350-401 QoS — Timed 30 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.

Question 11

A network engineer runs the following command on Router R2:

R2# show class-map
 Class Map match-any VOICE (id 1)
   Match ip dscp ef (46)
 Class Map match-any DATA (id 2)
   Match ip dscp af31 (26)
 Class Map match-any class-default (id 0)
   Match any

R2# show policy-map
  Policy Map QOS_POLICY
    Class VOICE
      priority level 1
      police cir 1000000 bc 15625 be 15625
    Class DATA
      bandwidth remaining percent 50
    Class class-default
      bandwidth remaining percent 50

R2# 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
        strict priority
        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: DATA (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: ip dscp af31 (26)
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 0/0
      bandwidth remaining percent 50 (0 kbps)

Class-map: class-default (match-any)
      100 packets, 10000 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 100/10000
      bandwidth remaining percent 50 (0 kbps)

Based on this output, what can be concluded?

Accepted Answer

All traffic is being handled by class-default, which gets 100% of the bandwidth.. The VOICE class uses strict priority queuing (priority level 1). However, with 0 packets matched, no voice traffic is being classified. The DATA class also shows 0 packets. All traffic falls into class-default. The bandwidth remaining percent commands allocate remaining bandwidth, but since only class-default has traffic, it effectively gets 100% of the bandwidth. The police command on VOICE is not affecting anything because no packets match.

Answer

Voice traffic is being prioritized with strict priority queuing and policed at 1 Mbps.

Answer

Data traffic is being guaranteed 50% of the remaining bandwidth.

Answer

The police command on VOICE is causing drops for voice traffic.

Question 12

A network engineer runs the following command on Router R3:

R3# show mls qos interface GigabitEthernet0/1
 GigabitEthernet0/1
  trust state: trust DSCP
  trust mode: trust dscp
  COS override: dis
  default COS: 0
  DSCP Mutation Map: default dscp mutation map
  trust device: none
  qos mode: port-based

R3# show mls qos
 QoS is enabled globally
  QoS global counters: total packets not matching QoS criteria = 0
  Total packets with known CoS = 0
  Total packets dropped by policing = 0

Based on this output, what can be concluded?

Accepted Answer

The interface trusts the DSCP markings of incoming packets.. The interface is configured to trust DSCP values. This means incoming packets will have their DSCP values preserved and used for QoS classification. The global QoS is enabled. The trust mode is 'trust dscp', indicating that the switch will trust the DSCP markings of received packets.

Answer

The interface is configured to trust CoS values.

Answer

The interface will overwrite incoming DSCP values with default CoS.

Answer

QoS is disabled globally.

Question 13

A network engineer runs the following command on Router R4:

R4# show policy-map interface GigabitEthernet0/1
 GigabitEthernet0/1

Service-policy input: SHAPE_POLICY

Class-map: class-default (match-any)
      1000 packets, 100000 bytes
      5 minute offered rate 100000 bps, drop rate 0 bps
      Match: any
      Queueing
      shape (average) cir 500000, bc 5000, be 5000
      target shape rate 500000

Based on this output, what can be concluded?

Accepted Answer

Traffic is being shaped to an average rate of 500 kbps.. The policy-map is applied to the input direction. Shaping is typically used on output to smooth traffic, but it can be applied to input as well, though it is less common. The shape command is shaping all traffic to an average CIR of 500 kbps. The offered rate is 100 kbps, which is below the shaping rate, so no drops occur.

Answer

Traffic is being policed to 500 kbps.

Answer

The policy is applied to output traffic.

Answer

The offered rate exceeds the shaping rate, causing drops.

Question 14

A network engineer runs the following command on Router R5:

R5# show queueing interface GigabitEthernet0/1
 Interface GigabitEthernet0/1 queueing strategy:  weighted fair
  Queueing on output: Weighted Fair Queueing
  Current fair queue configuration:
    Number of queues: 256
    Dynamic queues: 256
    Reserved queues: 0
  Current WFQ global configuration:
    Total dynamic queues: 256
    Total reserved queues: 0
  Class based weighted fair queueing: enabled
  Queueing on input: FIFO

Based on this output, what can be concluded?

