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new ccna training.When an ARP request is sent to an AP, it's an interesting process, and it's actually a little different from the process of sending it on a wired network. Keep in mind that on a wired network, the packet has only two MAC addresses in the header: the source address and the destination address. An 802.11 frame, on the other hand, can have four addresses: a Source Address (SA, Source Address), a Destination Address (DA, Destination Address), a Transmitter Address (TA, TransmitterAddress), and a Receiving Address (RA, ReceivingAddress). o In this example, SA is the MAC of the client sending the ARP request, DA is the broadcast address (for ARP requests), and RA is the AP. there is no TA in this example.The ARP request is shown in Figure 9-4.Frame Control Address 1 000c.0001.0101 Address 2 0000.0001.0001 Address 3 FFFF.FFFF.FFFF ARP RequestFigure 9~4 ARP requestThe AP receives the ARP and sees the MAC address in it. The AP verifies the Frame Check Sequence (FCS) in the frame and waits for the Short Interframe Space (SIFS) time. When the SIFS time is over, the AP sends back an ACK to the wireless client that sent the ARP request, which is not an ARP response, but an ACKO for the wireless frame transmission.The AP then forwards the frame to the WLC using the Lightweight Access Point Protocol (LWAPP),as shown in Figure 9.5. Figure 9-5 ARP forwarding in LWAPP framesLWAPP frames transmitted from the AP to the WLC are transmitted over the wired network. This raises the question: "What has happened to the 802.11 frame format? " LWAPP simply encapsulates the frame into a 6-byte header. This new 6-byte header takes the AP's IP address and MAC address as the source address and the WLC's IP address and MAC address as the destinationMAC: 0000.0001.00019.2 Basic Topics 113 address. This header encapsulates the original 802.11 frame with 3 MAC addresses (including the broadcast MAC address used for the ARP process). When the WLC receives an LWAPP frame, it opens this frame to display the ARP request and rewrites the ARP request in an 802.3 frame that can be transmitted over the wired network. The first address from the 802.11 frame is discarded, using the second address as the source address for the new 802.3 frame and the third address (the broadcast address) as the destination address. The WLC then forwards the ARP request over the wired network in 802.3 format, as shown in Figure 9.6. Here you can see how the frame appears between wireless client A and the AP, how the AP encapsulates the frame and sends it to the WLC, and how the WLC rewrites the frame and sends it to the wired network.