Native vlan mismatch

[todd.martin-02]

I had an interesting problem after upgrading to native on a core switch. I had several links that have native vlan mismatchs.

the network runs fine with these mismatchs. What exactly is the impact of this problem and if I set them to match on both sides (not vlan1), what can i expect?

[ashok_boin]

Hi,

Native VLAN mismatches won't effect your network unless your trunk configuration fails and you wanted the traffic to flow for atleast it's native vlan during the failure.

Thank you,

Regards...

-Ashok.

[amikat]

Hi,

I would recommend to match native vlans.

The impact of mismatch may even be merging vlans on some platforms, at majority both vlans are blocked by STP for user traffic.

Best regards,

Antonin

[mkato]

All the reporting switch is telling you is it has learned via CDP that the native (untagged) VLAN ID for the trunk in the other switch does not match the reporting switch's native VLAN ID for the trunk. If you map your IP subnets to VLAN ID's consistently across your network you could inadvertently connect two IP subnets together.

By the way, the native VLAN is not the same thing as the switchport access VLAN. The native VLAN is carried with untagged frames while the port is in trunking mode. If the port falls out of trunking mode, the port is assigned to the switchport access VLAN. The native VLAN and the access VLAN do not necessarily have to be the same.

[todd.martin-02]

hi,

my link between the switches is not a trunk. anytime talk about the aitive vlan is is in the contect of trunking. I am confused about how the native vlan works on a non-trunking link. below is a snip of my config.

Port Name Status Vlan Dupl

----- -------------------- ---------- ---------- ----

3/1 internal <23X.20> connected 239

Port Name Status Vlan Level Duplex Speed Type

----- ------------------ ---------- ---------- ------

2/11 16ESX-INT-A connected 1 norma

[amikat]

Hi,

You are quite right to expect the native vlan in 802.1q (dot1q) only.

The point is that the default port (interface) settings with Cat6K (I guess that is your box) is set to auto with CatOS (hybrid) but dynamic desirable with IOS. So the connection will form trunk AUTOMATICALLY unles you set to prevent (eg. static access).

You can check the status with the command "sh port trunk" (please check for 3/1 and 2/11) and look under the "status" header where it may say "trunking" in case of trunk.

I hope this makes sense.

Best regards,

Antonin

[milan.kulik]

Hi,

read this:

http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dd7c47f/2#selected_message

Regards,

Milan