350-501 MPLS and Segment Routing Practice Test 10 — 15 Questions

Question 1

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?

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

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

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iBGP

Answer

RSVP-TE

Question 2

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.

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Exact

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Loose

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Dynamic

Question 3

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.

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Apply BGP community strings to filter routes.

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Configure different route distinguishers for each VPN.

Question 4

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.

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The SID is an IPv6 address without any encoding.

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The SID consists of a prefix only.

Question 5

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

Accepted Answer

Push operation adds a label to the packet.. Option C is correct because the push operation inserts a new MPLS shim header (label) onto the packet's label stack at the ingress LER, encapsulating the IP packet with the label imposed by the forwarding entry. Option E is correct because the pop operation removes the top label from the stack, and when a router receives the reserved implicit-null label (value 3), it pops that label and forwards the packet as a plain IP packet to the adjacent router. Option A is incorrect because push can add more than one label at a time (e.g., label stacking for MPLS VPNs or TE), not strictly one. Option B is incorrect because a pop operation removes only the top label per operation; removing multiple labels requires multiple pops or a PHP-style action, not a single multi-label pop. Option D is incorrect because swap replaces the top label with a new label (one-for-one), it does not remove and replace two labels.

Answer

Push operation can only add one label at a time.

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Pop operation can remove multiple labels at once.

Answer

Swap operation removes and replaces two labels.

Question 6

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

Accepted Answer

MPLS forwarding capability on routers. Option B is correct because Segment Routing over MPLS still relies on the MPLS data plane, so routers must support MPLS forwarding (label push, swap, and pop operations) to carry SR-labeled packets. Option C is correct because SR-MPLS uses SIDs, specifically Node-SIDs (prefix SIDs) for shortest-path forwarding to a node and Adj-SIDs for forwarding over a specific adjacency, and these must be assigned and advertised for the forwarding instructions to work. Option D is correct because the IGP (OSPF or IS-IS) must be extended with Segment Routing extensions to advertise SID information and build the segment routing topology/label stack. Option A is not required because SR-MPLS does not depend on RSVP-TE signaling; SR replaces the need for RSVP-TE in many designs. Option E is not required because SR-MPLS does not rely on LDP for label distribution; labels/SIDs are distributed via the IGP SR extensions instead.

Answer

RSVP-TE signaling protocol

Answer

LDP for label distribution

Question 7

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.

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

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To carry the VPN label separately.

Question 8

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

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LDP

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RSVP-TE

Question 9

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

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

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The loopback interface does not have an ip address configured

Question 10

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?

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)

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LFA (Loop-Free Alternate)

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LDP FRR

Question 11

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

Accepted Answer

It relies on source routing. Option A is correct because SR-MPLS is fundamentally a source-routing architecture: the ingress node (headend) encodes the entire path as an ordered list of SIDs in the MPLS label stack, so intermediate nodes simply forward based on the top label without needing per-flow state. Option E is correct because Segment Routing is data-plane agnostic and can be instantiated over an MPLS data plane (SR-MPLS, using MPLS labels as SIDs) or over an IPv6 data plane (SRv6, using IPv6 addresses as SIDs), which is a defining characteristic of the SR architecture. Option B is incorrect because SR-MPLS does not require a centralized controller; distributed path computation via IGP (IS-IS/OSPF) extensions is sufficient, and a controller (e.g., PCE/SR-PCE) is optional for advanced TE. Option C is incorrect because SR-MPLS forwards based on the top MPLS label (SID) in the stack, not by performing a destination-IP lookup at each hop. Option D is incorrect because SR-MPLS explicitly removes the need for LDP; label/SID distribution is handled by IGP extensions (or BGP), making LDP unnecessary.

Answer

It requires a centralized controller for traffic engineering

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It forwards packets based on destination IP address at each hop

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It requires MPLS LDP for label distribution

Question 12

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

Accepted Answer

Create a VRF definition. Option B is correct because a VRF definition is the foundational step on a PE router: it creates the separate routing/forwarding table that isolates each customer's VPN routes and is referenced by the RD and route-targets. Option C is correct because the PE must configure the BGP address-family ipv4 vrf (per-VRF address family) to exchange the customer's IPv4 VPN routes with CE routers or other PEs, carrying the VPNv4 reachability. Option E is correct because the customer-facing interface must be assigned to the VRF (e.g., ip vrf forwarding <name>) so that received packets are placed into the correct VRF table and the interface participates in that VPN. Option A is not required because BGP on the CE is a customer-side routing choice, not a mandatory PE configuration step for MPLS L3VPN. Option D is not required because route-target import/export is configured directly under the VRF (route-target export/import), not via a route-map, so the stated method is incorrect.

Answer

Configure BGP on the CE router to advertise routes to PE

Answer

Configure route-target import/export using route-map

Question 13

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. Option A, 'show segment-routing traffic-eng policy', is correct because it directly displays the operational state of SR-TE policies, including whether each policy is up/active, its binding SID, and the candidate paths and segment lists in use, which confirms the policy is instantiated and installed. Option C, 'show mpls forwarding-table labels', is correct because it verifies the data plane by showing the incoming label, outgoing label, and next-hop for the policy's binding SID or segment list, proving traffic is actually being forwarded along the SR-TE path. Option B, 'show isis database verbose', only displays IS-IS link-state database contents and does not confirm SR-TE policy state or forwarding. Option D, 'show segment-routing traffic-eng segment-list', shows configured segment lists but not whether a policy is active or forwarding traffic. Option E, 'show bgp labels', displays BGP label bindings and is unrelated to SR-TE policy verification.

Answer

show isis database verbose

Answer

show segment-routing traffic-eng segment-list

Answer

show bgp labels

Question 14

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.. Segment Routing Traffic Engineering (SR-TE) policies allow an explicit path to be defined as an ordered list of segments (SIDs), which the headend router instantiates as a label stack. This steers traffic from Router A to Router D along a chosen path that avoids high-latency links, within a single IS-IS/MPLS domain. SR-TE is the purpose-built mechanism for explicit path steering in an SR deployment.

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 15

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.. Option A is correct because in an MPLS L3VPN, a PE only imports a VPNv4 route into a VRF when the route's route-target extended community matches the VRF's import route-target; if PE4's VRF import/export values are wrong, routes will not be installed or advertised correctly, so the RT configuration must be verified. Option C is correct because VPNv4 routes are exchanged over the BGP session between PE4 and the RR; if the BGP VPNv4 neighbor relationship (address-family vpnv4 unicast) is not Established, PE4 will neither send nor receive VPNv4 prefixes, which directly explains missing routes. Option B is not appropriate because OSPF is an IGP used for reachability inside the provider core, not for redistributing VPNv4 routes; adding it would not fix VPNv4 route distribution. Option D is less relevant because although physical/IGP reachability is needed for the BGP session, the scenario already implies the RR is functioning and the specific issue is VPN route propagation, making the BGP VPNv4 session and RT configuration the direct checks. Option E is not the right action because a route refresh only re-requests advertisements over an existing session and does not fix a missing or misconfigured VPNv4 session or incorrect route-targets.

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.