0001FIGURE 9-8 WLC RECEIVES ARP REPLY FROM GW AND CONVERTS I...

Figure 9-8 WLC Receives ARP Reply from GW and Converts It to LWAPP

The client, upon receiving the frame, sends an ACK after waiting the SIFS value.

The ARP process of the client now has a mapping to the GW MAC address and can dis-

patch the awaiting frame. Remember that it still must follow the rules, a backoff timer, and

a contention window and eventually transmit the frame following the ARP response.

Using VLANs to Add Control

Here is where things get a little tricky, which brings out the real purpose for this section.

According to the topology that this example is using, the client is trying to communicate

with another device that is connected to the same AP, but it just associates with a different

SSID and on a different subnet. The question is, “How do the AP and WLC keep the two

subnets separate when they are on the wired network?” The answer is VLANs. A VLAN is

a concept in switched networks that allows segmentation of users at a logical level. By us-

ing VLANs on the wired side of the AP and WLC, the client subnet can be logically seg-

mented, just as it is on the wireless space. The results look like this:

SSID = Logical Subnet = Logical VLAN or Logical Broadcast Domain

After the wireless frames move from the AP to the wired network, they must share a single

physical wire. You may think this is hard because having multiple BSSIDs means there is

more than one network, but it is not hard. The way this is accomplished is by using the