350-501 Practice Exam 4 — 20 Questions

Question 1

A service provider is implementing MPLS-TE with FRR (Fast Reroute) using one-to-one backup tunnels. After activation, they notice that the number of labels in the forwarding table has increased significantly. What is the most likely reason for this increase?

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

Accepted Answer

One-to-one backup creates a unique backup tunnel for each protected LSP. One-to-one backup tunnels in MPLS-TE FRR create a dedicated backup LSP for each protected primary LSP. This means every primary LSP gets its own unique set of labels for the backup path, leading to a significant increase in the number of labels in the forwarding table. In contrast, facility backup shares a single bypass tunnel among multiple protected LSPs, which conserves labels.

Answer

RSVP-TE signaling is not properly aggregating labels

Answer

Penultimate Hop Popping (PHP) is disabled

Answer

Facility backup is being used instead of one-to-one

Question 2

An engineer configures an RSVP-TE tunnel with a bandwidth of 100 Mbps and an explicit path that excludes a specific interface. The tunnel fails to come up. Which term describes the path computation method used by RSVP-TE to find a feasible path given these constraints?

Accepted Answer

CSPF (Constrained Shortest Path First). RSVP-TE uses Constrained Shortest Path First (CSPF) to compute a path that satisfies constraints such as bandwidth and explicit exclusions. CSPF is an extension of SPF that takes into account additional constraints like available bandwidth, link attributes, and administrative groups. Therefore, the path computation method is CSPF.

Answer

SPF (Shortest Path First)

Answer

Bellman-Ford

Answer

Dijkstra with ECMP

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.

Answer

Apply BGP community strings to filter routes.

Answer

Configure different route distinguishers for each VPN.

Question 4

An SP is deploying MPLS in the core/backbone to support L3VPN services. What is the purpose of MP-BGP in an MPLS VPN architecture?

Accepted Answer

To exchange customer VPN routes and VPN labels between PE routers. MP-BGP (Multiprotocol BGP) is used in MPLS L3VPNs to exchange VPN-IPv4 routes and their associated MPLS labels (VPN labels) between PE routers. It carries the route target extended communities that control import/export of routes into VRFs, enabling the PE routers to learn customer routes and the labels needed to forward traffic to the correct CE. Without MP-BGP, PEs would not have a scalable way to distribute customer routes across the provider core.

Answer

To forward data packets across the MPLS core

Answer

To establish IGP adjacencies between PE and CE routers

Answer

To distribute MPLS labels for LSPs

Question 5

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 6

In the context of SR-PCE, which protocol does the PCE use to communicate with the path computation client (PCC) to set up SR-TE policies?

Accepted Answer

PCEP. PCEP (Path Computation Element Communication Protocol) is the standard protocol used between a PCE and a Path Computation Client (PCC) to request and receive path computations and to set up SR-TE policies. In SR-PCE deployments, the PCE uses PCEP to download the segment list to the head-end router, which instantiates the SR-TE policy.

Answer

NETCONF

Answer

OSPF

Answer

BGP-LS

Question 7

An engineer is deploying Segment Routing in an MPLS network. To ensure that routers can forward packets based on SR-MPLS labels without requiring LDP, which requirement must be met?

Accepted Answer

IS-IS or OSPF must have the SR extension enabled.. Segment Routing (SR) can operate without LDP by encoding the forwarding path as a list of MPLS labels derived from the IGP. For routers to allocate and advertise these SR-MPLS labels, the IGP (IS-IS or OSPF) must have the Segment Routing extension enabled. This allows the IGP to distribute prefix-SIDs (labels) that routers use to forward packets, eliminating the need for LDP.

Answer

MPLS LDP must be enabled on all interfaces.

Answer

All routers must run BGP-LU.

Answer

The IGP must have the overload bit set.

Question 8

A service provider is designing a core network with IS-IS as the IGP. To support MPLS traffic engineering, which IS-IS extensions are required?

Accepted Answer

IS-IS TE extensions (RFC 5305). IS-IS TE extensions (RFC 5305) are required to carry MPLS Traffic Engineering information, such as link bandwidth, administrative groups, and TE metric, in Type-Length-Value (TLV) fields within IS-IS Link State PDUs. These extensions enable the IGP to flood TE attributes that MPLS TE uses for constraint-based path computation, such as with RSVP-TE or Segment Routing.

