When adding an external network in vCloud you receive an error: Port group has a conflicting VLAN (2003988)

Symptoms

• You cannot add an external network in vCloud
• You receive this error:
  
  Port group <portgroup_name> has a conflicting VLAN with another port group that is currently being used.

Cause

This issue can occur in the following scenarios:

• There is already an external network associated to a portgroup with the same VLAN that you are trying to use on a new external network.  By default, you can only associate one external network to a single VLAN in vCloud.  
• You have an existing external network associated to a portgroup with a VLAN that has been modified in vCenter and the external network was deleted in vCloud. 
• You have network pools that utilize the same VLAN or has a range of VLANs overlapping an external network VLAN.

Resolution

If you need to have two or more external networks associated to portgroups with the same VLAN IDs, you must enable this in vCloud.  To enable portgroups to share VLAN IDs:

1. Log into vCloud as a system administrator.
2. Select Administration from the System tab.
3. Select General under the System Settings category.
4. Enable Allow Overlapping External Networks.

If the VLAN of the portgroup that the external network is associated with has been modified, you can work around the issue:

1. Enable Allow Overlapping External Networks.
2. Recreate the external network
3. Disable Allow Overlapping External Networks.

 Source:-

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2003988&src=vmw_so_vex_ragga_1012%201%200%200%200%200%200