Cisco Cisco 2000 Series Wireless LAN Controller White Paper

A discovery request packet is 97 bytes, which includes the 4 byte FCS. A discovery response packet is 106
bytes, which includes the 4 byte FCS.

An LWAPP join request packet is used by the Access Point in order to inform the WLC that it wants to
service clients through the controller. The join request phase is also used in order to discover the MTU
supported by the transport. The initial join request sent by the Access Point is always padded with a test
element of 1596 bytes. Based on how the transport between the AP and the controller is set up, these join
request frames can be fragmented as well. If a join response is received for the initial request, the AP forwards
frames without any fragmentation. The join response also initiates the heartbeat timer (a 30−second value)
which, when it expires, deletes the WLC−AP session. The timer is refreshed upon the receipt of the Echo
Request or Acknowledgements.

If the initial join request does not yield any response, the AP sends out another join request with the test
element, which brings the total payload to 1500 bytes. If the second join request does not yield a response
either, the AP continues to cycle between the large and small packets and eventually times out to start over
from the Discovery phase.

Packet sizes for the join request and response messages vary based on the description but the packet exchange
captured for the purposes of this traffic−study between the AP and the WLC (ap−manager interface) is 3,000
bytes.