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CCNA Qos Questions

21 questions · Qos topic · All types, answers revealed

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1
Matchingmedium

Drag and drop each DSCP PHB on the left to its matching queue treatment on the right.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts
Matches

Expedited forwarding, strict priority queuing, low delay and jitter

Assured forwarding, four classes with three drop probabilities per class

Class selector, backward compatible with IP precedence, simple priority queuing

Best-effort, default queue, no guarantees

Default forwarding, same as best-effort (DSCP 0)

Why these pairings

EF PHB (DSCP 46) is for low-loss, low-latency traffic; AF PHBs (AF1x-AF4x) provide assured forwarding with four classes and three drop precedences; CS PHBs (CS1-CS7) are backward-compatible with IP precedence; BE (DSCP 0) is best-effort; DF (DSCP 0) is the default PHB.

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2
Matchingmedium

Drag and drop each DSCP value on the left to its matching Per-Hop Behavior (PHB) on the right.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts
Matches

EF

AF11

AF21

AF31

AF41

Why these pairings

DSCP 46 maps to EF (Expedited Forwarding), DSCP 10 maps to AF11 (Assured Forwarding class 1 low drop), DSCP 18 maps to AF21 (Assured Forwarding class 2 low drop), DSCP 26 maps to AF31 (Assured Forwarding class 3 low drop), DSCP 34 maps to AF41 (Assured Forwarding class 4 low drop).

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3
MCQhard

A large enterprise uses Cisco SD-WAN with multiple transport clouds (MPLS and Internet). The network team wants to ensure that voice traffic between two branch offices always uses the MPLS link, even if the Internet link has lower latency. The engineer creates a centralized data policy on the vSmart to match voice traffic based on DSCP EF and sets the preferred color to 'mpls'. After applying the policy, the engineer tests and finds that voice traffic is still using the Internet link. The vEdge routers show that the policy is received and active. What is the most likely reason for this failure?

A.The vEdge routers have not rebooted after the policy was applied.
B.The data policy was applied on the vEdge instead of the vSmart.
C.The DSCP EF marking is not supported in SD-WAN data policies.
D.The policy does not include a match condition for the correct VPN or site list.
AnswerD

Correct because the policy must be associated with the specific VPN and site list to apply to the traffic.

Why this answer

A centralized data policy on the vSmart must include match conditions for both the traffic (e.g., DSCP EF) and the scope of the policy (e.g., VPN list or site list). Without a site list or VPN list match, the policy may not apply to the specific branch-to-branch traffic, causing the vEdge to fall back to the default routing behavior (e.g., using the Internet link if it has lower latency). The vSmart distributes the policy to vEdges, but the vEdge only enforces it for matched traffic within the specified sites or VPNs.

Exam trap

Cisco often tests the misconception that a centralized data policy with only traffic match conditions (like DSCP) will automatically apply to all traffic, when in reality the policy must also include a site list or VPN list to define the scope of enforcement.

How to eliminate wrong answers

Option A is wrong because vEdge routers do not require a reboot for data policies to take effect; they are applied dynamically via the vSmart. Option B is wrong because the engineer explicitly created a centralized data policy on the vSmart, and applying it on the vEdge would be a localized policy, which is not the described scenario. Option C is wrong because DSCP EF (46) is fully supported in Cisco SD-WAN data policies for matching voice traffic; the issue is not a lack of support but missing scope conditions.

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4
Drag & Dropmedium

Drag and drop the steps of DSCP re-marking at enterprise WAN edge into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

At the enterprise WAN edge, traffic is first classified based on existing markings or other criteria. Then a policy-map is created to set the new DSCP value. The policy is applied outbound on the WAN interface.

The router re-marks packets as they exit. Finally, the new DSCP value is verified using show commands.

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5
Drag & Dropmedium

Drag and drop the steps of DSCP-to-CoS mapping at LAN boundary into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

First, trust the DSCP on the ingress interface. Then, map DSCP to CoS using a table-map. Apply the table-map in a policy-map.

Finally, apply the policy-map to the interface.

