350-501 Automation and Quality of Service — Timed 20 Questions

Question 1

A service provider is implementing QoS on an MPLS network to support voice, video, and data traffic. Which queuing mechanism provides the lowest latency for real-time traffic?

Accepted Answer

LLQ. LLQ (Low Latency Queuing) is the correct choice because it provides a strict priority queue specifically designed for real-time traffic like voice and video. By placing delay-sensitive packets into a dedicated priority queue that is serviced before all other queues, LLQ ensures minimal and predictable latency, which is essential for maintaining voice quality in an MPLS network.

Answer

FIFO

Answer

WRED

Answer

CBWFQ

Question 2

An engineer is troubleshooting a QoS policy on a Cisco router. The policy is intended to mark voice traffic with DSCP EF and video traffic with DSCP AF41. After applying the policy, voice traffic is correctly marked, but video traffic is marked as DSCP 0. What is the most likely cause?

Accepted Answer

The class map for video traffic does not match the traffic correctly.. Option A is correct because the most common reason for video traffic being marked as DSCP 0 (default) while voice traffic is correctly marked is that the class map for video traffic fails to match the intended packets. This could be due to an incorrect match statement (e.g., using the wrong ACL, protocol, or DSCP value) or a misconfigured match criterion that does not capture the video flows. Since voice traffic is marked correctly, the policy itself is applied and functional, isolating the issue to the video class map's matching logic.

Answer

The video traffic is being policed and dropped.

Answer

The trust boundary is set to 'trust dscp' and the incoming video traffic is not marked.

Answer

The policy is not applied to the correct interface direction.

Question 3

A service provider uses an MPLS-TE tunnel to carry voice and data traffic. The tunnel is experiencing packet loss during congestion. The engineer wants to ensure that voice traffic receives guaranteed bandwidth and low latency while data traffic uses remaining bandwidth. Which QoS configuration should be applied on the tunnel interface?

Accepted Answer

LLQ with a priority queue for voice and a default class for data. Option A is correct because Low Latency Queuing (LLQ) allows you to place voice traffic into a strict priority queue, ensuring guaranteed bandwidth and low latency during congestion, while the default class uses CBWFQ to allocate remaining bandwidth to data traffic. This matches the requirement of prioritizing voice without starving data entirely, as the priority queue is policed to prevent voice from consuming all bandwidth.

Answer

CBWFQ with bandwidth allocation for voice and data

Answer

Policing on voice traffic to limit its rate

Answer

Shaping on the tunnel to 75% of bandwidth with no queuing

Question 4

An engineer is configuring MPLS VPN and needs to ensure that customer traffic is automatically marked with a specific QoS policy based on the VPN. Which method should be used to propagate QoS markings across the MPLS network?

Accepted Answer

Use MPLS EXP bits to mark traffic at the ingress PE and map to QoS at egress. In an MPLS VPN environment, QoS markings must be preserved across the MPLS backbone. MPLS EXP (Experimental) bits are the standard mechanism to carry QoS information within the MPLS label stack. At the ingress PE, customer traffic is classified and marked with the appropriate EXP bits based on the VPN or other criteria. The egress PE then uses these EXP bits to map traffic to the correct QoS policy, ensuring end-to-end QoS treatment.

Answer

Use 802.1p CoS on the CE-PE link and preserve it across the MPLS backbone

Answer

Use IP ToS bits to mark traffic and rely on MPLS to preserve them

Answer

Set DSCP at the ingress PE and preserve it across the MPLS backbone

Question 5

A network administrator configures a class map to match VoIP traffic using 'match ip dscp ef' on a Cisco router. However, the QoS policy is not applying the expected marking to VoIP packets. What is a possible reason?

Accepted Answer

The VoIP traffic is not marked with DSCP EF from the source.. Option B is correct because the 'match ip dscp ef' command in the class map checks the DSCP value already present in the incoming VoIP packets. If the source device (e.g., an IP phone) does not mark the packets with DSCP EF (46), the class map will not match, and the QoS policy will not apply the expected marking. The policy can only re-mark packets that are already matched by the class map.

Answer

The policy is applied in the output direction instead of input.

Answer

The policy is applied to the wrong interface.

Answer

The class map uses the wrong match type.

Question 6

Which TWO QoS mechanisms are used to provide congestion avoidance? (Choose two.)

Accepted Answer

RED. RED (Random Early Detection) and WRED (Weighted Random Early Detection) are congestion avoidance mechanisms that proactively drop packets before a queue becomes full, signaling TCP senders to reduce their transmission rate. Unlike congestion management tools (like CBWFQ or LLQ) that queue packets during congestion, RED/WRED monitor average queue depth and drop packets probabilistically to prevent tail drops and global TCP synchronization.

Answer

Policing

Answer

CBWFQ

Answer

LLQ

Question 7

Which THREE statements about implementing QoS in an MPLS VPN environment are correct? (Choose three.)