Answer

IS-IS multi-topology (MT)

Answer

IS-IS wide metrics

Answer

IS-IS L1/L2 routing

Question 9

A service provider is designing an OSPF network with multiple areas. Which TWO area types allow external routes (Type 5 LSAs) to be injected into the area? (Choose two.)

Accepted Answer

Standard area. Option B (Standard area) is correct because a standard OSPF area accepts all LSA types, including Type 5 external LSAs originated by ASBRs, so external routes can be injected and flooded throughout it. Option D (Backbone area, Area 0) is correct because the backbone is a normal (non-stub) area that permits Type 5 LSAs; in fact, external routes are typically injected at an ASBR and propagated across Area 0 to other areas. The unmarked options do not belong: an NSSA (A) blocks Type 5 LSAs and instead uses Type 7 LSAs that are translated to Type 5 only at the ABR, a stub area (C) blocks Type 5 LSAs entirely, and a totally stubby area (E) blocks Type 5 LSAs as well as Type 3 and Type 4 LSAs, relying solely on a default route.

Answer

NSSA

Answer

Stub area

Answer

Totally stubby area

Question 10

An engineer is troubleshooting an SR-MPLS TE tunnel that is not coming up. The headend router is configured with an explicit candidate path that uses a segment list containing SIDs 16010, 16020, and 16030. The show segment-routing traffic-eng policy command indicates that the policy is down. Which of the following is the most likely cause?

Accepted Answer

One or more of the SIDs in the segment list are not reachable in the IGP, so the path cannot be resolved.. An SR-TE policy with an explicit segment list requires all SIDs in the list to be reachable via the IGP (or BGP-LS). If any SID is missing or unreachable, the headend cannot resolve the path, and the policy remains down. Troubleshooting should focus on verifying that each SID in the segment list is advertised and reachable, using show isis segment-routing prefix-sid or similar commands. Once the missing SID is resolved, the policy should come up.

Answer

The headend router does not have a BGP session with the other routers to learn the SIDs.

Answer

The policy is configured with a color that does not match any BGP color community.

Answer

The SRGB is not configured on the headend router.

Question 11

A service provider is designing a QoS policy for a business VPN service that carries voice, video, and data. The policy must ensure that voice (EF) is never dropped even during congestion, while video (AF41) and data (AF21) share the remaining bandwidth in a ratio 2:1. Which queuing strategy should be used?

Accepted Answer

Apply LLQ for voice and CBWFQ for video and data with bandwidth ratio 2:1.. LLQ provides strict priority queuing for voice (EF), ensuring it is never dropped during congestion, while CBWFQ allows the remaining bandwidth to be shared among video (AF41) and data (AF21) in a 2:1 ratio. LLQ is designed to guarantee low latency for real-time traffic, and CBWFQ enables explicit bandwidth allocation for non-real-time classes. This combination meets the requirement of zero-drop for voice and proportional sharing for video and data.

Answer

Use only LLQ for all three classes.

Answer

Apply CBWFQ with bandwidth percentages to all three classes.

Answer

Use FIFO queuing with WRED for all classes.

Question 12

In the MQC (Modular QoS CLI) framework, which component is used to match traffic based on DSCP values?

Accepted Answer

Class-map. Class-maps define match criteria (e.g., match ip dscp 46) to classify traffic in MQC.

Answer

Service-policy

Answer

Access-list

Answer

Policy-map

Question 13

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 14

A service provider is implementing QoS for voice traffic using the DiffServ model. Which DSCP value should be assigned to voice packets to ensure the highest priority treatment?

Accepted Answer

DSCP 46. DSCP 46 (binary 101110) corresponds to the Per-Hop Behavior (PHB) of Expedited Forwarding (EF), which is defined in RFC 3246 for low-loss, low-latency, and low-jitter treatment. Voice traffic requires the highest priority in a DiffServ domain, and EF (DSCP 46) is the standard marking for voice bearer packets to ensure they receive strict priority queuing.