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6
Matchingmedium

Drag and drop each DSCP PHB on the left to its matching queue treatment on the right.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts
Matches

Strict priority queue, low latency

Assured forwarding with drop precedence

Class selector, maps to IP precedence

Best-effort, no bandwidth guarantee

Default PHB, identical to BE

Why these pairings

EF is for low-latency traffic (strict priority). AF uses four classes with drop probabilities. CS is backward-compatible with IP precedence.

BE is best-effort with no guarantees. DF is the default PHB (same as BE).

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7
Drag & Dropmedium

Drag and drop the steps of traffic shaping vs policing configuration steps into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

First, identify the traffic to be shaped or policed using a class-map. Then configure the policy-map with either shape or police command. Apply the service-policy in the appropriate direction.

For shaping, the router buffers excess traffic; for policing, it drops or re-marks. Finally, verify using show policy-map interface.

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8
MCQhard

A network engineer runs the following command on Router R1: R1# show policy-map control-plane Control Plane Service-policy input: CoPP-POLICY Class-map: MGMT-CLASS (match-all) 100 packets, 5000 bytes 5 minute offered rate 1000 bps Match: access-group name MGMT-ACL police: cir 32000 bps, bc 4000 bytes, be 4000 bytes conformed 80 packets, 4000 bytes; actions: transmit exceeded 15 packets, 750 bytes; actions: drop violated 5 packets, 250 bytes; actions: drop Class-map: class-default (match-any) 200 packets, 10000 bytes 5 minute offered rate 2000 bps Match: any police: cir 64000 bps, bc 8000 bytes, be 8000 bytes conformed 200 packets, 10000 bytes; actions: transmit exceeded 0 packets, 0 bytes; actions: drop violated 0 packets, 0 bytes; actions: drop Based on this output, what can be concluded?

A.Management traffic to the control plane is being policed, and some packets are being dropped because they exceed the configured rate.
B.All management traffic is being transmitted without drops.
C.The policer is configured in the output direction.
D.The class-default is dropping packets.
AnswerA

The policer shows 80 conformed and 20 exceeded/violated packets, meaning 20 packets were dropped due to exceeding the CIR.

Why this answer

The output shows that the CoPP-POLICY policy map is applied to the control plane in the input direction. For the MGMT-CLASS class, the policer has a CIR of 32000 bps, and the counters show 15 packets exceeded and 5 packets violated, both with a drop action. This confirms that some management traffic is being dropped because it exceeds the configured rate, making option A correct.

Exam trap

Cisco often tests the distinction between 'input' and 'output' direction for CoPP, and the trap here is that candidates assume the policy is applied in the output direction or overlook the drop counters in the MGMT-CLASS, leading them to incorrectly select option B or C.

How to eliminate wrong answers

Option B is wrong because the output clearly shows 15 exceeded and 5 violated packets being dropped for the MGMT-CLASS, so not all management traffic is transmitted without drops. Option C is wrong because the command 'show policy-map control-plane' without specifying 'output' defaults to the input direction, and the output explicitly states 'Service-policy input: CoPP-POLICY', confirming it is applied in the input direction. Option D is wrong because the class-default counters show 0 exceeded and 0 violated packets, meaning no packets are being dropped in that class.

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9
MCQhard

A network engineer is configuring CoPP on a Cisco ASR 1000 router to protect the control plane from excessive traffic. The engineer wants to allow BGP traffic from a specific peer (10.0.0.1) while rate-limiting all other BGP traffic. The engineer creates an ACL that permits TCP port 179 from host 10.0.0.1 and denies all other BGP traffic. The CoPP class-map matches this ACL. However, after applying the policy, BGP sessions from other peers are still being established. What is the most likely reason?

A.The ACL denies all other BGP traffic, so CoPP does not match it, and it falls through to the default class, which permits it.
B.The ACL is applied in the wrong order; the deny statement should be before the permit statement.
C.BGP uses UDP port 179, not TCP, so the ACL does not match BGP traffic.
D.CoPP does not affect BGP sessions because they are established before the policy is applied.
AnswerA

Correct because CoPP only applies to traffic matched by the class-map; if the ACL denies traffic, it is not matched, and the default class (often permit) allows it.

