V-LAN Interview Questions

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

Answer: A VLAN is a group of devices on the same broadcast domain, such aslogical 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?

Answer: The VLAN number will be significant in the local switch. If trunking is
enabled, the VLAN number will be significant across the entire trunking domainIn other words, the VLAN will be transported to every switch that has a trunk linsupporting that VLAN.

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

Answer: Local VLAN
End-to-end VLAN

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

Answer: interface fastethernet 4/11
switchport mode access
switchport access vlan 2

5. Generally, what must be configuredfor a port-based VLAN?

Answer: The switch port

6. What is the default VLAN on all port

Answer: VLAN 1

7. What is a trunk link?

Answer: A trunk link is a cfrom multiple VLANs. Eacacross the trunk link.

8. What methods of EtheCatalyst switch trunk?

Answer: 802.1Q

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

Answer: 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)?

Answer: 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.

Answer: 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?

Answer: 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 _________________

Answer: 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?

Answer: switchport trunk native vlan 100

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

Answer: 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?

Answer: show interface gigabitethernet 4/4 switchport
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?

  1. interface fastethernet 0/12
  2. switchport trunk encapsulation dot1q
  3. switchport trunk native vlan 10
  4. switchport trunk allowed vlan 1-1005
  5. switchport mode trunk

Answer: 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.


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