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

Question 1

Refer to the exhibit. An engineer pushes this XML configuration via NETCONF, but the device rejects it with an error stating 'invalid policy-map structure'. What is the problem?

Accepted Answer

The class name must be defined in a class-map before being referenced in the policy-map. The error 'invalid policy-map structure' occurs because the policy-map references a class name that has not been defined in a class-map beforehand. In Cisco IOS, a class-map must exist before it can be used inside a policy-map; otherwise, the device rejects the configuration as structurally invalid.

Answer

The configuration is missing the class-default class

Answer

The YANG model does not support the 'police' keyword

Answer

The priority and police commands cannot coexist in the same class

Question 2

Refer to the exhibit. An engineer checks the policy and notices that the policing is not working as expected—traffic is not being dropped even when exceeding 1 Mbps. What could be the issue?

Accepted Answer

The shape command is interfering with the police command. The shape command can interfere with police by smoothing traffic, which may prevent the policer from seeing bursts that exceed the committed rate. If the shape rate is set higher than the police rate, the policer may not drop traffic because the shaper is already limiting the output to a lower rate. Additionally, the order of operations in a service policy matters: if shape is applied before police, the policer sees shaped traffic, which may not exceed the rate.

Answer

The service-policy is applied inbound, but police should be applied outbound to be effective

Answer

The police command is missing an exceed-action, so traffic is transmitted instead of dropped

Answer

The police rate is too high compared to interface speed

Question 3

An engineer is configuring QoS for voice traffic on a Cisco router. Which marking should be applied to voice packets to ensure proper end-to-end prioritization?

Accepted Answer

DSCP EF. Voice traffic requires low latency, jitter, and packet loss. DSCP EF (Expedited Forwarding, per-hop behavior value 46) is the standard marking for real-time voice payloads, ensuring strict priority queuing (PQ) across the network. RFC 3246 defines EF for this purpose, and Cisco routers use it to map voice to the priority queue.

Answer

DSCP AF41

Answer

DSCP CS3

Answer

IP Precedence 3

Question 4

A customer has a 100 Mbps access link and wants to limit traffic to 95 Mbps with burst allowance up to 100 Mbps. Which QoS action should be applied on the egress interface?

Accepted Answer

police. Police is the correct QoS action because it allows you to enforce a maximum traffic rate (95 Mbps) while permitting bursts up to 100 Mbps, dropping or remarking excess traffic. Unlike shaping, policing does not buffer traffic, so it can enforce a hard limit on egress without introducing delay, which matches the requirement to limit traffic with a burst allowance.

Answer

shape

Answer

priority

Answer

bandwidth

Question 5

An enterprise uses IPsec VPN to connect branch offices. They apply QoS policies on the tunnel interface but notice that original DSCP markings are not preserved after encryption. Which feature should be enabled to maintain end-to-end QoS?

Accepted Answer

QoS pre-classify. When IPsec encrypts a packet, the original IP header (including DSCP markings) is hidden inside the tunnel payload. The tunnel interface then applies a new outer IP header, and QoS policies applied to the tunnel interface classify based on the outer header's DSCP, which defaults to 0. Enabling 'qos pre-classify' on the crypto map or tunnel interface copies the original DSCP value to the outer IP header before encryption, preserving end-to-end QoS markings across the IPsec tunnel.

Answer

AutoQoS

Answer

MPLS TE

Answer

NBAR

Question 6

In an MPLS network, which field in the MPLS label header is used to carry QoS information between LSRs?

Accepted Answer

EXP bits. In MPLS, the EXP (Experimental) bits, also known as the Traffic Class (TC) field per RFC 5462, are 3 bits in the MPLS label header used to carry Quality of Service (QoS) information between Label Switch Routers (LSRs). These bits allow LSRs to apply per-hop behaviors (PHBs) such as queuing and scheduling based on the packet's QoS class, enabling differentiated services across the MPLS network.

Answer

ToS

Answer

IP Precedence

Answer

DSCP

Question 7

A network automation engineer needs to retrieve QoS policy statistics from a Cisco IOS XE device using RESTCONF. Which YANG module should be targeted?

