2.5 Configure, verify, and troubleshoot interswitch connectivity PART 3

Yup, I’m back for part 3. Upon checking the content, I think I miss something which is the native vlan.

After the bootcamp, all I thought that native vlan is just default VLAN. When I dig deeper to it, I’ve learned that native vlan is an accessory of 802.1q.

Here’s the thing:

ISL–> encapsulates frame with its vlan id regardless whether it is untagged or not.

802.1q –> Has something special encapsulation method wherein all untagged frames will be sent out to native VLAN. So as a summary, native VLAN exists only in 802.1q.

Can you elaborate it mate?

Before any configuration, all ports are assigned to VLAN 1 which is the default VLAN. But for 802.1q, once you tagged a VLAN’s port, all the other untagged frames will be sent out to native vlan. Native VLAN by default is the VLAN 1( default VLAN) but you can assign it to other VLAN#.

 

native

With  the topology above, we have 2 switches, those are not configured however just for presentation reasons,  I just would like to highlight here that if we configured a link as a trunk link, both trunk ports should have the same native vlan. In the event that ports has different native vlan, it will not forward the frame on either vlan 20 or vlan 15. It may notify us an error or either of the ports may go down. Native VLAN mismatch is one factor to consider

A sample configuration will be the following:

 

en
conf t
int fa 0/1
!switchport trunk native vlan ( vlan #)
switchport trunk native vlan 10
exit

 

To verify use show command:

show interfaces trunk

What is the purpose of native vlan?

The primary purpose of native vlan is backward compatibility because some of the old switches doesn’t know the concept of vlan tagging.

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