Interview Questions:CCNA & VLAN

1. What is a VLAN? When is it used?

A VLAN is a group of devices on the same broadcast domain, such as a logical subnet or segment. VLANs can span switch ports, switches within a switch block, or closets and buildings. VLANs group users and devices into common workgroups across geographical areas. VLANs help provide segmentation, security, and problem isolation.

2. When a VLAN is configured on a Catalyst switch port, in how much of the campus network will the VLAN number be unique and significant?

The VLAN number will be significant in the local switch. If trunking is enabled, the VLAN number will be significant across the entire trunking domain. In other words, the VLAN will be transported to every switch that has a trunk link supporting that VLAN.

3. Name two types of VLANs in terms of spanning areas of the campus network.

Local VLAN
End-to-end VLAN

4. What switch commands configure Fast Ethernet port 4/11 for VLAN 2?

interface fastethernet 4/11switchport mode accessswitchport access vlan 2

5. Generally, what must be configured (both switch and end-user device) for a port-based VLAN?

The switch port

6. What is the default VLAN on all ports of a Catalyst switch?


7. What is a trunk link?

A trunk link is a connection between two switches that transports traffic from multiple VLANs. Each frame is identified with its source VLAN during its trip across the trunk link.

8. What methods of Ethernet VLAN frame identification can be used on a Catalyst switch trunk?


9. What is the difference between the two trunking methods? How many bytes are added to trunked frames for VLAN identification in each method?

ISL uses encapsulation and adds a 26-byte header and a 4-byte trailer. 802.1Q adds a 4-byte tag field within existing frames, without encapsulation.

10. What is the purpose of the Dynamic Trunking Protocol (DTP)?

DTP allows negotiation of a common trunking method between endpoints of a trunk link.

11. What commands are needed to configure a Catalyst switch trunk port Gigabit 3/1 to transport only VLANs 100, 200 through 205, and 300 using IEEE 802.1Q?

(Assume that trunking is enabled and active on the port already. Also assume that the interface gigabit 3/1 command already has been entered.)

switchport trunk allowed vlan 100, 200-205, 300

12. Two neighboring switch trunk ports are set to the auto mode with ISL trunking encapsulation mode. What will the resulting trunk mode become?

Trunking will not be established. Both switches are in the passive auto state and are waiting to be asked to start the trunking mode. The link will remain an access link on both switches.

13. Complete the following command to configure the switch port to use DTP to actively ask the other end to become a trunk:switchport mode _________________

switchport mode dynamic desirable

14. Which command can set the native VLAN of a trunk port to VLAN 100 after the interface has been selected?

switchport trunk native vlan 100

15. What command can configure a trunk port to stop sending and receiving DTP packets completely?

switchport nonegotiate

16. What command can be used on a Catalyst switch to verify exactly what VLANs will be transported over trunk link gigabitethernet 4/4?

show interface gigabitethernet 4/4 switchportor show interface gigabitethernet 4/4 switchport trunk

17. Suppose that a switch port is configured with the following commands. A PC with a nontrunking NIC card then is connected to that port. What, if any, traffic will the PC successfully send and receive?

interface fastethernet 0/12switchport trunk encapsulation dot1qswitchport trunk native vlan 10switchport trunk allowed vlan 1-1005switchport mode trunk

The PC expects only a single network connection, using a single VLAN. In other words, the PC can't participate in any form of trunking. Only untagged or unencapsulated frames will be understood. Recall that an 802.1Q trunk's native VLAN is the only VLAN that has untagged frames. Therefore, the PC will be capable of exchanging frames only on VLAN 10, the native VLAN.