Hi Wilson,
The reason VLAN 1 became a special VLAN is that L2 devices needed to have a default VLAN to assign to their ports, including their management port(s). In addition to that, many L2 protocols such as CDP, PAgP, and VTP needed to be sent on a specific VLAN on trunk links. For all these purposes VLAN 1 was chosen.
As a consequence, VLAN 1 may sometimes end up unwisely spanning the entire network if not appropriately pruned and, if its diameter is large enough, the risk of instability can increase significantly.
So as a generic security rule the network administrator should prune VLAN 1 from all the trunks and from all the access ports that don't require it (including not connected and shutdown ports). i.e. always need not be member of trunk link.
http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a008013159f.shtml#wp39009
Rate if it does,
Rgs.
