Take a look at my 1941 example:
Does your 2851 work the same? Do you have to use the console on the router to switch to the AP? The AP is integrated in the router but in reality, these are two separate devices that are connected with an internal gigabit link.
In my 1941, the router uses the wlan-GI0/0 and the AP uses its Gi0/0 interface. This is a L2 interface…it’s the same thing as connecting a router to a switch.
We use VLANs so that you can separate traffic. In your example, you have BVI10, 20 and 30 with a different SSID each. You could attach an ACL to each VLAN so that your guest users can only access the Internet, admins can do anything, etc. If you don’t care about this then you can just create one SSID and one VLAN.
The bridge-group ties two interfaces together…correct. Normally on a switch, all interfaces in the same VLAN can communicate with each other. It’s easier to understand the bridge group when you think of a router…you can bridge two L3 interfaces on a router and then it will become one “L2 domain”. We do the same thing on the AP…we bridge the radio + gigabit interface together.
Encapsulation dot1q is configuring trunking. The interface will add a VLAN tag for all frames it sends. You have them on the radio interface which is not needed, unless you want to send tagged frames to the wireless client You need this on the internal interface towards the router…take a look at my 1941 example. It has this command on the AP so that it sends tagged frames to the router. Otherwise the router has no way to know to which VLAN the frames belong.
The bridge group “bridges” two interfaces together. You can use this on a router to bridge two L3 interfaces together so it becomes a single L2 domain or you can use it to bridge two different kind of interfaces. For example, the radio interface uses 802.11 Ethernet frames and the Gigabit interface uses regular Ethernet frames. The bridge group allows us to get IP packets from one type of interface to another.
You should probably put the NAT commands on the VLAN interfaces of the router (interface vlan). These are L3 interfaces that the router can use for everything. The BVI interfaces can probably only be used for simple stuff like telnetting/ssh’ing to the AP.
These routers with an internal AP are pretty complex to configure. It is a lot easier when you only have a radio interface on the router. The 1941 (and I think your 2851 as well) are two separate boxes in one device. You can create pretty complex configurations with multiple SSIDs, authentication types, access-lists and everything.