Answer

DSCP 26

Answer

DSCP 34

Answer

DSCP 0

Question 15

An MPLS L3VPN service provider wants to prevent label spoofing attacks where a customer could inject MPLS labels to bypass ACLs. Which configuration practice should be implemented on PE-CE links?

Accepted Answer

Disable MPLS on PE-CE links. To prevent label spoofing attacks where a customer injects MPLS labels, the PE-CE link should not have MPLS enabled. Customer edge devices should not send labeled packets to the PE; disabling MPLS on PE-CE links ensures that any labeled packets from the customer are dropped or treated as IP, preventing label spoofing.

Answer

Enable MPLS TTL propagation to detect spoofing

Answer

Enable MPLS on PE-CE links with TTL propagation disabled

Answer

Use explicit null labels on PE-CE links

Question 16

A service provider is implementing network slicing for 5G services. Which of the following is NOT a typical characteristic of a network slice?

Accepted Answer

Slices require dedicated physical infrastructure per slice.. Network slicing in 5G is built on network function virtualization (NFV) and software-defined networking (SDN), allowing multiple logical networks to share the same physical infrastructure. Each slice is a virtualized, isolated instance with its own resources and performance characteristics, so requiring dedicated physical infrastructure per slice contradicts the fundamental efficiency and flexibility that slicing provides. Therefore, option C is NOT a typical characteristic.

Answer

Each slice can have its own dedicated virtualized functions and resources.

Answer

Slices can be customized for specific service types like eMBB, uRLLC, or mMTC.

Answer

Slices are isolated from each other in terms of performance and security.

Question 17

An operator notices that a new MPLS-TE tunnel is not being established. The tunnel configuration includes a dynamic path option and a bandwidth of 100 Mbps. The network uses RSVP-TE with CSPF. The link-state database shows sufficient reservable bandwidth on all links along the calculated path. What is the most likely cause of the tunnel establishment failure?

Accepted Answer

RSVP-TE is not enabled on the transit interfaces. RSVP-TE must be explicitly enabled on every interface that will participate in MPLS-TE label-switched path (LSP) signaling. Even if the link-state database shows sufficient reservable bandwidth, without RSVP-TE enabled on transit interfaces, the PATH messages cannot be processed, and the tunnel will fail to establish. This is a common misconfiguration when deploying MPLS-TE.

Answer

The path option is misconfigured with a strict explicit path

Answer

MPLS LDP is not enabled on the core routers

Answer

The tunnel source interface is not configured with an IP address

Question 18

A service provider is implementing MAP-T (Mapping of Address and Port using Translation) to transition customers from IPv4 to IPv6. Which two characteristics of MAP-T are true? (Choose two.)

Accepted Answer

MAP-T employs a Border Router that performs translation between IPv4 and IPv6. MAP-T uses stateful or stateless translation between IPv4 and IPv6, and it uses a Border Router (BR) that performs translation. MAP-T does not use encapsulation; it uses translation (NAT). MAP-T also uses a Domain Name System (DNS) but that is not unique. The key characteristics: it uses a BR for translation and can be stateless.

Answer

MAP-T is primarily used for IPv6-to-IPv4 communication only

Answer

MAP-T requires a dedicated IPv6 prefix for each customer

Answer

MAP-T uses encapsulation to transport IPv4 packets over an IPv6 network

Question 19

In IOS XR, which process is responsible for managing the configuration database and commit operations?

Accepted Answer

confd. In IOS XR, the confd process is responsible for managing the configuration database and commit operations. It handles configuration changes, validation, and commits, ensuring that the system configuration is consistent and applied atomically.

Answer

fman

Answer

sysmgr

Answer

rib

Question 20

Which MEF service type defines a point-to-point Ethernet connection, often used for business connectivity?

Accepted Answer

E-Line. E-Line is the MEF service type that defines a point-to-point Ethernet connection, providing a dedicated Ethernet Virtual Connection (EVC) between exactly two UNIs. It is the MEF equivalent of a point-to-point Ethernet private line and is commonly used for business connectivity such as site-to-site links and internet access. E-Line maps to services like Ethernet Private Line (EPL) and Ethernet Virtual Private Line (EVPL).

Answer

E-Tree

Answer

E-Access

Answer

E-LAN