VLAN Trunking Questions

Question 1

Refer to the exhibit.

Switch1#show int fastEthernet0/1 switchport Name: Fa0/1 Switchport: Enabled Administrative Mode: dynamic auto Operational Mode: static access Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: native Negotiation of Trunking: On Access Mode VLAN: 2 (VLAN0002) Trunking Native Mode VLAN: 3 (VLAN0003) Administrative Native VLAN tagging: enabled Voice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk Native VLAN tagging: enabled Administrative private-vlan trunk encapsulation: dot1q Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk private VLANs: none Operational private-vlan: none Trunking VLANs Enabled: ALL Pruning VLANs Enabled: 2-1001 Capture Mode Disabled Capture VLANs Allowed: ALL Protected: false Unknown unicast blocked: disabled Unknown multicast blocked: disabled Appliance trust: none

If a port is configured as shown and receives an untagged frame, of which VLAN will the untagged frame be a member?

A. VLAN 1
B. VLAN 2
C. VLAN 3
D. VLAN 4

Answer: B

Explanation

When typing:

Switch(config-if)#switchport mode ?

access Set trunking mode to ACCESS unconditionally

dynamic Set trunking mode to dynamically negotiate access or trunk mode

trunk Set trunking mode to TRUNK unconditionally

and

Switch(config-if)#switchport mode dynamic ?

auto Set trunking mode dynamic negotiation parameter to AUTO

desirable Set trunking mode dynamic negotiation parameter to DESIRABLE

So if we configure Fa0/1 as dynamic auto mode, it will not initiate any negotitation but waiting for the other end negotiate to be a trunk with DTP. If the other end does not ask it to become a trunk then it will become an access port. Therefore when using the “show interface fastEthernet0/1 switchport” command we will see two output lines “Administrative Mode: dynamic auto” and “Operational Mode: static access“

Note: To set this port to VLAN 2 as the output above just use one additional command: “switchport access vlan 2″.

Now back to our question, from the output we see that Fa0/1 is operating as an access port on VLAN 2 so if it receive untagged frame it will suppose that frame is coming from VLAN 2.

Question 2

Refer to the exhibit.

Switch1#show interfaces fastEthernet0/1 switchport Name: Fa0/1 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: On Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 3 (VLAN0003) Administrative Native VLAN tagging: enabled Voice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk Native VLAN tagging: enabled Administrative private-vlan trunk encapsulation: dot1q Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk private VLANs: none Operational private-vlan: none Trunking VLANs Enabled: 4-100 Pruning VLANs Enabled: 100-200 Capture Mode Disabled Capture VLANs Allowed: ALL Protected: false Unknown unicast blocked: disabled Unknown multicast blocked: disabled Appliance trust: none

Which VLANs are permitted to send frames out port FastEthernet0/1?

A. 100-200
B. 4-100
C. 1 and 4-100
D. 3 and 4-100

Answer: D

Explanation

Traffic on the native vlan does not get tagged as it crosses a trunk, so there is no dot1q tag in the first place to be filtered. And you don’t need to allow the native vlan. But if we force to tag the native vlan (with the “switchport trunk native vlan tag” command) then if the native vlan is not in the “allowed vlan” list it will be dropped.

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