSVP-TE with fast-reroute.

Question 7

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

Accepted Answer

Push operation adds a label to the packet.. The MPLS push operation adds a new label onto the top of the label stack, encapsulating the packet for forwarding through an LSP. Option E is correct because the pop operation removes the top label, and implicit-null (label value 3) triggers the penultimate hop to pop the label before forwarding the packet to the egress LSR, reducing the egress router's processing load.

Answer

Push operation can only add one label at a time.

Answer

Pop operation can remove multiple labels at once.

Answer

Swap operation removes and replaces two labels.

Question 8

Which three are required components for deploying Segment Routing in an MPLS network? (Choose three.)

Accepted Answer

MPLS forwarding capability on routers. B is correct because Segment Routing relies on the existing MPLS forwarding plane; routers must be capable of pushing, swapping, and popping MPLS labels based on the Segment Routing label stack. Without MPLS forwarding hardware or software support, the SR-MPLS data plane cannot function, even if the control plane is configured.

Answer

RSVP-TE signaling protocol

Answer

LDP for label distribution

Question 9

Which two are characteristics of the Anycast SID in Segment Routing? (Choose two.)

Accepted Answer

It provides redundancy and fast convergence by allowing traffic to be rerouted to another router.. Anycast SIDs are shared among multiple routers in the same anycast group, allowing any of those routers to process traffic destined for that SID. This provides redundancy and fast convergence because if one router fails, traffic is automatically rerouted to another router in the group without requiring control-plane signaling changes.

Answer

It identifies an adjacency between two routers.

Answer

It is typically used as an Adj-SID for load balancing.

Answer

It is unique per node and identifies a specific router.

Question 10

Refer to the exhibit. The show command output displays the LDP neighbor state. What does the 'Downstream' label mode indicate about label distribution?

Accepted Answer

Labels are advertised to all neighbors without a request.. In the context of LDP, 'Downstream' label mode means that a router advertises label bindings for all prefixes in its routing table to its LDP neighbors without waiting for an explicit request. This is the default label distribution mode for LDP, where labels are proactively sent to all peers. Option D correctly identifies this behavior.

Answer

Labels are sent only to the peer that advertises the prefix.

Answer

Labels are only distributed when explicitly requested.

Answer

The neighbor must request labels via a Label Request message.

Question 11

Refer to the exhibit. A network engineer is configuring a segment routing traffic engineering policy. The output shows two candidate paths. Why is path1 selected as the active path even though path2 has a bandwidth constraint?

Accepted Answer

path2 has a lower preference value.. Path1 is selected as the active path because it has a higher preference value (200) compared to path2 (100). In segment routing traffic engineering, the preference value is the primary tie-breaker for candidate paths; the path with the highest preference is chosen regardless of bandwidth constraints. Bandwidth constraints are only evaluated after preference, and a path with a lower preference is not considered for activation even if it has a bandwidth constraint configured.

Answer

path1 has a shorter segment list.

Answer

path2 is down due to insufficient bandwidth.

Answer

path1 is explicit and always preferred over dynamic paths.

Question 12

Refer to the exhibit. The output shows an MPLS forwarding entry with FRR protection. What is the purpose of the backup path shown?

Accepted Answer

To forward traffic if the primary outgoing interface fails.. The backup path in an MPLS Fast Reroute (FRR) configuration is precomputed and installed in the forwarding table to provide sub-50ms protection against link or node failures. When the primary outgoing interface fails, traffic is immediately switched to the backup path without waiting for IGP convergence, ensuring minimal packet loss.

Answer

To handle penultimate hop popping for the primary path.

Answer

To provide a load-balancing alternative for the primary path.

Answer

To carry the VPN label separately.

Question 13

A service provider is designing a new MPLS core network and wants to simplify traffic engineering without deploying a centralized controller. Which technology should be used to enable source-routed paths with explicit path control?

Accepted Answer

Segment Routing (SR-MPLS). Segment Routing (SR-MPLS) enables source-routed paths with explicit path control by encoding the path as a list of segment identifiers (SIDs) in the packet header, allowing the ingress router to steer traffic along a specific path without a centralized controller. This simplifies traffic engineering in an MPLS core network by eliminating the need for per-flow signaling and state maintenance on intermediate nodes, which is exactly what the question requires.

Answer

BGP-LU

Answer

LDP

Answer

RSVP-TE

Question 14

A network engineer is troubleshooting MPLS L3VPN issues on a PE router. The CE router is receiving routes from the PE, but the CE cannot ping the remote CE. The PE shows the remote prefix in its routing table and in the VRF. What is the most likely cause?

Accepted Answer

The LSP between the PEs is not operational. The CE can receive routes from the PE (indicating that the VRF and route-target import/export are functioning correctly), but the CE cannot ping the remote CE. This points to a failure in the data plane between the PEs. If the LSP (Label Switched Path) between the PEs is not operational, MPLS labels cannot be swapped or forwarded correctly, causing packets to be dropped even though the control plane (routing table and VRF) shows the remote prefix.

Answer

The VRF route-target import/export is misconfigured

Answer

The MTU on the core links is too small

Answer

The RD is different on the two PEs

Question 15

A service provider is migrating from LDP-based MPLS to Segment Routing (SR-MPLS) with IS-IS. After enabling SR on all routers, the adjacency segment IDs (ADJ-SIDs) are not being advertised. Which configuration is missing?

Accepted Answer

The `segment-routing mpls` command is not enabled under the IS-IS process. In IS-IS, Segment Routing (SR-MPLS) requires explicit activation under the IS-IS routing process with the `segment-routing mpls` command. Without this, the router will not generate or advertise adjacency SIDs (ADJ-SIDs) even if SR is enabled globally. This command ties the SR-MPLS data plane to the IS-IS control plane, enabling IS-IS to allocate and distribute SIDs for adjacencies.

Answer

The router is running OSPF instead of IS-IS

Answer

The interface has the `isis adjacency-sid` command incorrectly configured

Answer

The loopback interface does not have an ip address configured