350-401 QoS — 10 Questions

Question 1

A network engineer is configuring QoS on a Cisco Catalyst 3850 switch to prioritize voice traffic. The switch is connected to an IP phone and a PC using a single access port. The engineer applies a service policy on the access port that marks CoS 5 for voice and CoS 0 for data. However, the IP phone is not receiving any voice packets. What is the most likely cause?

Accepted Answer

The switchport is configured as an access port without 'mls qos trust cos'. The correct answer identifies that the switchport must be configured to trust the CoS markings from the IP phone. If the port is set to untrusted, the switch will re-mark all incoming CoS values to 0, overriding the phone's markings.

Answer

The IP phone is not configured with the correct VLAN for voice traffic

Answer

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

Answer

The switch does not support CoS marking on access ports

Question 2

An engineer is deploying QoS on a WAN link between two sites using a Cisco ISR 4451 router. The link is a 10 Mbps MPLS circuit. The engineer wants to ensure that voice traffic (EF) is never dropped, even during congestion. The current policy uses a single class map for voice with a policer that drops excess traffic. During peak hours, users report choppy voice calls. What change should the engineer make?

Accepted Answer

Change the policer to a shaper and apply it to the voice class. The correct answer is to replace the policer with a shaper or use a low-latency queue (LLQ) to provide strict priority queuing. A policer drops excess traffic, which can cause voice packet loss. Using LLQ ensures voice gets priority without dropping.

Answer

Increase the policer rate to 20 Mbps to accommodate voice bursts

Answer

Remove the policer and rely on FIFO queuing

Answer

Apply the policy in the output direction only

Question 3

A network engineer is troubleshooting QoS on a Cisco Nexus 9000 switch. The switch is configured with a policy map that uses a class-default with a bandwidth remaining percent of 100. However, during congestion, traffic in a priority queue (class-map for EF) is experiencing drops even though the priority queue is not fully utilized. What is the most likely cause?

Accepted Answer

The priority queue is implicitly policed to a default rate on Nexus switches. The correct answer is that the priority queue is policed by default on Nexus switches to prevent starvation of other queues. The priority queue has a policer that drops traffic if it exceeds a certain rate, even if the queue is not congested.

Answer

The class-default bandwidth remaining percent should be set to 0

Answer

The priority queue is not configured with a queue-limit

Answer

The switch is using strict priority queuing without any shaping

Question 4

An engineer is configuring QoS on a Cisco ASR 1000 router to support three traffic classes: voice (EF), video (AF41), and data (default). The link is a 50 Mbps Ethernet circuit. The engineer wants to guarantee 10 Mbps for voice, 20 Mbps for video, and the remaining for data. The current policy uses bandwidth percent statements. During congestion, voice traffic is not receiving its guaranteed bandwidth. What is the most likely cause?

Accepted Answer

The interface bandwidth command is not set to 50000 kbps. The correct answer is that bandwidth percent is based on the interface bandwidth, which may not match the actual link speed if the interface bandwidth is not set correctly. The engineer should use bandwidth remaining percent or shape the traffic.

Answer

The voice class should use priority instead of bandwidth

Answer

The video class should use bandwidth remaining percent

Answer

The policy map is applied in the input direction

Question 5

A network engineer is troubleshooting voice quality issues on a Cisco Catalyst 9300 switch. The switch is configured with auto QoS for voice, which enabled trust on the access ports. However, voice packets are being marked with DSCP EF but are still experiencing jitter. The engineer checks the interface queue statistics and sees that the priority queue is not being used. What is the most likely reason?

Accepted Answer

Auto QoS does not create a priority queue; a manual policy is required. The correct answer is that auto QoS for voice enables trust but does not automatically create a priority queue on the switch; the engineer must also configure a service policy that includes a priority queue for EF traffic.

Answer

The switch does not support DSCP-based queuing

Answer

The voice VLAN is not configured on the access port

Answer

The switch is using default CoS-to-queue mapping which maps EF to a non-priority queue

Question 6

An engineer is configuring QoS on a Cisco ISR 4331 router for a site-to-site VPN tunnel. The tunnel interface is configured with a service policy that uses a class map matching DSCP EF. The engineer notices that the policy is not shaping traffic as expected; the tunnel bandwidth is 20 Mbps but the shaper is set to 10 Mbps. However, traffic still exceeds 10 Mbps. What is the most likely cause?