Accepted Answer

The interface uses Weighted Fair Queueing for output with 256 queues.. The output shows that the output queueing strategy is Weighted Fair Queueing (WFQ) with 256 queues. Input queueing is FIFO. WFQ is a flow-based queuing method that provides fair bandwidth allocation among flows.

Answer

The interface uses FIFO queuing for output.

Answer

The interface uses Class-Based Weighted Fair Queueing (CBWFQ).

Answer

The interface uses Priority Queuing.

Question 15

A network engineer runs the following command on Router R6:

R6# show policy-map interface GigabitEthernet0/1
 GigabitEthernet0/1

Service-policy output: QOS_POLICY

Class-map: VOICE (match-any)
      500 packets, 50000 bytes
      5 minute offered rate 50000 bps, drop rate 0 bps
      Match: ip dscp ef (46)
      Queueing
        strict priority
        queue limit 64 packets
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 500/50000
      police cir 1000000 bc 15625 be 15625
        conformed 500 packets, 50000 bytes; actions:
          transmit
        exceeded 0 packets, 0 bytes; actions:
          drop
        violated 0 packets, 0 bytes; actions:
          drop

Class-map: DATA (match-any)
      1000 packets, 100000 bytes
      5 minute offered rate 100000 bps, drop rate 0 bps
      Match: ip dscp af31 (26)
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 1000/100000
      bandwidth remaining percent 50

Class-map: class-default (match-any)
      2000 packets, 200000 bytes
      5 minute offered rate 200000 bps, drop rate 0 bps
      Match: any
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 2000/200000
      bandwidth remaining percent 50

Based on this output, what can be concluded?

Accepted Answer

Data traffic is being guaranteed 50% of the remaining bandwidth.. The VOICE class uses strict priority queuing and is policing at 1 Mbps. The offered rate for voice is 50 kbps, well below the police rate, so all voice packets are conforming and transmitted. The DATA and class-default classes share remaining bandwidth equally (50% each). The total offered rate is 350 kbps, and since the interface bandwidth is likely higher, no drops occur.

Answer

Voice traffic is being dropped because it exceeds the police rate.

Answer

All traffic is being shaped to a CIR of 1 Mbps.

Answer

The policy-map is applied to input traffic.

Question 16

A network engineer runs the following command on Router R7:

R7# show mls qos interface GigabitEthernet0/1 statistics
 GigabitEthernet0/1
  Ingress statistics:
    dscp: incoming no_change classified policed dropped
    default: 0 0 0 0 0
    ef: 100 100 100 0 0
    af31: 200 200 200 0 0
    other: 300 300 300 0 0

Egress statistics:
    dscp: queued dropped
    default: 0 0
    ef: 100 0
    af31: 200 0
    other: 300 0

Based on this output, what can be concluded?

Accepted Answer

All packets are being queued without drops at egress.. The ingress statistics show that packets with DSCP EF, AF31, and other values are being classified and no policing drops occur. The egress statistics show that all packets are queued without drops. This indicates that the QoS policy is not dropping any traffic, and the interface is likely not congested.

Answer

Packets with DSCP EF are being dropped at ingress.

Answer

Policing is dropping packets with DSCP AF31.

Answer

The interface is not using any QoS policy.

Question 17

A network engineer runs the following command on Router R8:

R8# 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
        strict priority
        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: DATA (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: ip dscp af31 (26)
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 0/0
      bandwidth remaining percent 50

Class-map: class-default (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 0/0
      bandwidth remaining percent 50

Based on this output, what can be concluded?

Accepted Answer

The interface is not passing any traffic.. All classes show zero packets, zero bytes, and zero rates. This indicates that no traffic has been processed through this interface. The policy-map is applied but no packets have been seen.

Answer

Voice traffic is being prioritized but not policed.

Answer

Data traffic is being dropped due to policing.

Answer

The policy-map is applied to input traffic.

