350-501 Practice Test 42 — 15 Questions

Question 1

In the context of YANG modeling for network automation, which statement about 'must' expressions is true?

Accepted Answer

They enforce a constraint that must be true for the data to be valid.. 'must' is an XPath expression that enforces a constraint on the data; if the condition is false, validation fails.

Answer

They conditionally make a leaf mandatory.

Answer

They are used to refine types.

Answer

They are used to define default values.

Question 2

An SP engineer is implementing LLQ on a PE router to prioritize voice. Which additional configuration is recommended to prevent voice traffic from starving other queues?

Accepted Answer

Configure a policer on the LLQ class to limit its rate.. Policing within the LLQ class limits the amount of traffic that can use the priority queue, preventing starvation.

Answer

Use CBWFQ without LLQ.

Answer

Disable queueing on other classes.

Answer

Set the priority queue bandwidth to 100%.

Question 3

A service provider is implementing hierarchical QoS on an edge router. The goal is to shape the aggregate traffic to 100 Mbps and then apply different queuing policies for voice and data within that shaped pipe. Which configuration approach is correct?

Accepted Answer

Configure a parent policy-map with shape and a child policy-map with queuing, then apply the child under the parent's class.. Hierarchical QoS uses a parent policy for shaping and a child policy for classification and queuing. The parent policy applies shape, and the child policy is attached to a class within the parent.

Answer

Apply shaping on the interface and queuing on a subinterface separate from the policy-map.

Answer

Use a child policy-map for shaping and a parent policy-map for queuing.

Answer

Apply a single policy-map with both shape and queue commands under the same class.

Question 4

In NSO, which statement best describes the function of the CDB (Configuration Database)?

Accepted Answer

It holds the configuration and operational data of managed devices, acting as a centralized datastore.. CDB stores the intended configuration and operational state for managed devices, enabling NSO to compare and sync with actual device configurations.

Answer

It translates NETCONF RPCs to device-specific CLI commands.

Answer

It stores YANG models for service definition.

Answer

It provides a northbound API for OSS/BSS integration.

Question 5

An SP network is migrating from IS-IS narrow metrics to wide metrics. What is a key advantage of using wide metrics?

Accepted Answer

Wide metrics support up to 2^24-1 link cost and TE metrics.. Wide metrics support values up to 2^24-1 (16,777,215) and allow Traffic Engineering (TE) metrics, unlike narrow metrics which are limited to 6 bits (max 63).

Answer

Wide metrics are backward compatible with narrow metrics without any configuration.

Answer

Wide metrics reduce SPF computation time.

Answer

Wide metrics enable BFD for faster convergence.

Question 6

An SP uses traffic policing and shaping to manage bandwidth. Which statement best describes the difference between the two mechanisms?

Accepted Answer

Shaping buffers excess traffic to smooth bursts, while policing drops or re-marks excess traffic.. Shaping buffers excess packets to smooth traffic and reduce bursts, while policing drops or re-marks excess packets to enforce a strict rate limit.

Answer

Both mechanisms buffer excess traffic but policing uses a smaller buffer.

Answer

Shaping drops excess traffic while policing buffers it.

Answer

Policing buffers excess traffic while shaping drops it.

Question 7

An SP wants to secure management access to IOS XR routers. Which two measures should be implemented? (Choose two.)

Accepted Answer

Implement AAA with TACACS+. SSH provides encrypted remote access, and AAA with TACACS+ provides centralized authentication and authorization. SNMPv3 is also secure but not a management access method per se; the question asks for access measures.

Answer

Enable Telnet for remote access

Answer

Use SNMPv2c for monitoring

Answer

Disable all logging

Question 8

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

Question 9

Which TWO statements accurately describe the behavior of the QoS policy shown in the exhibit?

Accepted Answer

During congestion, video traffic is allocated 30% of the remaining bandwidth after voice is served.. The policy uses the 'bandwidth remaining percent' command under the video class, which allocates 30% of the remaining bandwidth after the voice class (which is in a strict priority queue) has been served. This is the standard behavior for class-based weighted fair queuing (CBWFQ) when a priority queue is present: the priority traffic is serviced first, and then the remaining bandwidth is distributed according to the 'bandwidth remaining percent' values assigned to the non-priority classes.

Answer

The policy shapes the total output traffic to 30 Mbps.

Answer

Video traffic is placed in a strict priority queue.

Answer

The policy polices voice traffic to a maximum of 30% of the interface bandwidth.

Question 10

An SP implements Carrier-Grade NAT (CGNAT) to conserve IPv4 addresses. For legal compliance, what additional function must be enabled to log subscriber IP-port mappings?

Accepted Answer

NAT logging (syslog or other). CGNAT logging is required for lawful intercept and auditing. ALG support is for application layer protocols, not logging. Port allocation is part of NAT operation. DPI classifies traffic.

Answer

Port allocation algorithm (deterministic or random)

Answer

Application Layer Gateway (ALG) support

Answer

Deep Packet Inspection (DPI)

Question 11

A network engineer needs to ensure that a specific customer's traffic is not adversely affected by other customers' traffic in a shared MPLS core. Which technology should be used?

Accepted Answer

QoS policies on PE routers with shaping and policing. QoS policies on PE routers with shaping and policing allow the service provider to enforce traffic contracts per customer, ensuring that one customer's traffic does not consume bandwidth allocated to another. This is achieved by applying ingress policing to limit customer traffic to the Committed Information Rate (CIR) and egress shaping to smooth bursts, preventing congestion in the MPLS core from affecting other customers.

Answer

iBGP route reflectors

Answer

MPLS Traffic Engineering

Answer

802.1Q VLANs

Question 12

During multicast troubleshooting, the command 'show ip mroute' shows that the RPF neighbor for a particular (S,G) entry is incorrect. What is the most likely cause?

Accepted Answer

The unicast routing table does not have a route back to the source.. The RPF (Reverse Path Forwarding) neighbor for an (S,G) entry is determined by the unicast routing table: the router expects to reach the source S via the interface and next-hop that the unicast route provides. If the unicast routing table does not have a route back to the source, the router cannot determine the correct RPF interface and neighbor, leading to an incorrect RPF neighbor in the 'show ip mroute' output. This is the most common cause of RPF failures in multicast troubleshooting.

Answer

The multicast routing protocol is not enabled globally.

Answer

TTL threshold is set too high on the incoming interface.

Answer

IGMP snooping is disabled on the switch.

Question 13

A service provider is implementing security for BGP peering. Which two methods help prevent BGP route hijacking? (Choose two.)

Accepted Answer

RPKI origin validation. Prefix filtering (with prefix-lists and route-maps) ensures only expected prefixes are accepted from peers. RPKI validation provides cryptographic verification of origin AS. Both prevent hijacking.

Answer

BGP next-hop-self

Answer

BGP multipath

Answer

BGP GSHUT

Question 14

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, MPLS should be disabled on PE-CE links so that customers cannot send labeled packets. Additionally, disabling TTL propagation prevents label stack visibility.

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 15

A service provider is implementing Segment Routing (SR) with MPLS. Which three statements are true regarding SR-MPLS? (Choose three.)

Accepted Answer

SR-MPLS does not require LDP or RSVP-TE. B is correct because SR-MPLS uses the MPLS data plane but replaces the control plane protocols LDP and RSVP-TE with IGP extensions (IS-IS or OSPF) to distribute segment information. This eliminates the need for these traditional label distribution protocols, simplifying network operations and reducing protocol overhead.

Answer

SR-MPLS supports traffic engineering only with a central controller

Answer

Adjacency SIDs are advertised via BGP-LS