Accepted Answer

Cisco-IOS-XE-QoS. The Cisco-IOS-XE-QoS YANG module is the native Cisco module that provides the data model for QoS policy configuration and operational statistics on IOS XE devices. Since the engineer is using RESTCONF to retrieve QoS statistics from a Cisco IOS XE device, this module is the correct target because it is specifically designed for and supported on IOS XE platforms.

Answer

ietf-qos

Answer

Cisco-NX-OS-device

Answer

openconfig-qos

Question 8

Which congestion avoidance technique drops packets probabilistically before the queue becomes full?

Accepted Answer

WRED. WRED (Weighted Random Early Detection) is a congestion avoidance mechanism that monitors the average queue depth and, when it exceeds a configured threshold, begins dropping packets probabilistically before the queue becomes completely full. This proactive dropping signals TCP senders to reduce their transmission rates, thereby preventing tail drop and global synchronization.

Answer

FIFO

Answer

Priority Queuing

Answer

Custom Queuing

Question 9

Which TWO conditions cause a router to drop packets when a police action is configured?

Accepted Answer

exceed action is drop. When a police action is configured, the router uses a token bucket model to meter traffic. If traffic exceeds the committed information rate (CIR) and burst size, it falls into the 'exceed' or 'violate' category depending on the configured conform and exceed burst parameters. The 'drop' action for exceed or violate conditions explicitly instructs the router to discard those packets, enforcing traffic shaping or policing at the hardware or software level.

Answer

conform action is drop

Answer

no bandwidth is reserved

Answer

queue is full

Question 10

Which THREE QoS mechanisms can be used for traffic policing?

Accepted Answer

dual-rate three-color. Traffic policing uses token bucket mechanisms to measure and enforce traffic rates. Dual-rate three-color (RFC 2698) uses two token buckets (CIR/PIR) to mark packets as green, yellow, or red, allowing separate policing of committed and peak rates. Single-rate two-color (RFC 2697) uses one token bucket to mark packets as either conforming (green) or exceeding (red). Both are standard policing implementations.

Answer

WRED

Answer

shaping

Question 11

A service provider operates a large MPLS network with hundreds of PE routers. They have deployed QoS policies to ensure real-time traffic (voice and video) receives priority. The policy uses LLQ with a strict priority queue for voice (DSCP EF) and another for video (DSCP AF41). Each priority queue has a policer to prevent starvation of other classes. Recently, a customer reports that their video calls are experiencing jitter and packet loss. The engineer examines the PE router connected to this customer. The interface is GigabitEthernet0/0/1 with the policy applied in the output direction. Show commands indicate that the video priority queue is frequently dropping packets, while the voice queue has no drops. The police for the video queue is set to 10 Mbps. The customer's contract guarantees 20 Mbps for video traffic. However, the actual video traffic is averaging 15 Mbps with bursts to 18 Mbps. The engineer notes that the video class also has a bandwidth command of 10 Mbps under the priority statement. What is the most likely cause of the video packet loss?

Accepted Answer

The police rate for the video queue is too low; increase it to at least 20 Mbps.. The video priority queue is dropping packets because the police rate (10 Mbps) is lower than the actual traffic rate (15 Mbps average, bursts to 18 Mbps). In LLQ, the priority queue uses a policer to enforce a maximum rate; traffic exceeding the police rate is dropped. The bandwidth command under priority only reserves bandwidth for the queue in the absence of congestion or for shaping purposes, but the policer is the active enforcement mechanism that causes drops. Increasing the police rate to at least 20 Mbps aligns with the customer's guaranteed rate and prevents drops.

Answer

The bandwidth command under priority is too low; increase it to 20 Mbps.

Answer

The video queue is using the wrong queue limit; increase the queue limit to accommodate bursts.

Answer

The video traffic is not being classified correctly; check the class-map match criteria.