Question 18

A network engineer runs the following command on Router R9:

R9# show queueing interface GigabitEthernet0/1
 Interface GigabitEthernet0/1 queueing strategy:  class-based weighted fair
  Queueing on output: Class-based Weighted Fair Queueing
  Queueing on input: FIFO

R9# show policy-map interface GigabitEthernet0/1
 GigabitEthernet0/1

Service-policy output: QOS_POLICY

Class-map: VOICE (match-any)
      100 packets, 10000 bytes
      5 minute offered rate 10000 bps, drop rate 0 bps
      Match: ip dscp ef (46)
      Queueing
        strict priority
        queue limit 64 packets
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 100/10000
      police cir 1000000 bc 15625 be 15625
        conformed 100 packets, 10000 bytes; actions:
          transmit
        exceeded 0 packets, 0 bytes; actions:
          drop
        violated 0 packets, 0 bytes; actions:
          drop

Class-map: DATA (match-any)
      200 packets, 20000 bytes
      5 minute offered rate 20000 bps, drop rate 0 bps
      Match: ip dscp af31 (26)
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 200/20000
      bandwidth remaining percent 50

Class-map: class-default (match-any)
      300 packets, 30000 bytes
      5 minute offered rate 30000 bps, drop rate 0 bps
      Match: any
      Queueing
        (queue depth/total drops/no-buffer drops) 0/0/0
        (pkts output/bytes output) 300/30000
      bandwidth remaining percent 50

Based on this output, what can be concluded?

Accepted Answer

Data traffic is guaranteed 50% of the remaining bandwidth after priority queuing.. The output queueing strategy is CBWFQ. The VOICE class uses strict priority and is policed at 1 Mbps. All voice packets conform (100 packets). DATA and class-default share remaining bandwidth equally. The total offered rate is 60 kbps, well below any congestion, so no drops occur.

Answer

The interface uses WFQ for output queuing.

Answer

Voice traffic is being dropped because it exceeds the police rate.

Answer

The policy-map is applied to input traffic.

Question 19

Examine the following configuration snippet on a Cisco IOS-XE router:

interface GigabitEthernet0/1
 service-policy output QOS_POLICY

policy-map QOS_POLICY
 class VOICE
  priority percent 10
 class VIDEO
  bandwidth percent 30
 class class-default
  fair-queue

What is the effect of this configuration?

Accepted Answer

VOICE traffic is guaranteed 10% of the interface bandwidth with strict priority queuing, VIDEO traffic is guaranteed 30%, and all other traffic shares the remaining bandwidth using fair-queuing.. The policy-map applies a priority queue for VOICE traffic, guarantees bandwidth for VIDEO, and uses fair-queue for all other traffic on the output interface.

Answer

VOICE traffic is limited to 10% of bandwidth, VIDEO to 30%, and all other traffic is dropped if the interface is congested.

Answer

VOICE traffic is given priority over VIDEO, but VIDEO can use up to 30% of bandwidth only if VOICE is not using its allocation.

Answer

The policy-map is invalid because 'priority' and 'bandwidth' cannot be used together in the same policy-map.

Question 20

Consider the following configuration on a Cisco router:

class-map match-any CRITICAL_DATA
 match ip dscp af21 af22 af23

policy-map QOS
 class CRITICAL_DATA
  bandwidth remaining percent 50
 class class-default
  fair-queue

interface GigabitEthernet0/0
 service-policy output QOS

Which statement about this configuration is true?

Accepted Answer

Traffic matching DSCP AF21, AF22, or AF23 is guaranteed 50% of the interface bandwidth, and all other traffic is subject to fair-queuing.. The class-map matches DSCP values AF21, AF22, and AF23 (which are Assured Forwarding classes). The policy-map allocates 50% of remaining bandwidth to this class after any priority queues, and uses fair-queue for the default class.

Answer

Only traffic with DSCP AF21 is matched; AF22 and AF23 are ignored because 'match-any' requires all conditions to be true.

Answer

The configuration is invalid because 'bandwidth remaining percent' cannot be used with 'fair-queue' in the same policy-map.

Answer

The policy-map will only be applied to incoming traffic on GigabitEthernet0/0.