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Behavior of trunk native VLAN vs. access VLAN

My switch (2942XL, 12.0(5)WC15) has a behaviour I cannot get my head around. Maybe I am missing something trivial.

I have a server connected to an access port on VLAN 1. That works OK. (Let us not get into a discussion of the merits or otherwise of VLAN 1! I know!)

This server is going to host several virtual servers, all on different VLANs, so I need to convert the switchport into a trunk. So, as a first step, without changing anything on the server, I convert it into a trunk with native VLAN 1. I lose contact with the server, and I cannot work out why. Put the switchport mode back to access, and it works OK again.

If I have a trunk carrying VLAN 1 as native, then I am not tagging VLAN 1. The server is not tagging its frames, and so I should interpret them as VLAN 1. In other words, as far as VLAN 1 is concerned, it should behave just like an access port. So what is going on?

I set the trunk with a switchport mode trunk, and the only encapsulation available on that switch is dot1q. I waited enough time for the port to go into forwarding, so that is not the problem.

When I do a show mac-address int F0/15 in trunk mode, I do not see any dynamic CAM entries. When I do the same in access mode, I do see them.

The server is a Sun. I have done this procedure for a VMware machine and it worked OK.

Can anyone work out what is going on?

Kevin Dorrell

Luxembourg

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Behavior of trunk native VLAN vs. access VLAN

Hi Kevin, could this be DTP? Try configuing "switchport nonegotiate" to turn DTP off. Also is the server capable of running dot1q? If so I would set the encapsulation type to dot1q.

Cisco Employee

Re: Behavior of trunk native VLAN vs. access VLAN

Hi Kevin,

When you configure your interface as trunk you loose the connectivity and that si a very expected behavior because NATIVE VLAN will only come in scenario when TRUNK is UP.

And when you cofigure yor port as trunk and SUN server does not suppot trunking or tagging the trunk status on switch should show you down and if trunking id down there is no concept of native vlan so traffic will not pass.

Can you check the status of trunk after it is configured as trunk port with command

"sh interface fa0/15 switchport" and check the status of trunking.

HTH

Ankur

3 REPLIES

Re: Behavior of trunk native VLAN vs. access VLAN

Hi Kevin, could this be DTP? Try configuing "switchport nonegotiate" to turn DTP off. Also is the server capable of running dot1q? If so I would set the encapsulation type to dot1q.

Cisco Employee

Re: Behavior of trunk native VLAN vs. access VLAN

Hi Kevin,

When you configure your interface as trunk you loose the connectivity and that si a very expected behavior because NATIVE VLAN will only come in scenario when TRUNK is UP.

And when you cofigure yor port as trunk and SUN server does not suppot trunking or tagging the trunk status on switch should show you down and if trunking id down there is no concept of native vlan so traffic will not pass.

Can you check the status of trunk after it is configured as trunk port with command

"sh interface fa0/15 switchport" and check the status of trunking.

HTH

Ankur

Re: Behavior of trunk native VLAN vs. access VLAN

Silly me! I knew it was going to be something trivial.

I got confused between my switches. The 2950s support dot1q only, but the 2900XLs support ISL and dot1q, with ISL being the default. You can guess the rest!

Thank you Mark, thank you Ankur, that has got to be worth 5 points each.

Kevin Dorrell

Luxembourg

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