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CCNA Wireless Official Exam Certification Guide part 19 is a best of breed Cisco® exam study guide that focuses specifically on the objectives for the CCNA® Wireless IUWNE exam. Senior instructor Brandon Carroll shares preparation hints and test-taking tips, helping you identify areas of weakness and improve both your conceptual knowledge and hands-on skills. Material is presented in a concise manner, focusing on increasing your understanding and retention of exam topics. | 152 CCNA Wireless Official Exam Certification Guide At some point the frame will be received by a Layer 3 device hopefully the default gateway. In Figure 9-7 the router has received the ARP request and will respond to it with its MAC address. Key Topic DESTINATION SOURCE ARP 0000.0000.0001 000c.0A0A.1111 REQUEST 000000 Client A 10.99.99.1 0000.0000.0001 Figure 9-7 Gateway Responds to ARP That ARP response is sent back as a unicast message so the switches in the path are going to forward it directly to the port that leads back to the wireless client rather than flooding the frame out all ports. Eventually the frame is received by the WLC and it must be rebuilt as an 802.11 frame. When the WLC rewrites the frame it places the DA as address 1 the SA as address 3 and the TA as address 2 which is the SSID of the AP. Figure 9-8 illustrates this process. As illustrated in Figure 9-9 the newly formed 802.11 frame is placed inside an LWAPP header where the AP IP and MAC is the destination and the WLC IP and MAC is the source. The LWAPP frame is forwarded to the AP. Next the AP must remove the LWAPP header exposing the 802.11 frame. The 802.11 frame is buffered and the process of sending a frame on the wireless network begins. The AP starts a backoff timer and begins counting down. If a wireless frame is heard during the countdown the reservation in the heard frame is added to the countdown and the AP continues. Eventually the timer expires and the frame can be sent an 802.11 frame. Chapter 9 Delivering Packets from the Wireless to Wired Network 153 Key Topic 000000 Client A 10.99.99.1 0000.0000.0001 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 dispatch the awaiting frame. Remember that it still must follow the rules a backoff timer and a contention window and eventually transmit the frame following