Question 12

A large enterprise recently implemented centralized QoS policies using Cisco DNA Center. The policies are pushed via RESTCONF to the branch routers. After the deployment, the branch office reports that critical business applications (like ERP) are being delayed. The network team verifies that the QoS policy is applied correctly on the WAN interface (Serial0/0/0) with a shape of 10 Mbps. The policy-map has a class for 'critical-data' with bandwidth 5 Mbps and another class for 'transactional-data' with bandwidth 3 Mbps. The remaining traffic is in class-default with fair-queue. The branch router's CPU utilization is normal. The interface output shows that the queue for critical-data is rarely full, but packets are experiencing high latency. The engineer pings from the branch server to the central site and sees 200 ms RTT normally, but up to 500 ms during peak hours. What is the most likely cause of the high latency for critical-data?

Accepted Answer

The shape rate is too low; increase it to match the access link speed (e.g., 20 Mbps).. The shape rate of 10 Mbps is the bottleneck. Even though the critical-data class has a bandwidth guarantee of 5 Mbps, the overall interface is shaped to 10 Mbps. During peak hours, when the sum of all traffic (critical, transactional, and default) exceeds 10 Mbps, packets are queued at the shaper. This queueing introduces additional delay (up to 500 ms) for all classes, including critical-data, because the shaper enforces a single token bucket for the entire interface. Increasing the shape rate to match the actual access link speed (e.g., 20 Mbps) would reduce the queuing delay by allowing more traffic to be transmitted immediately.

Answer

The critical-data class needs a priority command to reduce latency.

Answer

The queue-limit for critical-data is too high; reduce it to force early drops.

Answer

The bandwidth command in critical-data should be increased above 5 Mbps.

Question 13

A network engineer is troubleshooting QoS on a Cisco ASR 1000 router. The router has a service-policy applied on the ingress interface GigabitEthernet0/0/0. The policy uses a class-map to match traffic based on NBAR protocol discovery for 'cisco-jabber'. The goal is to mark the traffic with DSCP AF41. However, when the engineer checks the policy statistics, the class 'jabber' shows zero matches, even though the users are actively using Cisco Jabber. The NBAR protocol discovery is enabled globally and on the interface. The engineer verifies that the NBAR protocol pack is up-to-date. What is the most likely reason for the class-map not matching?

Accepted Answer

Cisco Jabber traffic is encrypted, so NBAR cannot identify it. Cisco Jabber uses encrypted signaling and media (SRTP/TLS), which prevents NBAR from performing deep packet inspection to identify the application. Even with an up-to-date protocol pack, NBAR cannot match encrypted traffic unless decryption is performed elsewhere. Therefore, the class-map matching 'cisco-jabber' via NBAR protocol discovery will show zero matches.

Answer

The service-policy should be applied on the egress interface instead

Answer

The class-map is using 'match access-group' instead of 'match protocol'

Answer

The NBAR protocol pack is not activated on this interface

Question 14

Which TWO actions are required to implement QoS trust boundaries correctly in a service provider network?

Accepted Answer

Configure classification at the edge. Classification must be performed at the network edge to establish the trust boundary and enforce QoS policies. Option D is correct because setting trust to CoS on access ports allows the switch to accept the priority markings from trusted devices like IP phones or routers, which is essential for maintaining the trust boundary. Option B is incorrect because Auto QoS is not a mandatory requirement for trust boundaries; it is an optional feature that automates QoS configuration. Option C is incorrect because shaping is applied at customer premises and is not directly related to trust boundary implementation. Option E is incorrect because MPLS EXP bits are used for core marking, not for configuring trust boundaries at the access layer.

Answer

Enable auto QoS on all interfaces

Answer

Apply shaping at customer premises

Answer

Use MPLS EXP bits for core marking

Question 15

Based on the exhibit, which statement is true about the applied QoS policy?

Accepted Answer

Video traffic is being dropped at a rate of 5 kbps.. The exhibit shows that the video class has a police rate of 20 kbps and an exceed rate of 5 kbps, indicating that 5 kbps of video traffic is being dropped due to the policer. Option A is false because the voice class has no drops. Option B is false because the policy does not use low-latency queuing (LLQ); it uses policing instead. Option D is false because voice traffic shows zero drops.

Answer

The default class matches only best-effort traffic.

Answer

The policy uses low-latency queuing for voice.

Answer

Voice traffic is experiencing drops due to policer.