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New Member

vlan mismatch

Hi all, which reason can be the source of this problem?..

CDP-4-NVLANMISMATCH:Native vlan mismatch detected on port 2/3

9 REPLIES
New Member

Re: vlan mismatch

Native VLAN of one switch is different than the Native VLAN of other switch.

regards

Neo

Cisco Employee

Re: vlan mismatch

Hi,

The switch generates this message whenever the switch port is physically connected to another switch or router. This message appears on the switch because the configured native VLAN on the port is different than the native VLAN on the connecting switch/router port.

A trunk port that you have configured with IEEE 802.1Q tagging can receive both tagged and untagged traffic. By default, the switch forwards untagged traffic with the native VLAN that is configured for the port. If a packet has a VLAN ID that is the same as the outgoing port native VLAN ID, the switch transmits the packet untagged. Otherwise, the switch transmits the packet with a tag.

Ensure that the native VLAN for an 802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk is different from the native VLAN on the other end, the traffic of the native VLANs on both sides cannot transmit correctly on the trunk.

HTH,

Bobby

*Please rate helpful posts.

Re: vlan mismatch

Hi,

I would like to ask one question with regard to the above.

If I have more than 1 native vlan,needs to be trunked from switch to another is it possibl??

And if I currently has the same Scenario with (missmatch native vlan log) does this cause any problem future wise??

Appreciate your feedback,

thanks,

Re: vlan mismatch

HI

I think there can be only one native vlan of a port at a time.

native vlans means the vlan the switch port returnd when it is not trunking.

I think there is not problem caused due to this it is only CDP info messages.as the concept of native vlan has already been explained in the above posts.

u can trunk with different native vlans but it is recommeneded to have the same native vlan at both the ends.

Thanks

mahmood

Re: vlan mismatch

Hi Friend,

No it is not possible. You can have only one native vlan on one trunk.

Actually native vlan mismatch as per many documents says it is not a problem but as per me it is a problem because the 2 vlans which are having native vlan mismatch will not be able to communicate to their respective vlans.

Lets take it this way..2 switches (switch 1 & switch 2) connected via trunk and on switch 1 natve vlan is 1 and on another switch 2 native vlan is 2.

Now any frame for vlan 1 which will be send out from switch 1 will be send out untagged and as it reaches switch 2 because switch 2 expect vlan 1 to be tagged it will drop traffic for vlan 1 because switch 2 expects vlan 2 as untagged vlan. I have tried and tested this many a times. So native vlan mismatch is a problem in network.

HTH, if yes please rate the post.

Ankur

Re: vlan mismatch

Hi Ankur,

with regard to ur post above, I would like to inform and share with u what was happening in my situation,

I have native vlan 703 from switch 1 probagated to switch 2 (native vlan 1), but guess what?? they are communicating very well.

I dont understand how? but as far as I know & following some cisco documentation, they said any untagged traffic could be associated or merg with other untagged traffic on other switch, thats why different untagged traffic ID's are recognizing & communictes with each other at both ends,

Am I wrong or do have any ideas that could guide me to the right direction?

thanks in advance,

Re: vlan mismatch

Hi Friend,

To be frank it should not work and if it is working its a leakage of technology.

WHY I am saying is sometime if you connect 2 switches together with 2 different vlan on both the sides it talks WHY because when frame moves out of access port it has no tagging and no recognition of which vlan it belogs to and once it reaches other end it gets the vlan id configured on other side. And it seems something of this sort is happeneing in your network.

But if we look at concept it should not work on trunk ports becuase when untagged traffic reaches the other end of the trunk connection remote end switche expects it to be tag and not able to recognise it so it should be dropped.

Regards,

Ankur

Cisco Employee

Re: vlan mismatch

Can you confirm if this is indeed a trunk port (carrying multiple vlans) or an access port (carrying just one vlan)?

Cisco Employee

Re: vlan mismatch

Can you confirm if this is indeed a trunk port (carrying multiple vlans) or an access port (carrying just one vlan)?

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