Accepted Answer

DSCP values can be set at the PE router to classify customer traffic into different classes.. Option A is correct because on a PE router, DSCP values can be set or remarked to classify incoming customer traffic into distinct service classes. This classification is performed at the ingress edge of the MPLS VPN network, allowing the provider to apply appropriate per-hop behaviors (PHBs) before the traffic is label-switched.

Answer

QoS policies cannot be applied to MPLS interfaces due to label encapsulation.

Answer

802.1p CoS marking is preserved across the MPLS backbone by default.

Question 8

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

Accepted Answer

The VOICE class is allocated a strict priority queue with a bandwidth limit of 10% of the interface bandwidth.. The exhibit shows a QoS policy where the VOICE class is configured with the 'priority' command and a bandwidth statement of 10 percent. In Cisco IOS, the 'priority' command under a class map creates a strict priority queue (LLQ), and the bandwidth percentage defines the maximum amount of interface bandwidth that can be used by this queue, ensuring low-latency treatment for voice traffic. Therefore, option C is correct.

Answer

The class-default will use weighted fair queuing (WFQ).

Answer

The VIDEO class will be limited to 30% of the total interface bandwidth.

Answer

The VIDEO class is guaranteed at least 30% of the total interface bandwidth.

Question 9

A service provider is deploying QoS on a Cisco ASR 9000 router to support a triple-play service (voice, video, data) over an Ethernet access network. The network engineer must ensure that voice traffic (EF) is prioritized, video traffic (AF41) is treated with low loss, and data traffic (default) gets best-effort service. Additionally, the engineer must implement a hierarchical QoS policy to shape the aggregate subscriber traffic to 100 Mbps on a GigabitEthernet interface, with the following per-class bandwidth allocations: voice 10 Mbps, video 40 Mbps, and data 50 Mbps. During testing, voice packets are being dropped under congestion, and video traffic is experiencing jitter. The current QoS configuration is as follows:

policy-map CHILD
 class VOICE
  priority level 1
  police rate 10 mbps
 class VIDEO
  bandwidth remaining ratio 40
 class DATA
  bandwidth remaining ratio 50
!
policy-map PARENT
 class class-default
  shape average 100 mbps
  service-policy CHILD
!

Which action should the engineer take to resolve the issues while maintaining the design objectives?

Accepted Answer

Remove the police from the voice class and add 'priority level 1' without police; this ensures voice is priority queued without dropping.. Option B is correct because the current configuration uses a police rate of 10 Mbps under the voice priority class, which causes voice packets to be dropped when they exceed that rate, even though priority queuing should guarantee low latency. Removing the police and keeping 'priority level 1' allows voice traffic to be strictly prioritized without a hard rate limit, ensuring no drops for voice while still allowing the parent shaper to enforce the aggregate 100 Mbps. This resolves the voice drops and, by preventing voice from being policed, reduces jitter for video traffic that might otherwise be affected by bursty voice drops.

Answer

Change the video class to 'priority level 2' to give it preferential treatment over data.

Answer

Add a 'police' to the video class with a CIR of 40 Mbps and 'conform-action transmit exceed-action drop' to protect voice.

Answer

Change the voice class to use 'bandwidth remaining percent 10' and remove the police.

Question 10

A service provider is designing a QoS policy for a multi-service MPLS VPN network that carries voice, video, and data traffic. The network uses DiffServ and MPLS EXP markings. The design must ensure that voice traffic is given priority over video and data, while video traffic should have better treatment than data but not at the expense of voice. The provider plans to use a hierarchical QoS (HQoS) policy at the PE-CE interfaces. Which configuration approach best meets these requirements?

Accepted Answer

Apply a parent policy with a shape for the total bandwidth and a child policy with LLQ for voice and CBWFQ for video and data.. Option C is correct because hierarchical QoS (HQoS) allows the service provider to enforce a total bandwidth shape at the parent level while using a child policy to apply LLQ for voice (ensuring strict priority) and CBWFQ for video and data (ensuring video gets better treatment than data without starving voice). This meets the requirement that video should not degrade voice, as the parent shape prevents any single class from monopolizing the link, and the child policy’s LLQ guarantees voice priority over all other traffic.

Answer

Mark voice traffic with EXP 5, video with EXP 4, and data with EXP 0, and rely on the core to prioritize based on EXP.

Answer

Use a single-level policy with LLQ for voice and video together, and CBWFQ for data.

Answer

Apply class-based shaping to each traffic class separately on the interface.

Question 11

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.. Option B is correct because 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 12

A service provider has deployed a new MPLS L3VPN service for a customer with multiple sites. The customer reports intermittent voice quality issues during peak hours. The provider uses a DiffServ QoS model with MPLS EXP markings. The PE routers apply the following policy on the customer-facing interfaces:

policy-map CUSTOMER-OUT
 class VOICE
  priority percent 10
  set mpls experimental topmost 5
 class VIDEO
  bandwidth remaining percent 30
  set mpls experimental topmost 4
 class DATA
  bandwidth remaining percent 70
  set mpls experimental topmost 0

The core network has a simple policy that maps EXP 5 to PQ, EXP 4 to a low-latency queue, and EXP 0 to best effort. The provider monitors the network and finds that the PE-CE interfaces are not congested (average utilization is 40%), but the voice packets are experiencing jitter. The customer's voice traffic is about 5% of the link capacity. Which action is most likely to reduce the jitter?

