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VLAN & Trunking Questions

March 16th, 2020 Go to comments

Note: If you are not sure about Virtual Local Area Network (VLAN), please read our Virtual Local Area Network VLAN Tutorial.

Quick summary about VLAN:

Be default all access ports belong to VLAN 1. If we want to assign a new VLAN, we have to use the command “switchport access vlan <vlan-id>” under interface mode.

VLAN Benefits:

– Logically group devices by department/function, not location so it provides more efficient use of bandwidth
– Separate broadcast domains, thus reducing their sizes and help mitigate broadcast storms
– Provide additional security because it separates sensitive data traffic from other traffic

The following commands are used to a create a VLAN (for example VLAN 10) and assign it to an interface (for example fa0/1) on a switch:

Switch(config)#vlan 10 //Create VLAN 10 first
Switch(config)#interface fa0/1
Switch(config-if)#switchport access vlan 10 //assign Fa0/1 interface to VLAN 10

To verify a VLAN or check which ports belong to which VLAN, use the “show vlan” command (or “show vlan id <vlan-id>” for a specific VLAN)

Question 1


From the output we see the native VLAN of Switch1 on Gi0/1 interface is VLAN 1 while that of Switch2 is VLAN 99 so there would be a native VLAN mismatch.

Question 2


Maybe this question is missing the “SW2 is set to Dynamic Auto” part so we assume this part to find out the best answer. Dynamic Desirable + Dynamic Desirable/Dynamic Auto/Trunk will form a trunk link.

Question 3


The trunk still forms with mismatched native VLANs and the traffic can actually flow between mismatched switches. But it is absolutely necessary that the native VLANs on both ends of a trunk link match; otherwise a native VLAN mismatch occurs, causing the two VLANs to effectively merge. For example with the above configuration, SW1 would send untagged frames for VLAN 999. SW2 receives them but would think they are for VLAN 99 so we can say these two VLANs are merged.

Comments (3) Comments
  1. Fernando
    June 24th, 2020

    Am new to Cisco and will to do an exam of ccna and need help

  2. Chris Jones
    August 8th, 2020

    I have a question. In Question 1 we see the mismatched native VLANs not allowing traffic to pass but in question three we see that the mismatch VLANs will allow traffic to pass. Can somebody explain the difference?

  3. Cindy
    August 14th, 2020

    How come the questions arent shown on most of these?

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