Hey Pete,
Have a read of this good doc, here is an excerpt;
The routers and switches that make up the physical infrastructure of a network are managed in a different method than the client PCs that attach to that physical infrastructure. The VLAN these router and switch interfaces are members of is called the Native VLAN (by default, VLAN 1). Client PCs are members of a different VLAN, just as IP telephones are members of yet another VLAN. The administrative interface of the access point or bridge (interface BVI1) are considered and numbered a part of the Native VLAN regardless of what VLANs or SSIDs pass through that wireless device.The switchport config might look like this;
switchport mode trunk
switchport trunk encapsulation dot1q
switchport trunk native vlan 1
switchport trunk allowed vlan 1,10,30
Where vlan 1 is Native and vlan 10 and 30 will be associated with SSID's.
When you use an IEEE 802.1Q trunk port, all frames are tagged except those on the VLAN configured as the "native VLAN" for the port. Frames on the native VLAN are always transmitted untagged and are normally received untagged. Therefore, when an AP is connected to the switchport, the native VLAN configured on the AP must match the native VLAN configured on the switchport.
Note: If there is a mismatch in the native VLANs, the frames are dropped.
This scenario is better explained with an example. If the native VLAN on the switchport is configured as VLAN 12 and on the AP, the native VLAN is configured as VLAN 1, then when the AP sends a frame on its native VLAN to the switch, the switch considers the frame as belonging to VLAN 12 since the frames from the native VLAN of the AP are untagged. This causes confusion in the network and results in connectivity problems. The same happens when the switchport forwards a frame from its native VLAN to the AP.
From this good doc;
Using VLANs with Cisco Aironet Wireless Equipment
http://www.cisco.com/en/US/products/hw/wireless/ps4570/products_configuration_example09186a00801d0815.shtml#vlanap
Hope this helps!
Rob
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