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Community Member

1 VLAN over 2 VTP domains



1. a network with 3548's (edges) and 2 x 6509 + 2 x 3550 (core, redundant mashed network).

2. Two vtp-domains; one on the 'left' and one on the 'right'.

Is it possible to create a vlan with the same name and tagging over the two vtp-domains; e.g. an edge of the left can communicate with an edge on the 'right' in the same vlan (so with 802.1q without routing)?

I hope this question makes any sense.



Community Member

Re: 1 VLAN over 2 VTP domains

If you're switches are all connected via trunks which allow the VLAN that you are creating then the answer is YES, but you will have to manually configure the VLANs in both domains. Also make sure you are not running VTP pruning on the switches.

Basically, if a vlan exists on all of the switches with the same VLAN number and the links that are connecting the switches are trunks that allow the given VLAN to be carried, then it doesn't matter what the VLAN name or the VTP domain name is.

VTP is only used to "synchronize" VLANs between switches so that a vlan will exist on any switches that are trunking common VLANs. What will not happen in your case is that the VLANs will not be automatically created on the intermediate switches. In other words you will have to configure the VLANs to be the same on all the switches in question. The VLAN name is only a reference tag for the administrator, or processes like VMPS or 802.1X vlan assignments. Neither of these has any bearing on the broadcast domain of the VLAN.

Re: 1 VLAN over 2 VTP domains

Yes, it's possible. But a little dangerous (not recommended by Cisco).

1) You need to know exactly where is the border between these two VTP domains and never forget it. If your topology is not simple (i.e. the border goes between more than two switches) it might be difficult to remember.

2) You have to configure trunks to "no negotiation" on the border. (If you leave them to negotiate, they will never start trunking - requires the same VTP domain.)

3) You have to use the same native VLAN on both border thrunk sides. If not, you'll connect two different VLANs together

4) i'm not sure what VTP prunning could do on the border - it might or not prune the VLANs on the border (I don't think so, but test it).

So it is quite complicated.

And it might be easier to move all switches to VTP transparent mode and configure all VLANs manually.



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