350-501 Automation and Quality of Services Practice Test 4 — 15 Questions

Question 1

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 2

An SP engineer is configuring hierarchical QoS on a PE router. The parent policy shapes the traffic to 10 Mbps, and the child policy applies LLQ for voice and CBWFQ for data. What is the primary purpose of using policing within the LLQ child policy?

Accepted Answer

To drop excess voice traffic and prevent starvation of other queues. In hierarchical QoS (HQoS), the parent policy shapes the aggregate traffic to 10 Mbps, while the child policy applies LLQ for voice and CBWFQ for data. The primary purpose of policing within the LLQ child policy is to enforce a strict rate limit on voice traffic, dropping excess packets to prevent voice from consuming bandwidth beyond its allocated share. This prevents voice traffic from starving other queues (e.g., data) within the shaped parent rate, ensuring fairness and protecting other classes from congestion.

Answer

To re-mark voice traffic to a lower DSCP

Answer

To police the parent shape rate

Answer

To buffer excess voice traffic

Question 3

In MPLS networks, the EXP bits are used for QoS. How are the 3-bit EXP values typically derived from DSCP markings at the ingress PE router?

Accepted Answer

The 3 most significant bits of DSCP are copied to EXP. A common mapping is to copy the three most significant bits of DSCP to EXP, which maps DSCP PHBs into corresponding EXP values.

Answer

EXP is set by the egress PE router

Answer

EXP is set to the same binary as the entire DSCP byte

Answer

EXP is set to a static value per customer

Question 4

An SP uses NSO for service orchestration. A new service requires YANG data validation to ensure that the 'bandwidth' leaf does not exceed 10000 kbps when the 'service-type' is 'premium'. Which YANG statement is appropriate?

Accepted Answer

must '. <= 10000' { error-message 'Bandwidth too high'; }. A 'must' expression in YANG provides a XPath condition that must evaluate to true for the data to be valid.

Answer

when '. <= 10000'

Answer

leafref bandwidth { path '../bandwidth-limit'; }

Answer

range '0..10000';

Question 5

Which NETCONF operation is used to retrieve the entire running configuration from a network device?

Accepted Answer

get-config. The NETCONF `get-config` operation is specifically designed to retrieve a configuration datastore, such as the running configuration. Unlike the `get` operation, which retrieves both configuration and state data, `get-config` targets only the configuration data, making it the precise operation for fetching the entire running configuration from a network device.

Answer

get

Answer

commit

Answer

edit-config

Question 6

An SP is deploying SR-TE policies using PCEP. What is the role of the SR-PCE in this architecture?

Accepted Answer

It computes explicit paths and sends them to routers via PCEP. SR-PCE acts as a central controller that computes paths for SR-TE policies and communicates them via PCEP.

Answer

It forwards MPLS packets based on labels

Answer

It stores all routing tables in the network

Answer

It replaces the IGP for routing

Question 7

An engineer configures a class-map to match traffic with DSCP AF41. What is the numerical DSCP value for AF41?

Accepted Answer

34. AF41 is Assured Forwarding class 4, low drop probability, with DSCP decimal value 34 (binary 100010).

Answer

38

Answer

36

Answer

32

Question 8

In a hierarchical QoS policy, the parent policy shapes to 20 Mbps. The child policy has a class for voice with priority and police rate 2 Mbps, and a class for data with bandwidth 10 Mbps. What is the maximum bandwidth voice traffic can consume under congestion?

Accepted Answer

2 Mbps. Voice is in LLQ with a police rate of 2 Mbps; even though the parent shape is 20 Mbps, policing limits voice to 2 Mbps to prevent starvation.

Answer

8 Mbps

Answer

20 Mbps

Answer

10 Mbps

Question 9

What is the primary function of the NED (Network Element Driver) in NSO?

Accepted Answer

Abstract the device configuration and map YANG to device CLI. The NED (Network Element Driver) in Cisco NSO is the component that abstracts the native device configuration and translates YANG data models into device-specific CLI commands or protocols (e.g., SNMP, NETCONF). This allows NSO to manage heterogeneous devices using a unified YANG-based service model, making option A correct.

Answer

Provide RESTCONF endpoints

Answer

Store the configuration database

Answer

Handle northbound API requests

Question 10

Which RESTCONF HTTP method should be used to replace the entire configuration of a device interface?

Accepted Answer

PUT. RESTCONF uses PUT to replace a resource entirely; PATCH is for partial updates.

Answer

GET

Answer

DELETE

Answer

POST

Question 11

An SP network experiences congestion on a core link. The QoS policy uses CBWFQ with classes for voice (LLQ), video (bandwidth 30%), and data (bandwidth 50%). During congestion, voice traffic spikes to 25% of the link. What happens to video and data traffic?

Accepted Answer

Video and data share the remaining 75% as per their bandwidth percentages. LLQ is strict priority, so voice gets priority up to its police rate; if voice exceeds, it may be dropped. Video and data get their allocated bandwidths, but if voice exceeds its allowed priority, it may be policed, preventing starvation.

Answer

Video and data are dropped entirely until voice subsides

Answer

Video and data are unaffected

Answer

Video and data are treated as best effort

Question 12

What is the main difference between traffic shaping and traffic policing?

Accepted Answer

Shaping buffers excess traffic; policing drops or re-marks. Traffic shaping buffers excess traffic that exceeds the configured rate, smoothing bursts and delaying packets to conform to the desired rate. Traffic policing, on the other hand, immediately drops or re-marks packets that exceed the rate limit, without buffering. This fundamental difference in handling excess traffic—buffering versus dropping/re-marking—is the key distinction between the two QoS mechanisms.

Answer

Shaping is only for ingress; policing for egress

Answer

Shaping drops excess traffic; policing buffers it

Answer

Both perform the same function

Question 13

Which TWO statements correctly describe the DiffServ model? (Choose two.)

Accepted Answer

DSCP uses 6 bits, providing 64 possible values. The DiffServ model uses the 6-bit Differentiated Services Code Point (DSCP) field in the IP header to mark packets, providing 2^6 = 64 possible values (0-63). This allows for up to 64 distinct per-hop behaviors (PHBs), enabling scalable QoS classification beyond the limited 8 classes of the older IP Precedence model.

Answer

AF classes use DSCP values 0-7

Answer

Best Effort is marked with DSCP 46

Answer

DiffServ uses a 3-bit field for QoS marking

Question 14

An SP is designing MQC policies. Which THREE statements about MQC are true? (Choose three.)

Accepted Answer

MQC uses a three-step process: define class-map, policy-map, and apply service-policy. MQC uses class-map, policy-map, and service-policy; class-map can match multiple criteria; policy-map defines actions.

Answer

In a policy-map, the class class-default is optional

Answer

A policy-map can contain only one class

Question 15

An SP uses NSO for automation. Which TWO protocols can NSO use as southbound interfaces to communicate with network devices? (Choose two.)

Accepted Answer

RESTCONF. RESTCONF is a southbound interface that NSO uses to communicate with network devices. It is a RESTful protocol that uses HTTP methods (GET, POST, PUT, DELETE) to access YANG-defined data models, making it suitable for modern programmable devices. NSO leverages RESTCONF to configure and manage devices that support this protocol, providing a standardized, stateless interface for automation.

Answer

gRPC

Answer

SNMP

Answer

HTTP