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 access and voice VLAN commands convert the port into a trunk allowing every configured VLAN
   • PC data is untagged in VLAN 100; phone voice is tagged in VLAN 101 on the access port
   • 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 (Address Resolution Protocol)

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
   • Selects VLAN 101's 802.1Q-tagged frames for the subinterface
   • Converts the physical router interface into a Layer 2 access port in VLAN 101
   • 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, identifying the TFTP server
   • 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 faulty because native VLAN frames must carry the same tag as voice frames
   • PCs always send tagged traffic while phones never do
   • Native data VLAN is untagged; the separate voice VLAN is tagged
   • 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?

   • Phones require a separate VLAN because their Ethernet hardware cannot receive PC broadcast frames
   • Separate addressing and broadcast domains help identify voice and apply QoS
   • 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?

   • Reserves an end-to-end circuit for each identified application flow
   • Groups traffic by match criteria
   • Encrypts traffic according to priority
   • Changes the speed negotiated on the physical link

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

   • Reserves a dedicated end-to-end circuit for the marked flow
   • Tell every router to accept the packet without inspection
   • Signals traffic class to downstream QoS policies
   • Ensures the marked packet cannot be dropped during congestion

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

    • When the interface experiences contention or congestion
    • Whenever an Ethernet frame carries a source MAC address
    • 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 drops or remarks excess; shaping buffers it to smooth the rate
    • Both implement strict-priority queuing, but policing uses DSCP and shaping uses CoS
    • Shaping can only operate on received traffic

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

    • Prevent sustained priority traffic from starving other classes
    • To convert voice traffic into TCP
    • Make endpoint DSCP markings trusted throughout every router
    • 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 the endpoint's marking and bypass ingress classification for high-priority packets
    • Drop all marked traffic
    • Classify it only by destination MAC
    • Validate or remark by access policy before downstream trust