Accepted Answer

The shaper should be applied to the physical interface instead of the tunnel interface. The correct answer is that the shaper must be applied to the physical interface, not the tunnel interface, because the tunnel interface does not have a direct view of the underlying bandwidth. Shaping on the tunnel interface is ineffective.

Answer

The shaper rate should be set to 20 Mbps to match the tunnel bandwidth

Answer

The class map should match on the outer IP header instead of the inner DSCP

Answer

The service policy should be applied in the input direction

Question 7

A network engineer is deploying QoS on a Cisco Catalyst 4500 switch to support four queues per port. The engineer wants to assign voice traffic to queue 1 (priority), video to queue 2, critical data to queue 3, and best-effort to queue 4. The switch is configured with the default CoS-to-queue mapping. However, video traffic is being placed in queue 1 along with voice. What should the engineer do to separate them?

Accepted Answer

Modify the CoS-to-queue mapping using the 'mls qos srr-queue output cos-map' command. The correct answer is to modify the CoS-to-queue mapping using the 'mls qos srr-queue output cos-map' command to assign different CoS values to different queues.

Answer

Change the video traffic marking to DSCP AF41 and rely on DSCP-to-queue mapping

Answer

Apply a service policy that uses a priority queue for voice only

Answer

Increase the number of queues to eight

Question 8

An engineer is troubleshooting QoS on a Cisco ASR 1002 router. The router is configured with a policy map that includes a class for voice with a priority command. During congestion, the engineer notices that voice traffic is being dropped even though the priority queue is not congested. The router logs show 'QoS: priority queue overflow'. What is the most likely cause?

Accepted Answer

The priority queue has a default policer that drops traffic exceeding a certain rate. The correct answer is that the priority queue has a default policer that limits the amount of traffic that can be sent as priority. When the voice traffic exceeds this policer, it is dropped, even if the queue is not full.

Answer

The interface bandwidth is set too low, causing the priority queue to be under-provisioned

Answer

The class map is not matching the voice traffic correctly

Answer

The router is using WRED on the priority queue

Question 9

A network engineer is configuring QoS on a Cisco Catalyst 2960-X switch to support marking of traffic based on VLAN. The switch has two VLANs: VLAN 10 (voice) and VLAN 20 (data). The engineer wants to mark all traffic from VLAN 10 with CoS 5 and all traffic from VLAN 20 with CoS 0. The engineer applies a policy map that matches on VLAN using a class map. However, the marking is not being applied. What is the most likely reason?

Accepted Answer

The switch does not support VLAN-based classification in QoS. The correct answer is that Catalyst 2960-X switches do not support matching on VLAN in a class map for QoS; they only support matching on CoS, DSCP, or IP precedence. The engineer must use a different matching criterion.

Answer

The policy map is applied in the wrong direction

Answer

The class map should use 'match access-group' instead of 'match vlan'

Answer

The switch requires the 'mls qos' command to be enabled globally

Question 10

A network engineer runs the following command on Router R1:

R1# show policy-map interface GigabitEthernet0/1
 GigabitEthernet0/1

Service-policy output: QOS_POLICY

Class-map: VOICE (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: ip dscp ef (46)
      Queueing
        queue limit 64 packets
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 0/0
      police cir 1000000 bc 15625 be 15625
        conformed 0 packets, 0 bytes; actions:
          transmit
        exceeded 0 packets, 0 bytes; actions:
          drop
        violated 0 packets, 0 bytes; actions:
          drop

Class-map: class-default (match-any)
      100 packets, 10000 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any
      Queueing
        queue limit 64 packets
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 100/10000

Based on this output, what can be concluded?

Accepted Answer

Voice traffic is not being classified because no packets match the VOICE class.. The output shows that the VOICE class is matching DSCP EF (46) and has a police command with a CIR of 1 Mbps. Since no packets have been matched (0 packets, 0 bytes), the voice traffic is not being classified. This indicates that either no voice traffic is present or the DSCP marking is not EF. The class-default is handling all traffic.

Answer

Voice traffic is being marked with DSCP EF and is being policed at 1 Mbps.

Answer

All traffic is being dropped due to the police action.

Answer

The policy-map is applied in the input direction.