Why this answer

The ACL is designed to match BGP traffic from host 10.0.0.1 (permit) and deny all other BGP traffic. However, CoPP class-maps match traffic based on permit statements in the ACL; a deny statement in the ACL causes the traffic to not be matched by that class. As a result, BGP traffic from other peers is not classified into the rate-limiting class and falls through to the default class, which typically permits all traffic.

This explains why BGP sessions from other peers are still being established.

Exam trap

Cisco often tests the misconception that a deny statement in an ACL used with CoPP will drop traffic, when in reality it only prevents the traffic from being matched by that class, allowing it to fall through to the default class which typically permits everything.

How to eliminate wrong answers

Option B is wrong because the order of permit and deny statements in an ACL is critical, but here the permit for host 10.0.0.1 is correctly placed before the deny all; the issue is not about order but about how CoPP treats deny entries. Option C is wrong because BGP uses TCP port 179, not UDP; this is a fundamental protocol fact. Option D is wrong because CoPP affects all traffic arriving at the control plane after the policy is applied, regardless of when sessions were established; existing sessions are still subject to rate-limiting.

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10
Matchingmedium

Drag and drop each DSCP value on the left to its matching Per-Hop Behavior (PHB) on the right.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts
Matches

Expedited Forwarding

Assured Forwarding class 4, low drop probability

Class Selector 3

Assured Forwarding class 2, medium drop probability

Best-effort

Why these pairings

DSCP values map to specific PHBs: EF is for expedited forwarding, AF41 is Assured Forwarding class 4 low drop, CS3 is class selector 3, AF21 is Assured Forwarding class 2 medium drop, and BE (0) is best-effort.

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11
MCQhard

A network engineer runs the following command on Router R1: R1# show policy-map control-plane Control Plane Service-policy input: CoPP-POLICY Class-map: BGP-CLASS (match-all) 50 packets, 2500 bytes 5 minute offered rate 500 bps Match: access-group name BGP-ACL police: cir 64000 bps, bc 8000 bytes, be 8000 bytes conformed 50 packets, 2500 bytes; actions: transmit exceeded 0 packets, 0 bytes; actions: drop violated 0 packets, 0 bytes; actions: drop Class-map: SNMP-CLASS (match-all) 200 packets, 10000 bytes 5 minute offered rate 2000 bps Match: access-group name SNMP-ACL police: cir 16000 bps, bc 2000 bytes, be 2000 bytes conformed 150 packets, 7500 bytes; actions: transmit exceeded 40 packets, 2000 bytes; actions: drop violated 10 packets, 500 bytes; actions: drop Class-map: class-default (match-any) 100 packets, 5000 bytes 5 minute offered rate 1000 bps Match: any police: cir 32000 bps, bc 4000 bytes, be 4000 bytes conformed 100 packets, 5000 bytes; actions: transmit exceeded 0 packets, 0 bytes; actions: drop violated 0 packets, 0 bytes; actions: drop Based on this output, what can be concluded?

A.SNMP traffic to the control plane is experiencing drops due to exceeding its policer rate, while BGP traffic is within its rate.
B.BGP traffic is being dropped because it exceeds the CIR.
C.All traffic to the control plane is being dropped.
D.The control-plane policy is applied in the output direction.
AnswerA

The SNMP class shows 150 conformed and 50 dropped (exceeded+violated), while BGP shows all 50 conformed.

Why this answer

The output shows that for the SNMP-CLASS, 40 packets exceeded and 10 packets violated the policer, resulting in drops, while the BGP-CLASS had 0 exceeded and 0 violated packets, meaning all BGP traffic conformed to its CIR of 64000 bps. This confirms that SNMP traffic is being dropped due to exceeding its policer rate, while BGP traffic is within its rate.

Exam trap

The trap here is that candidates may misinterpret the 'exceeded' and 'violated' counters as indicating that all traffic in a class is being dropped, when in fact only packets that exceed the policer thresholds are dropped, while conforming traffic is still transmitted.

How to eliminate wrong answers

Option B is wrong because the BGP-CLASS shows 0 exceeded and 0 violated packets, indicating no drops; it is not exceeding its CIR. Option C is wrong because the output shows that conformed packets for all classes are being transmitted, so not all traffic is dropped. Option D is wrong because the command 'show policy-map control-plane' and the output explicitly state 'Service-policy input: CoPP-POLICY', meaning the policy is applied in the input direction, not output.

