VLAN

Unanswered Question
Jan 25th, 2008

I have two switches with following config.

switch A = vlan 1 ( Deafult, Active) , vlan 13 (native, Active) , Interface vlan 13 = 10.11.11.11 / 24

switch B = vlan 1 ( Deafult, Active) , vlan 13 (native, Active) , Interface vlan 13 = 10.11.11.12 / 24

Trunk is b/w these switches with native vlan 13 and Allowd all vlans.both switches vlan 13 IPs can ping eachother with this config.

here is my qs.

i have config vlan 13 as native, but assigned vlan 1 as native on trunk. Now which vlan will be native on the trunk ?

i am a bit confused abut the usage of native vlans on two switches which sit in a enterprise network.

one more thing. let assume we have three siwtches, A, B, and C. I have vlan 100 on switch A and switch C and no vlan 100 on switch C. can i b able to reach till switch c by using vlan 100 ?? or there will be a need of creating vlan 100 on the middle / passing by switch i.e switch B.

I have this problem too.
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Edison Ortiz Fri, 01/25/2008 - 12:51

You are only allowed to have one native Vlan on a trunk port and you posted that you want 13 and 1 to be Native.

You can verify your current native vlan on a trunk with the show interface trunk command.

If you have 3 switches, A&C with Vlan 100 and B as the middle switch w/o Vlan 100, the packets from/to A & C will be dropped until you create Vlan 100 on switch B.

HTH,

__

Edison.

munawar.zeeshan Sat, 01/26/2008 - 02:40

a bit mistake, as i mentioned i have created VLAN 13 with name native but on trunk i havent specially created any native vlan , so default vative vlan will be vlan 1 or vlan 13 ?? i guess it be vlan 1. then what about vlan 13 ? it will not be sued as native over the trunk ??

glen.grant Sat, 01/26/2008 - 03:15

If you have not specifically stated what the native vlan will be on the interface then it will be vlan 1 by default.

jorgenolla Sat, 01/26/2008 - 15:07

switch A = vlan 1 ( Deafult, Active) , vlan 13 (native, Active) , Interface vlan 13 = 10.11.11.11 / 24

switch B = vlan 1 ( Deafult, Active) , vlan 13 (native, Active) , Interface vlan 13 = 10.11.11.12 / 24

This configuration only shows that you have named Vlan 13 "native". This is just a name, and has nothing to do with the actual assigning of the native vlan.

If you wish to make vlan 13 your native (untagged) vlan, then you need to use the following command on the trunk links between the switches:

#switchport trunk native vlan 13

This command must be entered on all your trunks between the 2 switches. If you don't, you will get a native vlan mismatch message.

Best Regards

shrikar.dange Sun, 01/27/2008 - 19:46

You can bring up a trunk with different native VLANs on each end; however,both switches will log error messages about the mismatch, and the potential exists that traffic will not pass correctly between the two native VLANs.

The native VLAN mismatch is discovered through the exchange of CDP messages, not

through examination of the trunk itself.Also, the native VLAN is configured independently of the trunk encapsulation.

If you disable VLAN 1 your VTP/DTP/CDP trffic will traverse through the native vlan.

HTH,

regards,

shri :)

jorgenolla Mon, 01/28/2008 - 06:19

Hi shrika,

I'm a little confused by your statement. The native vlan can be assigned to be different on each trunk port. If you have multiple trunks to different switches, the native vlan can be different on each trunk, as long as it matches the native vlan of the switch at the end point of the trunk.

The CDP message comes because the protocol has examined the trunk settings at each end point, and has determined that the native vlan has been assigned differently in each end, thus the potential of creating a black hole or loop.

How do you disable Vlan 1, exactly? You cannot delete vlan 1 (Default Vlan)! And even if you assign the native vlan to be something else that the Default Vlan (Vlan1), control traffic will still traverse Vlan1 (Default Vlan), the only difference is that the control traffic will be tagged.

Best Regards

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