3-7Configuring an Emulated LAN3.3.3 Data TransferWhen LEC1 receives a network-layer packet from a higher layer protocol totransmit to some destination MAC address (for example, LEC2), LEC1 ini-tially does not know the corresponding ATM address of the destination. Con-sequently, LEC1 transmits an LE_ARP request to the LES.NOTE: The example shown in Figure 3.2 assumesthat LEC2 has already registered with theLES, and that connections similar to thosedescribed for LEC1 already exist.While waiting for the LES to respond, LEC1 forwards the packet to the BUS.The BUS broadcasts the packet to all LECs on the ELAN. This is done to avoiddata loss, and to minimize connection set-up latency (due to the LE_ARP pro-cess) that may not be acceptable to some network protocols.If the LE_ARP response is received, LEC1 establishes a data-direct connectionÏ to the destination address of LEC2. This path will be used for subsequentdata transfers. Before LEC1 begins to use this connection, it first sends a“flush” packet via the BUS to the destination, LEC2. When LEC2 acknowl-edges receipt of this packet, signifying that the BUS path is empty, only thendoes LEC1 begin to use the data-direct connection Ï for data transfer. Thisprocess ensures that the network protocol’s frames arrive in the proper order.If no response is received to the LE_ARP, LEC1 continues to send data via theBUS, while continuing to LE_ARP until a response is received and a data-direct connection to LEC2 is established.If LEC1 already has a data-direct connection to a MAC address it wishes toreach, it need not go through the LE_ARP process again. Instead, it continuesto use the current connection. This is possible because each LEC maintains acache of MAC address to ATM address mappings that it receives in responseto the LE_ARPs it has sent. Entries in this cache are “aged” out over a periodof time. Data-direct connections are also cleared if they remain inactive for aperiod of time.
