We have one GigEth and we configure many vlan initerfaces using this. when we allow a new vlan, other vlans allowed get disconnected for some seconds....To counter this problem, we put a comma at the end of the following command line and then add new vlan
switchport trunk allowed vlan 100-116,new_vlan_id
If I put '100-120' instead of '100-116,new_vlan_id', old vlans get disconnected..
What you do is called pruning. When the set of vlans on a trunk is changed, a spanning-tree event is forced. This results in the disconnections that are reported. Also you should keep in mind that manual configuration of spanning-tree related items as these increase the risk of a spanning-tree loop. You should know very well what you are doing.
Best solution would be not to prune any vlans at all. Simply allow all vlans on the backbone:
switchport trunk allowed vlan 1 - 4095
The system will be only slightly heavier loaded but in return, you would not have this problem when adding a new vlan on the network.
When changing the allowed vlans on the trunk, you should issue the following command to simply add the allowed vlan to the trunk:
switchport trunk allowed vlan add %vlan_id%
This way you are not completey changing the trunk and should only be initiating a spanning tree event to add the vlan to the trunk. The rest of the vlans on the trunk should not be affected. The command you are entering is basically deleting the current allowed vlans and recreating them.
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