Accepted Answer

Add a police command under the VOICE class to limit voice traffic to 5% and shape the priority queue.. The voice traffic is only 5% of link capacity, but the priority percent 10 command allows up to 10% of the link to be treated as priority. During peak hours, if other traffic (e.g., video or data) bursts and exceeds the remaining bandwidth, the priority queue can still be policed implicitly by the scheduler, but jitter arises because the priority queue is not rate-limited. Adding a police command under the VOICE class to limit voice to 5% and shaping the priority queue ensures that voice traffic does not exceed its actual rate, preventing microbursts that cause jitter in the priority queue.

Answer

Configure the core routers to map EXP 5 to a low latency queue instead of PQ.

Answer

Increase the priority percent to 20 for voice traffic.

Answer

Change the set mpls experimental topmost to set ip dscp ef for voice.

Question 13

Drag and drop the steps to configure MPLS LDP on a Cisco router into the correct order.

Question 14

Drag and drop the steps to configure a standard ACL on a Cisco router into the correct order.

Question 15

Match each BGP attribute to its category or purpose.

Question 16

Match each network automation tool to its primary use.

Question 17

A service provider needs to prioritize voice traffic over best-effort data in an MPLS VPN. The PE router uses a QoS policy applied to the ingress interface. Which action ensures that voice packets are marked with the correct DSCP value before entering the MPLS core?

Accepted Answer

Apply a policy-map that matches voice traffic using a class-map and the 'set dscp ef' action.. Option A is correct because the question specifies that the PE router must mark voice packets with the correct DSCP value before they enter the MPLS core. The 'set dscp ef' action in a policy-map applied to the ingress interface explicitly sets the DSCP field to EF (46) for voice traffic matched by a class-map, ensuring proper classification and treatment across the MPLS network. This is the standard method for marking IP packets at the edge before MPLS encapsulation.

Answer

Configure the ingress interface with 'mls qos trust dscp' to preserve the customer marking.

Answer

Use a policy-map with the 'set mpls experimental 5' command.

Answer

Apply a police action to drop traffic exceeding the voice bandwidth.

Question 18

A network operator deploys a QoS policy on a route reflector to classify traffic based on BGP community values. However, the policy is not matching the intended traffic. Which is the most likely cause?

Accepted Answer

The class-map uses a match statement referencing BGP community values, which are not visible at the QoS classification stage.. BGP community values are exchanged as part of the BGP routing update and are stored in the BGP table, but they are not carried in the IP packet header. QoS classification in Cisco IOS operates on fields within the Layer 2 or Layer 3 packet header (e.g., DSCP, IP precedence, CoS) and cannot inspect BGP attributes like community values. Therefore, a class-map using a match statement for BGP communities will never match traffic at the QoS classification stage, making this the most likely cause of the policy not working.

Answer

The policy-map is applied to the wrong interface.

Answer

The service-policy is applied in the wrong direction (input vs output).

Answer

The class-default class is consuming all traffic.

Question 19

An SP uses BGP FlowSpec to mitigate DDoS attacks. They also want to rate-limit the traffic per FlowSpec rule. Which configuration is required to enable policing within a FlowSpec action?

Accepted Answer

Include the 'rate-limit' action in the FlowSpec rule using the 'action' clause.. Option D is correct because BGP FlowSpec allows the inclusion of a 'rate-limit' action within the FlowSpec rule's 'action' clause to enforce policing. This action directly applies a traffic rate limit (in bits per second) to the matched flow, enabling DDoS mitigation without requiring external QoS policies or MPLS TE reservations.

Answer

Configure MPLS TE bandwidth reservation for the FlowSpec routes.

Answer

Use policy-based routing (PBR) to set QoS parameters.

Answer

Apply a QoS policy-map to the interface and match the FlowSpec destination.

Question 20

An SP's core router experiences excessive packet drops on a congested link. The QoS policy uses CBWFQ with 4 queues. The drops occur only in the queue for a premium business class. Which is the most likely cause?

Accepted Answer

The premium class is configured with a police action that drops traffic when it exceeds the committed information rate.. The premium business class queue is experiencing excessive packet drops because a police action configured with a 'drop' exceed-action is discarding traffic that exceeds the committed information rate (CIR). Unlike congestion management (queuing), policing is a congestion avoidance mechanism that drops packets immediately when the traffic rate surpasses the configured CIR, regardless of the queue depth or available bandwidth. This explains why drops occur only in the premium class queue, as the policer is explicitly dropping excess packets before they even enter the CBWFQ scheduler.

Answer

The 'bandwidth remaining' command is missing from the policy-map.

Answer

The interface is using MLP with interleaving.

Answer

The queue is using tail-drop without WRED.