QoS & Voice VLANs Practice Questions

Answers and explanations appear when you work through the set above.

1. A switchport has both switchport access vlan 100 and switchport voice vlan 101 configured. What is the effect of this combination?

   • The port becomes a trunk carrying every VLAN on the switch
   • Untagged traffic from an attached PC is placed in VLAN 100, while the IP phone's voice traffic is tagged and placed in VLAN 101, even though the port is not a full trunk
   • The PC's traffic is dropped since two VLANs cannot exist on one access port
   • VLAN 101 becomes the native VLAN for all traffic on the port

2. Which protocol allows a Cisco switch and an attached Cisco IP phone to automatically discover each other and exchange information such as the voice VLAN and power requirements?

   • CDP (Cisco Discovery Protocol)
   • STP (Spanning Tree Protocol)
   • DTP (Dynamic Trunking Protocol)
   • ARP

3. Which command displays how much power a switch is currently supplying to connected devices over each PoE-capable port?

   • show power inline
   • show interfaces status
   • show cdp neighbors
   • show ip interface brief

4. In a router-on-a-stick design for voice and data, a router subinterface is configured with encapsulation dot1q 101 to handle the Voice VLAN's traffic. What is the purpose of the dot1q encapsulation command here?

   • It enables PoE on the subinterface
   • It tells the subinterface to accept and process 802.1Q tagged frames belonging to VLAN 101 specifically
   • It converts the physical interface into an access port
   • It configures the DHCP option 150 value

5. Two DHCP pools are configured on a router, one handing out addresses to PCs and one to IP phones. What DHCP option in the phone pool tells the phones where to download their configuration file from?

   • Option 150, set to the TFTP server address
   • Option 66, set to the default gateway
   • default-router
   • dns-server

6. A packet capture on an 802.1Q trunk shows data-VLAN frames untagged and voice-VLAN frames tagged. The trunk configuration confirms the data VLAN is native and native tagging is disabled. Why is the capture expected?

   • The trunk is misconfigured and this indicates a fault
   • PCs always send tagged traffic while phones never do
   • The data VLAN is configured as the trunk's native VLAN, so its traffic passes untagged, while the voice VLAN is a separate tagged VLAN carried on the same trunk
   • It means QoS marking has failed for the PC traffic

7. Why is voice traffic typically placed in a separate VLAN from data traffic on the same access port, rather than mixing phone and PC traffic together in one VLAN?

   • IP phones cannot physically operate unless they are in their own VLAN
   • It allows voice traffic to be identified, prioritized with QoS, and given its own addressing and broadcast domain separate from PC data traffic
   • It is only done to save IP address space
   • Data VLANs do not support 802.1Q tagging

8. What does QoS classification do before a device applies differentiated treatment?

   • Guarantees bandwidth to every packet
   • Identifies traffic groups using fields or other match criteria
   • Encrypts traffic according to priority
   • Changes the physical link speed

9. What is the purpose of marking a packet with a DSCP value?

   • Reserve an end-to-end circuit
   • Tell every router to accept the packet without inspection
   • Signal a traffic class that downstream devices can use in their own QoS policies
   • Prevent the packet from being dropped

10. When does queuing policy materially affect which packets transmit next?

    • Only when an interface experiences contention or congestion
    • Whenever an Ethernet frame contains a source MAC
    • Only before traffic is classified
    • Whenever a route is learned by OSPF

11. Which statement best distinguishes policing from shaping when traffic exceeds a configured rate?

    • Policing always queues excess traffic; shaping always encrypts it
    • Policing typically drops or remarks excess traffic, while shaping buffers and delays it to smooth the output rate
    • They are different names for strict-priority queuing
    • Shaping can only operate on received traffic

12. Why is a low-latency priority queue normally policed or otherwise bounded?

    • To ensure priority traffic cannot starve every other class during sustained congestion
    • To convert voice traffic into TCP
    • To make DSCP markings globally trusted
    • To eliminate all voice packet loss

13. A packet arrives marked for high priority from an untrusted user port. What is a sound QoS boundary action?

    • Trust every marking because endpoints know best
    • Drop all marked traffic
    • Classify it only by destination MAC
    • Validate or remark it according to the access policy before trusting the mark downstream