Native VLAN Mismatch still works fine.....?

[poirot1967]

I have kind of an odd situation here.  The router and our uplink was configured with the native vlan being 839.  Our aggergate fiber switch, and all those attached to it, was configured with the native vlan being 829.  Ooooppps.  It has been like that for almost 10 years, I don't know why it works, but it does.  Oh, and I wasn't here 10 years ago.  We are getting the correct subnet, DHCP services, and are getting full access to the outside world where we shouldn't.  The uplink is a plain access port, no trunks.  I find it all a bit confusing.  I am not getting any mismatch errors, and cdp is running.  I can't see how the router is configured as it is managed by a different group.  Does anyone happen to know why this is working?  All the ports are configured as vlan 829 on our switches, so traffic isn't being carried by vlan1.  ???

Poirot

[Nagaraja Thanthry · Accepted solution]

Hello,

The VLAN tagging is applicable only on a trunk interface. So, when traffic moves from one switch to another, at that time your VLAN tagging comes into picture. One the traffic enters the switch, the TAG will be removed and will be forwarded based on the MAC addresses. So, in your scenario, between all your access switches, traffic goes tagged and the traffic belonging to native vlan (829) goes untagged. When the traffic hits the core switch, it gets routed and the traffic then goes to VLAN 839. So, there is no issue with the Native VLAN.

Hope this answers your questions.

Regards,

NT

[Senthil Rajendra]

Dear NT,

Is there any problem if we are having continuous Native VLAN Mismatch log in Switch or Router.

Tks&Rgds

Senthil

[Nagaraja Thanthry]

Hello,

It depends upon the functionality of the port you are getting the error on.

If it is an access-port, you can ignore that message (as long as the

connection is valid i.e. you have intentionally bridged two VLANs). But if

it is a trunk port, then you need to make sure that the native vlans match

on both ends.

Hope this helps.

Regards,

NT

[poirot1967]

Thanks for the reply.  It makes more sense now.  When we have another downtime I will fix it, just to be consistant.  But then again, it isn't exactly broke either.....

Thanks again

Poirot

[Mohamed Sobair]

Hi,

This behaviour is noticed on some IOS software releases, However , the documentation clearly states that if a native vlan mismatch on trunk ports , this could lead to spanning-tree loops and incosistency problems and IOS will immediately put the port into errdisable state and CDP message is logged.

So, I recommend setting it on both ends of the trunk to the same native VLANs.

HTH

Mohamed

[Senthil Rajendra]

Dear NT and Mohamed,

Thanks for your information..Its very useful.

Tks&Rgds

Senthil