Native vlan confusion

[Asif.oxy]

Hi,

I have some confusion regarding native vlan as per what i know all ports are in vlan 1 by default and operational mode is static access , suppose 2 switches are connected and there is native vlan mismatch on SW1 native vlan is 30 and on SW2 native vlan is 20 and we have clients in default vlan 1 on both switches in same subnet so would they will able to communicate(will ping work) and if yes then please help me understand it.

Thanks 

Asif 

[Giuseppe Larosa · Accepted solution]

Hello @Asif.oxy ,

the native VLAN concept applies to 802.1Q trunks, in creating the 802.1Q VLAN tagging standard there was the idea to provide some basic connectivity to a non trunking device if connected to a trunk port.

If the NIC does not handle VLAN tagged frames the native VLAN help the device to be able to participate to the native VLAN broadcast domain.

When creating a trunk between two switches the native VLAN should match on both ends of the link otherwise the two broadcast domains are joined.

Coming to your question / example :

>> suppose 2 switches are connected and there is native vlan mismatch on SW1 native vlan is 30 and on SW2 native vlan is 20 and we have clients in default vlan 1 on both switches in same subnet so would they will able to communicate(will ping work) and if yes then please help me understand it.

The answer is yes the native VLAN mismatch does not impact VLAN 1 as it is tagged with VLAN ID 1 on both ends of the trunk so communication in VLAN 1 is fine. Only users in VLAN 30 and 20 are impacted by native VLAN mismatch in this case.

The modern trend is to avoid to send user traffic over native VLAN on trunks between switches. It is preferred to send user traffic over tagged frames for many reasons like the presence of the CoS three bits in 802.1Q header.

Hope to help

Giuseppe

[marce1000]

 - Native vlan , has no local meaning and usually applies to trunk-port configuration settings, in your case default vlan1 is not effected and inter-vlan1-communications will work. If there is a trunk between SW1 and SW2 with a native vlan mismatch then  vlan30 and vlan20 between sw1 and sw2 will get connected.

 M.

[Asif.oxy]

Hi Marce,

Thanks for your reply.

[balaji.bandi]

Native VLAN for any untagged pakets in the trasport - by default Csico has default vlan 1

interface Port-channel X or interface gi x/x
switchport
switchport mode trunk

switchport trunk native vlan 1 -- (by default) if anything then you need to change this

switchport trunk allowed vlan 1, 20, 30

if you  can have native vlan configured if this was trunk, allowed required vlan in the trunk as exmaple  below :

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24063-pvid-inconsistency-24063.html#topic1