Cisco CCNA VLANs and 802.1Q Trunking Questions and Answers

Question 1

A network technician configures two PCs on a single Cisco switch. PC-A is on port Fa0/1 in VLAN 10, and PC-B is on port Fa0/2 in VLAN 20. Both PCs are assigned IP addresses in the 192.168.1.0/24 subnet. The PCs are unable to ping each other. What is the most likely cause of this issue?

Accepted Answer

A Layer 3 device is required to route between the VLANs.

Answer

VLANs create separate broadcast domains at Layer 2. Devices in different VLANs cannot communicate directly, even if they are on the same switch and in the same IP subnet. To enable communication between VLANs, a Layer 3 device, such as a router or a multilayer switch, must be used to perform inter-VLAN routing.

Question 2

What is the primary function of the native VLAN on an IEEE 802.1Q trunk port?

Accepted Answer

To carry untagged traffic.

Answer

The native VLAN is a specific VLAN assigned to an 802.1Q trunk port. Its primary purpose is to handle traffic that is not tagged with a VLAN ID. When a trunk port receives an untagged frame, it assumes the frame belongs to the native VLAN.

Question 3

A network administrator needs to configure a switch port to permanently operate in trunking mode and carry traffic for multiple VLANs between switches. Which Cisco IOS command should be used on the interface?

Accepted Answer

switchport mode trunk

Answer

The `switchport mode trunk` command statically configures an interface to be a Layer 2 trunk port. This forces the port into trunking mode, allowing it to pass traffic for multiple VLANs, as opposed to dynamic modes which negotiate the trunk status.

Question 4

An administrator adds VLAN 50 to two switches that are connected by a trunk link. Devices in VLAN 50 on the first switch cannot communicate with devices in VLAN 50 on the second switch. The administrator has confirmed that VLAN 50 exists on both switches. Which of the following is the most likely reason for the problem?

Accepted Answer

VLAN 50 has not been added to the allowed list on the trunk ports.

Answer

By default, a trunk port allows all VLANs to pass. However, if a trunk port has been configured with an allowed VLAN list using the `switchport trunk allowed vlan` command, any new VLAN must be explicitly added to that list. If VLAN 50 is not on the allowed list, its traffic will be blocked from crossing the trunk.

Question 5

The IEEE 802.1Q frame tag, used for identifying VLANs, is 4 bytes long and inserted into the Ethernet frame header. Which field within this tag is 12 bits long and specifies the VLAN membership of the frame?

Accepted Answer

VLAN Identifier (VID)

Answer

The 802.1Q tag contains four fields. The VLAN Identifier (VID) is a 12-bit field that specifies the VLAN to which the frame belongs. This 12-bit length allows for 4096 possible VLANs, with 4094 being usable.

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Cisco CCNA VLANs and 802.1Q Trunking Questions and Answers