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12
Matchingmedium

Drag and drop each traffic shaping or policing characteristic on the left to its correct description on the right.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts
Matches

Buffers excess traffic to smooth output rate; Introduces additional delay; Uses a token bucket to meter traffic

Drops or marks packets exceeding the rate; Typically applied inbound on an interface

Why these pairings

Traffic shaping buffers excess traffic to smooth output rate. Policing drops or marks excess traffic. Shaping introduces delay; policing does not.

Shaping uses a token bucket; policing uses a token bucket. Shaping is applied outbound; policing is typically inbound.

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13
MCQeasy

A network team must design QoS for a campus network that carries voice, video, and data traffic. The design must use the DiffServ model and ensure that voice traffic is prioritized over all other traffic classes. Which DSCP marking and queuing strategy should be used for voice?

A.Mark voice with AF41 and place in a weighted fair queue.
B.Mark voice with EF and place in a strict priority queue.
C.Mark voice with CS3 and place in a low-latency queue.
D.Mark voice with BE and rely on WRED for drop precedence.
AnswerB

EF (DSCP 46) is the standard marking for voice, and strict priority queue ensures minimal delay.

Why this answer

Voice traffic requires strict priority to ensure minimal jitter and latency. DSCP EF (Expedited Forwarding, per RFC 3246) is the standard marking for real-time traffic like voice, and placing it in a strict priority queue (LLQ) guarantees that voice packets are serviced before any other queue, which is essential for meeting QoS requirements in a DiffServ model.

Exam trap

The trap here is that candidates often confuse AF41 (used for video) with voice marking, or assume that any low-latency queue (LLQ) works regardless of DSCP value, but Cisco specifically tests that voice must use EF and strict priority queue, not just any low-latency queue.

How to eliminate wrong answers

Option A is wrong because AF41 (Assured Forwarding class 4, low drop probability) is designed for traffic that can tolerate some delay and jitter, such as video conferencing, not for voice which needs strict priority; weighted fair queue does not provide the absolute priority required for voice. Option C is wrong because CS3 (Class Selector 3) is a legacy marking that does not guarantee low latency or strict priority; while a low-latency queue (LLQ) is correct, the DSCP marking must be EF for voice, not CS3. Option D is wrong because BE (Best Effort, DSCP 0) is the default marking for non-priority traffic, and WRED (Weighted Random Early Detection) is a congestion avoidance mechanism that drops packets before queue overflow, which is unsuitable for voice as it introduces jitter and packet loss.

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14
Drag & Dropmedium

Drag and drop the steps of DSCP re-marking at enterprise WAN edge into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

The correct order for DSCP re-marking at the WAN edge begins with configuring a trust boundary on access ports to ensure that DSCP values are trusted from trusted sources. Next, define class maps to match existing DSCP values. Then create a policy map with set dscp commands to re-mark the DSCP values.

Apply the service policy to the WAN edge interface. Finally, verify the re-marking using the show policy-map interface command.

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15
Drag & Dropmedium

Drag and drop the steps of CoPP class-map match criteria and rate-limit application into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

CoPP configuration requires defining class-maps first, then policy-map with police statements, then applying to control-plane. The order ensures proper traffic classification and rate-limiting.

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16
MCQmedium

Consider the following partial configuration for QoS on a Cisco IOS-XE router: class-map match-all VOICE match ip dscp ef ! policy-map QOS_POLICY class VOICE priority 1000 class class-default fair-queue ! interface GigabitEthernet0/0 service-policy output QOS_POLICY What is the effect of the 'priority 1000' command under class VOICE?

A.Voice traffic is placed in a strict priority queue with a bandwidth limit of 1000 kbps.
B.Voice traffic is given a minimum bandwidth guarantee of 1000 kbps but no priority.
C.Voice traffic is dropped if it exceeds 1000 kbps.
D.Voice traffic is shaped to 1000 kbps.
AnswerA

The priority command creates a low-latency queue with a rate limit.

