Native VLAN mismatch between 2960 and 3560 switch

[Martijn de Loos]

Hello,

I am facing a very unusual, never seen, problem in our office network between a 2960 and 3560 switch. Both switches are connected to each other over a dot1q trunk link. Both are configured with native vlan 999. I verified this with "sh int trunk" and I checked the running config on both devices. Both show native VLAN is 999.

Here's the problem: The 2960 switch complains that the local native vlan is 1 and it mismatches with his neighbor which is on 999 and therefor, the link is down (LAN network on this switch becomes unreachable). Whenever I restart the switchport, network is up again for about 5 minutes then it displayes native vlan mismatch again and the trunk link goes down.

Here comes the funny part. The 2960 switch complains it is configured on native vlan 1 (while it's not) and the other end on 999. So I logon to the 3560 switch and configure its trunk port on native vlan 1. This should, in my opinion, solve the 2960's complaint. However, now the 2960 switch complains that its local port is configured on native vlan 999 and the other end on native vlan 1.

What am I missing here? 

[Jason Dance]

What happens if you change the native vlan to a number below 255 on both sides?

Regards,

Jason

[Martijn de Loos]

Hi Jason,

Thanks for your swift response.

I changed both sides to 99 now. Still the same issue. When both are configured on the same native vlan, the 2960 sees itself on native vlan 1. When I change the 3560 to vlan 1, the 2960 sees itself again at native vlan 99. I keep getting this conflict.

[Jason Dance]

Can you do a "show run interface" of the switch ports on the 2960 and 3560 and post it?

Perhaps also post the output from the "show int trunk" as well.

[Martijn de Loos]

Hi Jason,

I will do that tomorrow when I get back to the office. Thank you so far.

[Martijn de Loos]

Hi Jason,

Please see the attached text file for command outputs. On the 3560 I see from the sh int trunk command that gigabitethernet0/6 is on mode 'Auto' while the other trunk ports are on mode 'On'. Could that be something?

By the way, whenever the trunk link goes down, Gi0/6 disappears from the 'sh int trunk' output. It is only there for about 5 minutes after shutting down and re-enabling the port again (the amount of time the link actually works after a port reboot).

[Martijn de Loos]

The trunk port was set to 'Auto' and after I changed it to 'On' the link remains up. So this works now but I wonder what the difference is between trunk auto and trunk on?

[Jason Dance]

Hi Martijn.

These links may help you understand the differences between each trunking mode:

http://en.wikipedia.org/wiki/Dynamic_Trunking_Protocol

http://www.cisco.com/c/en/us/tech/lan-switching/dynamic-trunking-protocol-dtp/index.html

Regards,

Jason

[sajid_m123]

I faced a similar issue but one of the switches involved was vtp client and it needed vlans from the other switch (which was server). Just moved the vtp client to transparent, created native vlan locally, made it vtp client again and all worked as desired.

Malik