Why this answer

The 'priority' command under the class VOICE in the policy-map QOS_POLICY enables strict priority queuing (low-latency queue) for voice traffic matching DSCP EF. The 'priority 1000' sets a bandwidth limit of 1000 kbps for this queue, meaning voice traffic can use up to 1000 kbps in the strict priority queue; if it exceeds this rate, excess traffic is dropped (or reclassified, depending on configuration). This ensures voice traffic is serviced before other traffic but is limited to 1000 kbps.

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17
Drag & Dropmedium

Drag and drop the steps of DSCP-to-CoS mapping at LAN boundary into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

At the LAN boundary (switch port), DSCP is mapped to CoS for 802.1Q trunking. The order ensures proper trust, mapping, and queuing for consistent QoS across the campus network.

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18
Drag & Dropmedium

Drag and drop the steps of traffic shaping vs policing configuration steps into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

The correct order for configuring traffic shaping or policing is: first, define a class map to match the desired traffic (A); second, create a policy map that specifies the shape or police action (B); third, apply the service policy to the interface (C); fourth, verify the policy using the show policy-map interface command (D); and finally, adjust the shaping rate or policing burst as needed based on verification results (E). This sequence ensures that traffic classification and policy definition are completed before applying the policy, and verification is done before making adjustments.

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19
MCQmedium

Examine the following configuration: policy-map QUEUE class GOLD bandwidth percent 25 queue-limit 64 packets class SILVER bandwidth percent 25 queue-limit 128 packets class class-default fair-queue interface GigabitEthernet0/2 service-policy output QUEUE Which statement about this configuration is true?

A.The GOLD class has a smaller queue limit than SILVER, which may cause more packet drops for GOLD traffic under congestion.
B.The SILVER class will always receive more bandwidth than GOLD because of its larger queue limit.
C.The configuration is invalid because 'queue-limit' cannot be used with 'bandwidth percent' in the same class.
D.The 'fair-queue' command in class-default will override the bandwidth allocation for GOLD and SILVER.
AnswerA

Correct. A smaller queue limit means fewer packets can be buffered, increasing the likelihood of drops when the queue is full.

Why this answer

The GOLD class has a queue-limit of 64 packets, while the SILVER class has a queue-limit of 128 packets. Under congestion, the smaller queue for GOLD will fill up faster, leading to more tail drops for GOLD traffic, even though both classes are allocated the same bandwidth percentage. This demonstrates that queue-limit directly affects drop probability, not bandwidth allocation.

Exam trap

Cisco often tests the misconception that a larger queue-limit implies more bandwidth, when in fact queue-limit only affects buffer depth and drop behavior, not bandwidth allocation.

How to eliminate wrong answers

Option B is wrong because queue-limit does not affect bandwidth allocation; bandwidth is controlled by the 'bandwidth percent' command, which is set to 25% for both GOLD and SILVER, so they receive equal bandwidth under congestion. Option C is wrong because 'queue-limit' can be used with 'bandwidth percent' in the same class; they are independent QoS parameters that control different aspects (bandwidth guarantee vs. queue depth). Option D is wrong because 'fair-queue' in class-default only applies to the default class and does not override the explicit bandwidth allocation for GOLD and SILVER classes, which are configured with strict bandwidth percentages.

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20
Matchingmedium

Drag and drop each traffic shaping or policing characteristic on the left to its matching description on the right.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts
Matches

Buffers excess packets to send later, smoothing traffic rate

Drops or re-marks packets exceeding the configured rate

Uses a token bucket plus a queue to hold excess packets

Uses a token bucket without a queue; excess is dropped or re-marked

Typically applied inbound to enforce ingress rate limits

Why these pairings

Shaping buffers excess traffic to smooth bursts; policing drops or re-marks excess traffic; shaping uses a token bucket with a queue; policing typically uses a single or dual token bucket without queuing; policing can be applied inbound or outbound.

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21
Drag & Dropmedium

Drag and drop the steps of CoPP class-map match criteria and rate-limit application into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps
Order
1Step 1
2Step 2
3Step 3
4Step 4
5Step 5

Why this order

CoPP configuration requires defining match criteria in a class-map, then creating a policy-map with police commands, and finally applying the policy to the control plane. The order follows standard MQC (Modular QoS CLI) process.

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